Ribosome\u00a0Review<\/h1>\r\n<\/div>\r\nThe 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the [pb_glossary id=\"5551\"]ribosome[\/pb_glossary]<\/strong>, the\u00a0cell structure\u00a0where\u00a0[pb_glossary id=\"5813\"]proteins[\/pb_glossary]\u00a0are synthesized. The slender silver strand is the messenger [pb_glossary id=\"519\"]RNA[\/pb_glossary](mRNA)\u00a0bringing the code for a [pb_glossary id=\"5813\"]protein[\/pb_glossary] out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single [pb_glossary id=\"5551\"]ribosome[\/pb_glossary]), which can \"read\" the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0[pb_glossary id=\"5665\"]cells[\/pb_glossary]\u00a0\u2014 whether they are [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] or [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] \u2014 contain [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary], but only eukaryotic cells also contain a\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary]\u00a0and several other types of [pb_glossary id=\"5557\"]organelles[\/pb_glossary].\r\n\r\nWhat Are Organelles?<\/h1>\r\n<\/div>\r\nAn\u00a0[pb_glossary id=\"5557\"]organelle[\/pb_glossary]<\/strong>\u00a0is a structure within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] [pb_glossary id=\"5665\"]cell[\/pb_glossary] that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary],\u00a0[pb_glossary id=\"5783\"]mitochondria[\/pb_glossary],\u00a0[pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary],\u00a0[pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary], [pb_glossary id=\"5825\"]vesicles[\/pb_glossary], and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]. [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells.\r\n\r\nThe Nucleus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary]<\/strong> is the largest organelle in a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell, and it's considered the cell\u2019s control center. It contains most of the cell\u2019s [pb_glossary id=\"277\"]DNA[\/pb_glossary](which makes up chromosomes), and it is encoded with the genetic instructions for making [pb_glossary id=\"5813\"]proteins[\/pb_glossary]. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0[pb_glossary id=\"5569\"]nucleoplasm[\/pb_glossary]<\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the [pb_glossary id=\"5737\"]cytosol[\/pb_glossary] found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.\r\n\r\n[caption id=\"attachment_1636\" align=\"alignright\" width=\"459\"]![\"This](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Nucleus-1-2.png\")
Figure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em>[\/caption]\r\n\r\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the [pb_glossary id=\"5785\"]nuclear envelope[\/pb_glossary]<\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0[pb_glossary id=\"5789\"]nuclear pores[\/pb_glossary]<\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0[pb_glossary id=\"519\"]RNA[\/pb_glossary]\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]. After being produced in the [pb_glossary id=\"5793\"]nucleolus[\/pb_glossary], ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.\r\n\r\nMitochondria<\/h1>\r\n<\/div>\r\nThe\u00a0mitochondrion<\/strong>\u00a0(plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes\u00a0[pb_glossary id=\"5753\"]energy[\/pb_glossary]\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the \"power plants of the cell.\" They use energy from\u00a0organic compounds\u00a0(such as [pb_glossary id=\"5451\"]glucose[\/pb_glossary]) to make molecules of\u00a0[pb_glossary id=\"5549\"]ATP[\/pb_glossary] (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.\r\n\r\n[caption id=\"attachment_1634\" align=\"alignleft\" width=\"459\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Mitochondrion_structure.svg_-2.png\")
Figure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em>[\/caption]\r\n\r\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That's because they are!\r\n\r\nScientists think that mitochondria were once free-living organisms because they contain their own\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] cells, and the two organisms evolved a [pb_glossary id=\"5819\"]symbiotic[\/pb_glossary] relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as [pb_glossary id=\"5557\"]organelles[\/pb_glossary]\u00a0inside them. This theory is called\u00a0[pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary],<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span>\r\n\r\nEndoplasmic Reticulum<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary]<\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport [pb_glossary id=\"5813\"]proteins[\/pb_glossary] and [pb_glossary id=\"5651\"]lipids[\/pb_glossary]. There are two types of endoplasmic reticulum: [pb_glossary id=\"5603\"]rough endoplasmic reticulum[\/pb_glossary] (rER) and [pb_glossary id=\"5571\"]smooth endoplasmic reticulum[\/pb_glossary] (sER). Both types are shown in Figure 4.6.4.\r\n\r\n \t- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\r\n \t
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_1629\" align=\"aligncenter\" width=\"726\"]
![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Endomembrane_system_diagram_en.svg_-2.png\")
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed \"the endomembrane system\". All of the organelles in this system are composed of plasma membrane.<\/em>[\/caption]\r\n\r\n\r\n\r\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.\r\n\r\n<\/div>\r\n\r\nGolgi Apparatus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary]<\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the [pb_glossary id=\"5621\"]cell membrane[\/pb_glossary] for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.\r\n\r\nVesicles and Vacuoles<\/h1>\r\n<\/div>\r\nBoth\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport [pb_glossary id=\"5813\"]protein[\/pb_glossary] and [pb_glossary id=\"5651\"]lipid[\/pb_glossary] molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.\r\n\r\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don't leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span>\r\n\r\nCentrioles<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1637\" align=\"alignright\" width=\"442\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Centrioles-1-2.png\")
Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"5565\"]Centrioles[\/pb_glossary]<\/strong>\u00a0are organelles involved in\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]. The function of centrioles is to help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]<\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span>\r\n\r\n\r\n[caption id=\"attachment_1641\" align=\"alignleft\" width=\"333\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ribosome_shape-2.png\")
Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\n![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Ribosomal-Art-2.jpg\")
Figure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
What Are Organelles?<\/h1>\r\n<\/div>\r\nAn\u00a0[pb_glossary id=\"5557\"]organelle[\/pb_glossary]<\/strong>\u00a0is a structure within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] [pb_glossary id=\"5665\"]cell[\/pb_glossary] that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary],\u00a0[pb_glossary id=\"5783\"]mitochondria[\/pb_glossary],\u00a0[pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary],\u00a0[pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary], [pb_glossary id=\"5825\"]vesicles[\/pb_glossary], and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]. [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells.\r\n\r\nThe Nucleus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary]<\/strong> is the largest organelle in a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell, and it's considered the cell\u2019s control center. It contains most of the cell\u2019s [pb_glossary id=\"277\"]DNA[\/pb_glossary](which makes up chromosomes), and it is encoded with the genetic instructions for making [pb_glossary id=\"5813\"]proteins[\/pb_glossary]. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0[pb_glossary id=\"5569\"]nucleoplasm[\/pb_glossary]<\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the [pb_glossary id=\"5737\"]cytosol[\/pb_glossary] found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.\r\n\r\n[caption id=\"attachment_1636\" align=\"alignright\" width=\"459\"] Figure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em>[\/caption]\r\n\r\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the [pb_glossary id=\"5785\"]nuclear envelope[\/pb_glossary]<\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0[pb_glossary id=\"5789\"]nuclear pores[\/pb_glossary]<\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0[pb_glossary id=\"519\"]RNA[\/pb_glossary]\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]. After being produced in the [pb_glossary id=\"5793\"]nucleolus[\/pb_glossary], ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.\r\n\r\nMitochondria<\/h1>\r\n<\/div>\r\nThe\u00a0mitochondrion<\/strong>\u00a0(plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes\u00a0[pb_glossary id=\"5753\"]energy[\/pb_glossary]\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the \"power plants of the cell.\" They use energy from\u00a0organic compounds\u00a0(such as [pb_glossary id=\"5451\"]glucose[\/pb_glossary]) to make molecules of\u00a0[pb_glossary id=\"5549\"]ATP[\/pb_glossary] (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.\r\n\r\n[caption id=\"attachment_1634\" align=\"alignleft\" width=\"459\"] Figure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em>[\/caption]\r\n\r\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That's because they are!\r\n\r\nScientists think that mitochondria were once free-living organisms because they contain their own\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] cells, and the two organisms evolved a [pb_glossary id=\"5819\"]symbiotic[\/pb_glossary] relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as [pb_glossary id=\"5557\"]organelles[\/pb_glossary]\u00a0inside them. This theory is called\u00a0[pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary],<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span>\r\n\r\nEndoplasmic Reticulum<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary]<\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport [pb_glossary id=\"5813\"]proteins[\/pb_glossary] and [pb_glossary id=\"5651\"]lipids[\/pb_glossary]. There are two types of endoplasmic reticulum: [pb_glossary id=\"5603\"]rough endoplasmic reticulum[\/pb_glossary] (rER) and [pb_glossary id=\"5571\"]smooth endoplasmic reticulum[\/pb_glossary] (sER). Both types are shown in Figure 4.6.4.\r\n\r\n \t- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\r\n \t
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_1629\" align=\"aligncenter\" width=\"726\"] Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed \"the endomembrane system\". All of the organelles in this system are composed of plasma membrane.<\/em>[\/caption]\r\n\r\n\r\n\r\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.\r\n\r\n<\/div>\r\n\r\n
Golgi Apparatus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary]<\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the [pb_glossary id=\"5621\"]cell membrane[\/pb_glossary] for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.\r\n\r\nVesicles and Vacuoles<\/h1>\r\n<\/div>\r\nBoth\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport [pb_glossary id=\"5813\"]protein[\/pb_glossary] and [pb_glossary id=\"5651\"]lipid[\/pb_glossary] molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.\r\n\r\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don't leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span>\r\n\r\nCentrioles<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1637\" align=\"alignright\" width=\"442\"] Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"5565\"]Centrioles[\/pb_glossary]<\/strong>\u00a0are organelles involved in\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]. The function of centrioles is to help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]<\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span>\r\n\r\n\r\n[caption id=\"attachment_1641\" align=\"alignleft\" width=\"333\"] Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
The Nucleus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5797\"]nucleus[\/pb_glossary]<\/strong> is the largest organelle in a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell, and it's considered the cell\u2019s control center. It contains most of the cell\u2019s [pb_glossary id=\"277\"]DNA[\/pb_glossary](which makes up chromosomes), and it is encoded with the genetic instructions for making [pb_glossary id=\"5813\"]proteins[\/pb_glossary]. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0[pb_glossary id=\"5569\"]nucleoplasm[\/pb_glossary]<\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the [pb_glossary id=\"5737\"]cytosol[\/pb_glossary] found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.\r\n\r\n[caption id=\"attachment_1636\" align=\"alignright\" width=\"459\"] Figure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em>[\/caption]\r\n\r\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the [pb_glossary id=\"5785\"]nuclear envelope[\/pb_glossary]<\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0[pb_glossary id=\"5789\"]nuclear pores[\/pb_glossary]<\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0[pb_glossary id=\"519\"]RNA[\/pb_glossary]\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]. After being produced in the [pb_glossary id=\"5793\"]nucleolus[\/pb_glossary], ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.\r\n\r\nMitochondria<\/h1>\r\n<\/div>\r\nThe\u00a0mitochondrion<\/strong>\u00a0(plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes\u00a0[pb_glossary id=\"5753\"]energy[\/pb_glossary]\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the \"power plants of the cell.\" They use energy from\u00a0organic compounds\u00a0(such as [pb_glossary id=\"5451\"]glucose[\/pb_glossary]) to make molecules of\u00a0[pb_glossary id=\"5549\"]ATP[\/pb_glossary] (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.\r\n\r\n[caption id=\"attachment_1634\" align=\"alignleft\" width=\"459\"] Figure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em>[\/caption]\r\n\r\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That's because they are!\r\n\r\nScientists think that mitochondria were once free-living organisms because they contain their own\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] cells, and the two organisms evolved a [pb_glossary id=\"5819\"]symbiotic[\/pb_glossary] relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as [pb_glossary id=\"5557\"]organelles[\/pb_glossary]\u00a0inside them. This theory is called\u00a0[pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary],<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span>\r\n\r\nEndoplasmic Reticulum<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary]<\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport [pb_glossary id=\"5813\"]proteins[\/pb_glossary] and [pb_glossary id=\"5651\"]lipids[\/pb_glossary]. There are two types of endoplasmic reticulum: [pb_glossary id=\"5603\"]rough endoplasmic reticulum[\/pb_glossary] (rER) and [pb_glossary id=\"5571\"]smooth endoplasmic reticulum[\/pb_glossary] (sER). Both types are shown in Figure 4.6.4.\r\n\r\n \t- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\r\n \t
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_1629\" align=\"aligncenter\" width=\"726\"] Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed \"the endomembrane system\". All of the organelles in this system are composed of plasma membrane.<\/em>[\/caption]\r\n\r\n\r\n\r\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.\r\n\r\n<\/div>\r\n\r\n
Golgi Apparatus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary]<\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the [pb_glossary id=\"5621\"]cell membrane[\/pb_glossary] for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.\r\n\r\nVesicles and Vacuoles<\/h1>\r\n<\/div>\r\nBoth\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport [pb_glossary id=\"5813\"]protein[\/pb_glossary] and [pb_glossary id=\"5651\"]lipid[\/pb_glossary] molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.\r\n\r\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don't leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span>\r\n\r\nCentrioles<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1637\" align=\"alignright\" width=\"442\"] Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"5565\"]Centrioles[\/pb_glossary]<\/strong>\u00a0are organelles involved in\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]. The function of centrioles is to help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]<\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span>\r\n\r\n\r\n[caption id=\"attachment_1641\" align=\"alignleft\" width=\"333\"] Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
Mitochondria<\/h1>\r\n<\/div>\r\nThe\u00a0mitochondrion<\/strong>\u00a0(plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes\u00a0[pb_glossary id=\"5753\"]energy[\/pb_glossary]\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the \"power plants of the cell.\" They use energy from\u00a0organic compounds\u00a0(such as [pb_glossary id=\"5451\"]glucose[\/pb_glossary]) to make molecules of\u00a0[pb_glossary id=\"5549\"]ATP[\/pb_glossary] (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.\r\n\r\n[caption id=\"attachment_1634\" align=\"alignleft\" width=\"459\"] Figure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em>[\/caption]\r\n\r\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That's because they are!\r\n\r\nScientists think that mitochondria were once free-living organisms because they contain their own\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] cells, and the two organisms evolved a [pb_glossary id=\"5819\"]symbiotic[\/pb_glossary] relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as [pb_glossary id=\"5557\"]organelles[\/pb_glossary]\u00a0inside them. This theory is called\u00a0[pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary],<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span>\r\n\r\nEndoplasmic Reticulum<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary]<\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport [pb_glossary id=\"5813\"]proteins[\/pb_glossary] and [pb_glossary id=\"5651\"]lipids[\/pb_glossary]. There are two types of endoplasmic reticulum: [pb_glossary id=\"5603\"]rough endoplasmic reticulum[\/pb_glossary] (rER) and [pb_glossary id=\"5571\"]smooth endoplasmic reticulum[\/pb_glossary] (sER). Both types are shown in Figure 4.6.4.\r\n\r\n \t- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\r\n \t
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_1629\" align=\"aligncenter\" width=\"726\"] Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed \"the endomembrane system\". All of the organelles in this system are composed of plasma membrane.<\/em>[\/caption]\r\n\r\n\r\n\r\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.\r\n\r\n<\/div>\r\n\r\n
Golgi Apparatus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary]<\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the [pb_glossary id=\"5621\"]cell membrane[\/pb_glossary] for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.\r\n\r\nVesicles and Vacuoles<\/h1>\r\n<\/div>\r\nBoth\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport [pb_glossary id=\"5813\"]protein[\/pb_glossary] and [pb_glossary id=\"5651\"]lipid[\/pb_glossary] molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.\r\n\r\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don't leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span>\r\n\r\nCentrioles<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1637\" align=\"alignright\" width=\"442\"] Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"5565\"]Centrioles[\/pb_glossary]<\/strong>\u00a0are organelles involved in\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]. The function of centrioles is to help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]<\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span>\r\n\r\n\r\n[caption id=\"attachment_1641\" align=\"alignleft\" width=\"333\"] Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
Endoplasmic Reticulum<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary]<\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport [pb_glossary id=\"5813\"]proteins[\/pb_glossary] and [pb_glossary id=\"5651\"]lipids[\/pb_glossary]. There are two types of endoplasmic reticulum: [pb_glossary id=\"5603\"]rough endoplasmic reticulum[\/pb_glossary] (rER) and [pb_glossary id=\"5571\"]smooth endoplasmic reticulum[\/pb_glossary] (sER). Both types are shown in Figure 4.6.4.\r\n\r\n \t- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\r\n \t
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_1629\" align=\"aligncenter\" width=\"726\"] Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed \"the endomembrane system\". All of the organelles in this system are composed of plasma membrane.<\/em>[\/caption]\r\n\r\n\r\n\r\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.\r\n\r\n<\/div>\r\n\r\n
Golgi Apparatus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary]<\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the [pb_glossary id=\"5621\"]cell membrane[\/pb_glossary] for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.\r\n\r\nVesicles and Vacuoles<\/h1>\r\n<\/div>\r\nBoth\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport [pb_glossary id=\"5813\"]protein[\/pb_glossary] and [pb_glossary id=\"5651\"]lipid[\/pb_glossary] molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.\r\n\r\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don't leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span>\r\n\r\nCentrioles<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1637\" align=\"alignright\" width=\"442\"] Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"5565\"]Centrioles[\/pb_glossary]<\/strong>\u00a0are organelles involved in\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]. The function of centrioles is to help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]<\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span>\r\n\r\n\r\n[caption id=\"attachment_1641\" align=\"alignleft\" width=\"333\"] Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed \"the endomembrane system\". All of the organelles in this system are composed of plasma membrane.<\/em>[\/caption]\r\n\r\nGolgi Apparatus<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary]<\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the [pb_glossary id=\"5621\"]cell membrane[\/pb_glossary] for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.\r\n\r\nVesicles and Vacuoles<\/h1>\r\n<\/div>\r\nBoth\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport [pb_glossary id=\"5813\"]protein[\/pb_glossary] and [pb_glossary id=\"5651\"]lipid[\/pb_glossary] molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.\r\n\r\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don't leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span>\r\n\r\nCentrioles<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1637\" align=\"alignright\" width=\"442\"] Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"5565\"]Centrioles[\/pb_glossary]<\/strong>\u00a0are organelles involved in\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]. The function of centrioles is to help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]<\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span>\r\n\r\n\r\n[caption id=\"attachment_1641\" align=\"alignleft\" width=\"333\"] Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
Vesicles and Vacuoles<\/h1>\r\n<\/div>\r\nBoth\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport [pb_glossary id=\"5813\"]protein[\/pb_glossary] and [pb_glossary id=\"5651\"]lipid[\/pb_glossary] molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.\r\n\r\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don't leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span>\r\n\r\nCentrioles<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1637\" align=\"alignright\" width=\"442\"] Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"5565\"]Centrioles[\/pb_glossary]<\/strong>\u00a0are organelles involved in\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]. The function of centrioles is to help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]<\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span>\r\n\r\n\r\n[caption id=\"attachment_1641\" align=\"alignleft\" width=\"333\"] Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
Centrioles<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1637\" align=\"alignright\" width=\"442\"] Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"5565\"]Centrioles[\/pb_glossary]<\/strong>\u00a0are organelles involved in\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]. The function of centrioles is to help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]<\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span>\r\n\r\n\r\n[caption id=\"attachment_1641\" align=\"alignleft\" width=\"333\"] Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em>[\/caption]\r\nRibosomes<\/h1>\r\n<\/div>\r\nRibosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each [pb_glossary id=\"5551\"]ribosome[\/pb_glossary] is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.\r\n\r\n4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
4.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\nNucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\nFigure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n\nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\nFigure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\nAs you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\nFigure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\nMitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n\n- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n\nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\nThere are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n\nCentrioles<\/h1>\n<\/div>\nFigure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\nCentrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\nRibosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n\n\n4.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n4.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
- \r\n \t
- An [pb_glossary id=\"5557\"]organelle[\/pb_glossary] is a structure within the cytoplasm of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell that is enclosed within a membrane and performs a specific job. Although [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\r\n \t
- The mitochondrion (plural, [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary]) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted [pb_glossary id=\"5481\"]endosymbiotic theory[\/pb_glossary], mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\r\n \t
- The [pb_glossary id=\"5749\"]endoplasmic reticulum[\/pb_glossary] (ER) is an organelle that helps make and transport proteins and lipids. [pb_glossary id=\"5603\"]Rough endoplasmic reticulum[\/pb_glossary] (rER) is studded with ribosomes. [pb_glossary id=\"5571\"]Smooth endoplasmic reticulum[\/pb_glossary] (sER) has no ribosomes.<\/li>\r\n \t
- The [pb_glossary id=\"5485\"]Golgi apparatus[\/pb_glossary] is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\r\n \t
- Both\u00a0[pb_glossary id=\"5825\"]vesicles[\/pb_glossary] and [pb_glossary id=\"5821\"]vacuoles[\/pb_glossary]\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\r\n \t
- [pb_glossary id=\"5565\"]Centrioles[\/pb_glossary] are organelles located near the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] that help organize the\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0before\u00a0[pb_glossary id=\"5633\"]cell division[\/pb_glossary]\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"5551\"]Ribosomes[\/pb_glossary]\u00a0are small structures where proteins are made. They are found in both\u00a0[pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] and [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cells. They may be found alone or in groups within the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary] or on the rER.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 4.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- What is an\u00a0organelle?<\/li>\r\n \t
- Describe the structure and function of the nucleus.<\/li>\r\n \t
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\r\n \t
- Why are mitochondria referred to as the \"power plants of the cell\"?<\/li>\r\n \t
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\r\n \t
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\r\n \t
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\r\n \t
- [h5p id=\"473\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 4.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&t=121s\r\n\r\nBiology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be\r\n\r\nDavid Bolinsky: Visualizing the wonder of a living cell, TED, 2007.\r\n\r\n<\/div>\r\n<\/div>\r\nAttributes<\/h2>\r\nFigure 4.6.1\u00a0<\/strong>\r\n\r\nRibosomes at Work<\/a> by Pedrik<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 4.6.2<\/strong>\r\n\r\nNucleus<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.3\u00a0<\/strong>\r\n\r\nMitochondrion_structure.svg<\/a> by Kelvinsong<\/a>; modified by Sowlos<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 4.6.4<\/strong>\r\n\r\nEndomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [LadyofHats]<\/a> on Wikimedia Commons is released into the public domain <\/a>(https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.6.5<\/span><\/strong>\r\n\r\nCentrioles<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\u00a0<\/span>\r\n\r\nFigure 4.6.6<\/strong>\r\n\r\nRibosome_shape<\/a> by Vossman<\/a> on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n
References<\/h2>\r\n
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine<\/i>\u00a01\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436. https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.https:\/\/en.wikiversity.org\/wiki\/WikiJournal_of_Medicine\/Medical_gallery_of_Blausen_Medical_2014<\/p>\r\n
Nucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https:\/\/www.youtube.com\/watch?v=URUJD5NEXC8&feature=youtu.be<\/p>\r\n
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https:\/\/www.youtube.com\/watch?v=Id2rZS59xSE&feature=youtu.be<\/p>\r\n \r\n\r\n ","rendered":"
<\/p>\n
Figure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.\u00a0 This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.<\/em><\/figcaption><\/figure>\n \nRibosome\u00a0Review<\/h1>\n<\/div>\n
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.\u00a0 This artwork brings to life an important structure in living cells: the ribosome<\/a><\/strong>, the\u00a0cell structure\u00a0where\u00a0proteins<\/a>\u00a0are synthesized. The slender silver strand is the messenger RNA<\/a>(mRNA)\u00a0bringing the code for a protein<\/a> out into the cytoplasm.\u00a0 The purple and green structures are ribosomal subunits (which together form a single ribosome<\/a>), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.\u00a0\u00a0All living\u00a0cells<\/a>\u00a0\u2014 whether they are prokaryotic<\/a> or eukaryotic<\/a> \u2014 contain ribosomes<\/a>, but only eukaryotic cells also contain a\u00a0nucleus<\/a>\u00a0and several other types of organelles<\/a>.<\/p>\n
\nWhat Are Organelles?<\/h1>\n<\/div>\n
An\u00a0organelle<\/a><\/strong>\u00a0is a structure within the cytoplasm<\/a> of a eukaryotic<\/a> cell<\/a> that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the\u00a0nucleus<\/a>,\u00a0mitochondria<\/a>,\u00a0endoplasmic reticulum<\/a>,\u00a0Golgi apparatus<\/a>, vesicles<\/a>, and vacuoles<\/a>. Ribosomes<\/a>\u00a0are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic<\/a> cells.<\/p>\n
\nThe Nucleus<\/h1>\n<\/div>\n
The\u00a0nucleus<\/a><\/strong> is the largest organelle in a eukaryotic<\/a> cell, and it’s considered the cell\u2019s control center. It contains most of the cell\u2019s DNA<\/a>(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins<\/a>. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called\u00a0nucleoplasm<\/a><\/strong>, which\u00a0is similar in\u00a0composition\u00a0to the cytosol<\/a> found in the cytoplasm outside the nucleus.\u00a0Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red\u00a0blood\u00a0cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.<\/p>\n
Figure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.<\/em><\/figcaption><\/figure>\n As you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope<\/a><\/strong>. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called\u00a0nuclear pores<\/a><\/strong>\u00a0allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and\u00a0RNA<\/a>\u00a0molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model\u00a0below\u00a0is mainly involved in the assembly of ribosomes<\/a>. After being produced in the nucleolus<\/a>, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.<\/p>\n
\nMitochondria<\/h1>\n<\/div>\n
The\u00a0mitochondrion<\/strong>\u00a0(plural, mitochondria<\/a>) is an organelle that makes\u00a0energy<\/a>\u00a0available to the cell. This is why\u00a0mitochondria\u00a0are sometimes referred to as the “power plants of the cell.” They use energy from\u00a0organic compounds\u00a0(such as glucose<\/a>) to make molecules of\u00a0ATP<\/a> (adenosine triphosphate)<\/strong>, an energy-carrying molecule that is used almost universally inside cells for\u00a0energy.<\/p>\n
Figure 4.6.3 Mitochondria contain their own DNA and ribosomes!<\/em><\/figcaption><\/figure>\n Mitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.\u00a0 In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.\u00a0 Does this sound similar to the requirements to be considered a cell?\u00a0 That’s because they are!<\/p>\n
Scientists think that mitochondria were once free-living organisms because they contain their own\u00a0DNA<\/a>. They theorize that ancient\u00a0prokaryotes\u00a0infected (or were engulfed by) larger prokaryotic<\/a> cells, and the two organisms evolved a symbiotic<\/a> relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra\u00a0energy\u00a0from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles<\/a>\u00a0inside them. This theory is called\u00a0endosymbiotic theory<\/a>,<\/strong> and it is widely accepted by biologists today. (See the video in section 4.3<\/a> to learn all about endosymbiotic theory.)<\/span><\/p>\n
\nEndoplasmic Reticulum<\/h1>\n<\/div>\n
The\u00a0endoplasmic reticulum<\/a><\/strong>\u00a0(ER)<\/strong> is an organelle that helps make and transport proteins<\/a> and lipids<\/a>. There are two types of endoplasmic reticulum: rough endoplasmic reticulum<\/a> (rER) and smooth endoplasmic reticulum<\/a> (sER). Both types are shown in Figure 4.6.4.<\/p>\n
- \n
- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.<\/li>\n
- sER looks smooth because it does not have ribosomes. sER makes\u00a0lipids, stores substances, and plays other roles.<\/li>\n<\/ul>\n
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.<\/em><\/figcaption><\/figure>\n \nThe Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.<\/p>\n<\/div>\n
\nGolgi Apparatus<\/h1>\n<\/div>\n
The\u00a0Golgi apparatus<\/a><\/strong> (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane<\/a> for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.<\/p>\n
\nVesicles and Vacuoles<\/h1>\n<\/div>\n
Both\u00a0vesicles<\/a><\/strong>\u00a0and\u00a0vacuoles<\/a>\u00a0<\/strong>are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein<\/a> and lipid<\/a> molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.<\/p>\n
There are some vesicles which are specialized to carry out specific functions.\u00a0 Lysosomes, which use\u00a0enzymes\u00a0to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.\u00a0 Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.\u00a0<\/span><\/p>\n
\nCentrioles<\/h1>\n<\/div>\n
Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.<\/em><\/figcaption><\/figure>\n Centrioles<\/a><\/strong>\u00a0are organelles involved in\u00a0cell division<\/a>. The function of centrioles is to help organize the\u00a0chromosomes<\/a><\/span>\u00a0before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a\u00a0<\/span>protein\u00a0named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured\u00a0<\/span>in Figure 4.6.5.<\/span><\/p>\n
\nFigure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.<\/em><\/figcaption><\/figure>\n Ribosomes<\/h1>\n<\/div>\n
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome<\/a> is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in\u00a0 Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.<\/p>\n
\n\n 4.6 Summary<\/span><\/h1>\n<\/header>\n
\n- \n
- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- The mitochondrion (plural, mitochondria<\/a>) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory<\/a>, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.<\/li>\n
- The endoplasmic reticulum<\/a> (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum<\/a> (rER) is studded with ribosomes. Smooth endoplasmic reticulum<\/a> (sER) has no ribosomes.<\/li>\n
- The Golgi apparatus<\/a> is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.<\/li>\n
- Both\u00a0vesicles<\/a> and vacuoles<\/a>\u00a0are sac-like organelles that may be used to store and transport materials in the cell or as chambers for\u00a0biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.<\/li>\n
- Centrioles<\/a> are organelles located near the nucleus<\/a> that help organize the\u00a0chromosomes<\/a>\u00a0before\u00a0cell division<\/a>\u00a0so each daughter cell receives the correct number of chromosomes.<\/li>\n
- Ribosomes<\/a>\u00a0are small structures where proteins are made. They are found in both\u00a0prokaryotic<\/a> and eukaryotic<\/a> cells. They may be found alone or in groups within the cytoplasm<\/a> or on the rER.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n
\n\n 4.6 Review Questions<\/span><\/h1>\n<\/header>\n
\n- \n
- What is an\u00a0organelle?<\/li>\n
- Describe the structure and function of the nucleus.<\/li>\n
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.<\/li>\n
- Why are mitochondria referred to as the “power plants of the cell”?<\/li>\n
- What roles are played by\u00a0vesicles and vacuoles?<\/li>\n
- Why\u00a0do all cells need ribosomes \u2014 even prokaryotic cells that lack a nucleus and other\u00a0cell organelles?<\/li>\n
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?<\/li>\n
- \n\n
- The nucleus<\/a> is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls\u00a0gene expression, including controlling which proteins the cell makes.<\/li>\n
- An organelle<\/a> is a structure within the cytoplasm of a eukaryotic<\/a> cell that is enclosed within a membrane and performs a specific job. Although ribosomes<\/a>\u00a0are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.<\/li>\n
![Mitochondrion_structure.svg<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Mitochondrion_structure.svg\"){kind=link}
![Endomembrane_system_diagram_en.svg<\/a> by Mariana Ruiz [](\"https:\/\/commons.wikimedia.org\/wiki\/File:Endomembrane_system_diagram_en.svg\"){kind=link}
![Ribosome_shape<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Ribosome_shape.png\"){kind=link}