Bring on the S'mores!<\/h1>\r\n<\/div>\r\nThis inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of [pb_glossary id=\"5753\"]energy[\/pb_glossary]\u00a0that are released when a fuel like wood is burned. The\u00a0[pb_glossary id=\"5665\"]cells\u00a0[\/pb_glossary]of living things also get\u00a0energy\u00a0by \"burning.\" They \"burn\" [pb_glossary id=\"5451\"]glucose[\/pb_glossary] in\u00a0a\u00a0process called[pb_glossary id=\"5725\"] cellular respiration[\/pb_glossary].\r\n\r\nWhat Is Cellular Respiration?<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary]<\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules and release\u00a0[pb_glossary id=\"5753\"]energy[\/pb_glossary]. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0[pb_glossary id=\"5549\"]ATP[\/pb_glossary]<\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:\r\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\r\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0[pb_glossary id=\"1796\"]aerobic<\/strong>[\/pb_glossary] process.\r\n\r\nCellular respiration occurs in the [pb_glossary id=\"5665\"]cells[\/pb_glossary] of all living things, both [pb_glossary id=\"1708\"]autotrophs[\/pb_glossary] and [pb_glossary id=\"1716\"]heterotrophs[\/pb_glossary]. All of them burn [pb_glossary id=\"5451\"]glucose[\/pb_glossary] to form [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. The reactions of [pb_glossary id=\"5725\"]cellular respiration[\/pb_glossary] can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.\r\n\r\n[caption id=\"attachment_1797\" align=\"aligncenter\" width=\"720\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Carbohydrate_Metabolism-2-scaled.jpg\")
Figure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\r\n<\/div>\r\nThe first stage of cellular respiration is\u00a0[pb_glossary id=\"1798\"]glycolysis[\/pb_glossary]<\/strong>,\u00a0which happens\u00a0in the [pb_glossary id=\"5737\"]cytosol[\/pb_glossary] of the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary].\r\nSplitting Glucose<\/h2>\r\nThe word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0[pb_glossary id=\"5757\"]Enzymes[\/pb_glossary]\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.\r\n\r\n[caption id=\"attachment_1800\" align=\"aligncenter\" width=\"784\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Glycolysis-2.png\")
Figure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em>[\/caption]\r\nResults of Glycolysis<\/h2>\r\nEnergy is needed at the start of [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.\r\nTransition Reaction<\/h2>\r\nBefore pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb's Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).\r\n\r\n[caption id=\"attachment_9093\" align=\"aligncenter\" width=\"747\"]![\"In](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Transition-Reaction-4-10-14-2.png\")
Figure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em>[\/caption]\r\n\r\n\r\nStructure of the Mitochondrion<\/h2>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1804\" align=\"alignleft\" width=\"496\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Mitochondrion-2.png\")
Figure 4.10.5 Labelled mitochondrion structure. <\/em>[\/caption]\r\n\r\nBefore you read about the last two stages of cellular respiration, you need to know more about the [pb_glossary id=\"5783\"]mitochondrion[\/pb_glossary], where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.\r\n\r\n\r\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.\r\n\r\n<\/div>\r\nAs you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0[pb_glossary id=\"5629\"]intermembrane space[\/pb_glossary]<\/strong>. The space enclosed by the inner membrane is called the\u00a0[pb_glossary id=\"1812\"]matrix[\/pb_glossary]<\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.\r\n\r\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\r\n<\/div>\r\nRecall that [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0[pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.\r\n\r\n[caption id=\"attachment_1809\" align=\"alignnone\" width=\"835\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/krebs-cycle-2.png\")
Figure 4.10.6 Reactants and products of the Krebs Cycle.<\/em>[\/caption]\r\n\r\n\r\n\r\nSteps of the Krebs Cycle<\/span>\r\n\r\n<\/div>\r\nThe [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.\r\n\r\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.\r\n\r\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.\r\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\r\nAfter glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:\r\n\r\n \t- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\r\n \t
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\r\n \t
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\r\n<\/ul>\r\nThe events of cellular respiration up to this point are\u00a0[pb_glossary id=\"993\"]exergonic reactions[\/pb_glossary]<\/span>- they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an [pb_glossary id=\"5511\"]endergonic reaction[\/pb_glossary].\u00a0 Using an exothermic reaction to power an endothermic reaction is known as [pb_glossary id=\"5981\"]energy coupling[\/pb_glossary].<\/span>\r\n\r\n
Cellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1820\" align=\"alignright\" width=\"489\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/ETC-2.png\")
Figure 4.10.7. Reactants and products of the electron transport chain.<\/em>[\/caption]\r\n\r\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.\r\nTransporting Electrons<\/h2>\r\nThe [pb_glossary id=\"5473\"]Electron transport chain[\/pb_glossary]<\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0[pb_glossary id=\"1817\"]electrochemical gradient[\/pb_glossary]\u00a0that drives the synthesis of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_1819\" align=\"aligncenter\" width=\"811\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/The_Electron_Transport_Chain-2.jpg\")
Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\n\r\n\r\nMaking ATP<\/span>\r\n\r\n<\/div>\r\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\n![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/SMores-2-scaled.jpg\")
Figure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
What Is Cellular Respiration?<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary]<\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules and release\u00a0[pb_glossary id=\"5753\"]energy[\/pb_glossary]. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0[pb_glossary id=\"5549\"]ATP[\/pb_glossary]<\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:\r\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\r\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0[pb_glossary id=\"1796\"]aerobic<\/strong>[\/pb_glossary] process.\r\n\r\nCellular respiration occurs in the [pb_glossary id=\"5665\"]cells[\/pb_glossary] of all living things, both [pb_glossary id=\"1708\"]autotrophs[\/pb_glossary] and [pb_glossary id=\"1716\"]heterotrophs[\/pb_glossary]. All of them burn [pb_glossary id=\"5451\"]glucose[\/pb_glossary] to form [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. The reactions of [pb_glossary id=\"5725\"]cellular respiration[\/pb_glossary] can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.\r\n\r\n[caption id=\"attachment_1797\" align=\"aligncenter\" width=\"720\"] Figure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\r\n<\/div>\r\nThe first stage of cellular respiration is\u00a0[pb_glossary id=\"1798\"]glycolysis[\/pb_glossary]<\/strong>,\u00a0which happens\u00a0in the [pb_glossary id=\"5737\"]cytosol[\/pb_glossary] of the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary].\r\nSplitting Glucose<\/h2>\r\nThe word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0[pb_glossary id=\"5757\"]Enzymes[\/pb_glossary]\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.\r\n\r\n[caption id=\"attachment_1800\" align=\"aligncenter\" width=\"784\"] Figure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em>[\/caption]\r\nResults of Glycolysis<\/h2>\r\nEnergy is needed at the start of [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.\r\nTransition Reaction<\/h2>\r\nBefore pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb's Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).\r\n\r\n[caption id=\"attachment_9093\" align=\"aligncenter\" width=\"747\"] Figure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em>[\/caption]\r\n\r\n\r\nStructure of the Mitochondrion<\/h2>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1804\" align=\"alignleft\" width=\"496\"] Figure 4.10.5 Labelled mitochondrion structure. <\/em>[\/caption]\r\n\r\nBefore you read about the last two stages of cellular respiration, you need to know more about the [pb_glossary id=\"5783\"]mitochondrion[\/pb_glossary], where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.\r\n\r\n\r\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.\r\n\r\n<\/div>\r\nAs you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0[pb_glossary id=\"5629\"]intermembrane space[\/pb_glossary]<\/strong>. The space enclosed by the inner membrane is called the\u00a0[pb_glossary id=\"1812\"]matrix[\/pb_glossary]<\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.\r\n\r\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\r\n<\/div>\r\nRecall that [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0[pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.\r\n\r\n[caption id=\"attachment_1809\" align=\"alignnone\" width=\"835\"] Figure 4.10.6 Reactants and products of the Krebs Cycle.<\/em>[\/caption]\r\n\r\n\r\n\r\nSteps of the Krebs Cycle<\/span>\r\n\r\n<\/div>\r\nThe [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.\r\n\r\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.\r\n\r\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.\r\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\r\nAfter glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:\r\n\r\n \t- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\r\n \t
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\r\n \t
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\r\n<\/ul>\r\nThe events of cellular respiration up to this point are\u00a0[pb_glossary id=\"993\"]exergonic reactions[\/pb_glossary]<\/span>- they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an [pb_glossary id=\"5511\"]endergonic reaction[\/pb_glossary].\u00a0 Using an exothermic reaction to power an endothermic reaction is known as [pb_glossary id=\"5981\"]energy coupling[\/pb_glossary].<\/span>\r\n\r\n
Cellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1820\" align=\"alignright\" width=\"489\"] Figure 4.10.7. Reactants and products of the electron transport chain.<\/em>[\/caption]\r\n\r\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.\r\nTransporting Electrons<\/h2>\r\nThe [pb_glossary id=\"5473\"]Electron transport chain[\/pb_glossary]<\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0[pb_glossary id=\"1817\"]electrochemical gradient[\/pb_glossary]\u00a0that drives the synthesis of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_1819\" align=\"aligncenter\" width=\"811\"] Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\n\r\n\r\nMaking ATP<\/span>\r\n\r\n<\/div>\r\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
Figure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em>[\/caption]\r\n\r\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\r\n<\/div>\r\nThe first stage of cellular respiration is\u00a0[pb_glossary id=\"1798\"]glycolysis[\/pb_glossary]<\/strong>,\u00a0which happens\u00a0in the [pb_glossary id=\"5737\"]cytosol[\/pb_glossary] of the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary].\r\nSplitting Glucose<\/h2>\r\nThe word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0[pb_glossary id=\"5757\"]Enzymes[\/pb_glossary]\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.\r\n\r\n[caption id=\"attachment_1800\" align=\"aligncenter\" width=\"784\"] Figure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em>[\/caption]\r\nResults of Glycolysis<\/h2>\r\nEnergy is needed at the start of [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.\r\nTransition Reaction<\/h2>\r\nBefore pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb's Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).\r\n\r\n[caption id=\"attachment_9093\" align=\"aligncenter\" width=\"747\"] Figure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em>[\/caption]\r\n\r\n\r\nStructure of the Mitochondrion<\/h2>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1804\" align=\"alignleft\" width=\"496\"] Figure 4.10.5 Labelled mitochondrion structure. <\/em>[\/caption]\r\n\r\nBefore you read about the last two stages of cellular respiration, you need to know more about the [pb_glossary id=\"5783\"]mitochondrion[\/pb_glossary], where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.\r\n\r\n\r\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.\r\n\r\n<\/div>\r\nAs you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0[pb_glossary id=\"5629\"]intermembrane space[\/pb_glossary]<\/strong>. The space enclosed by the inner membrane is called the\u00a0[pb_glossary id=\"1812\"]matrix[\/pb_glossary]<\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.\r\n\r\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\r\n<\/div>\r\nRecall that [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0[pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.\r\n\r\n[caption id=\"attachment_1809\" align=\"alignnone\" width=\"835\"] Figure 4.10.6 Reactants and products of the Krebs Cycle.<\/em>[\/caption]\r\n\r\n\r\n\r\nSteps of the Krebs Cycle<\/span>\r\n\r\n<\/div>\r\nThe [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.\r\n\r\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.\r\n\r\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.\r\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\r\nAfter glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:\r\n\r\n \t- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\r\n \t
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\r\n \t
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\r\n<\/ul>\r\nThe events of cellular respiration up to this point are\u00a0[pb_glossary id=\"993\"]exergonic reactions[\/pb_glossary]<\/span>- they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an [pb_glossary id=\"5511\"]endergonic reaction[\/pb_glossary].\u00a0 Using an exothermic reaction to power an endothermic reaction is known as [pb_glossary id=\"5981\"]energy coupling[\/pb_glossary].<\/span>\r\n\r\n
Cellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1820\" align=\"alignright\" width=\"489\"] Figure 4.10.7. Reactants and products of the electron transport chain.<\/em>[\/caption]\r\n\r\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.\r\nTransporting Electrons<\/h2>\r\nThe [pb_glossary id=\"5473\"]Electron transport chain[\/pb_glossary]<\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0[pb_glossary id=\"1817\"]electrochemical gradient[\/pb_glossary]\u00a0that drives the synthesis of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_1819\" align=\"aligncenter\" width=\"811\"] Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\n\r\n\r\nMaking ATP<\/span>\r\n\r\n<\/div>\r\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
Figure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em>[\/caption]\r\nResults of Glycolysis<\/h2>\r\nEnergy is needed at the start of [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.\r\nTransition Reaction<\/h2>\r\nBefore pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb's Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).\r\n\r\n[caption id=\"attachment_9093\" align=\"aligncenter\" width=\"747\"] Figure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em>[\/caption]\r\n\r\n\r\nStructure of the Mitochondrion<\/h2>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1804\" align=\"alignleft\" width=\"496\"] Figure 4.10.5 Labelled mitochondrion structure. <\/em>[\/caption]\r\n\r\nBefore you read about the last two stages of cellular respiration, you need to know more about the [pb_glossary id=\"5783\"]mitochondrion[\/pb_glossary], where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.\r\n\r\n\r\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.\r\n\r\n<\/div>\r\nAs you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0[pb_glossary id=\"5629\"]intermembrane space[\/pb_glossary]<\/strong>. The space enclosed by the inner membrane is called the\u00a0[pb_glossary id=\"1812\"]matrix[\/pb_glossary]<\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.\r\n\r\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\r\n<\/div>\r\nRecall that [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0[pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.\r\n\r\n[caption id=\"attachment_1809\" align=\"alignnone\" width=\"835\"] Figure 4.10.6 Reactants and products of the Krebs Cycle.<\/em>[\/caption]\r\n\r\n\r\n\r\nSteps of the Krebs Cycle<\/span>\r\n\r\n<\/div>\r\nThe [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.\r\n\r\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.\r\n\r\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.\r\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\r\nAfter glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:\r\n\r\n \t- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\r\n \t
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\r\n \t
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\r\n<\/ul>\r\nThe events of cellular respiration up to this point are\u00a0[pb_glossary id=\"993\"]exergonic reactions[\/pb_glossary]<\/span>- they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an [pb_glossary id=\"5511\"]endergonic reaction[\/pb_glossary].\u00a0 Using an exothermic reaction to power an endothermic reaction is known as [pb_glossary id=\"5981\"]energy coupling[\/pb_glossary].<\/span>\r\n\r\n
Cellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1820\" align=\"alignright\" width=\"489\"] Figure 4.10.7. Reactants and products of the electron transport chain.<\/em>[\/caption]\r\n\r\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.\r\nTransporting Electrons<\/h2>\r\nThe [pb_glossary id=\"5473\"]Electron transport chain[\/pb_glossary]<\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0[pb_glossary id=\"1817\"]electrochemical gradient[\/pb_glossary]\u00a0that drives the synthesis of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_1819\" align=\"aligncenter\" width=\"811\"] Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\n\r\n\r\nMaking ATP<\/span>\r\n\r\n<\/div>\r\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
Figure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em>[\/caption]\r\n\r\nStructure of the Mitochondrion<\/h2>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1804\" align=\"alignleft\" width=\"496\"] Figure 4.10.5 Labelled mitochondrion structure. <\/em>[\/caption]\r\n\r\nBefore you read about the last two stages of cellular respiration, you need to know more about the [pb_glossary id=\"5783\"]mitochondrion[\/pb_glossary], where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.\r\n\r\n\r\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.\r\n\r\n<\/div>\r\nAs you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0[pb_glossary id=\"5629\"]intermembrane space[\/pb_glossary]<\/strong>. The space enclosed by the inner membrane is called the\u00a0[pb_glossary id=\"1812\"]matrix[\/pb_glossary]<\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.\r\n\r\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\r\n<\/div>\r\nRecall that [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0[pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.\r\n\r\n[caption id=\"attachment_1809\" align=\"alignnone\" width=\"835\"] Figure 4.10.6 Reactants and products of the Krebs Cycle.<\/em>[\/caption]\r\n\r\n\r\n\r\nSteps of the Krebs Cycle<\/span>\r\n\r\n<\/div>\r\nThe [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.\r\n\r\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.\r\n\r\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.\r\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\r\nAfter glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:\r\n\r\n \t- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\r\n \t
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\r\n \t
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\r\n<\/ul>\r\nThe events of cellular respiration up to this point are\u00a0[pb_glossary id=\"993\"]exergonic reactions[\/pb_glossary]<\/span>- they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an [pb_glossary id=\"5511\"]endergonic reaction[\/pb_glossary].\u00a0 Using an exothermic reaction to power an endothermic reaction is known as [pb_glossary id=\"5981\"]energy coupling[\/pb_glossary].<\/span>\r\n\r\n
Cellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1820\" align=\"alignright\" width=\"489\"] Figure 4.10.7. Reactants and products of the electron transport chain.<\/em>[\/caption]\r\n\r\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.\r\nTransporting Electrons<\/h2>\r\nThe [pb_glossary id=\"5473\"]Electron transport chain[\/pb_glossary]<\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0[pb_glossary id=\"1817\"]electrochemical gradient[\/pb_glossary]\u00a0that drives the synthesis of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_1819\" align=\"aligncenter\" width=\"811\"] Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\n\r\n\r\nMaking ATP<\/span>\r\n\r\n<\/div>\r\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
Cellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\r\n<\/div>\r\nRecall that [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary] produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0[pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.\r\n\r\n[caption id=\"attachment_1809\" align=\"alignnone\" width=\"835\"] Figure 4.10.6 Reactants and products of the Krebs Cycle.<\/em>[\/caption]\r\n\r\n\r\n\r\nSteps of the Krebs Cycle<\/span>\r\n\r\n<\/div>\r\nThe [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary] itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.\r\n\r\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.\r\n\r\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.\r\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\r\nAfter glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:\r\n\r\n \t- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\r\n \t
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\r\n \t
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\r\n<\/ul>\r\nThe events of cellular respiration up to this point are\u00a0[pb_glossary id=\"993\"]exergonic reactions[\/pb_glossary]<\/span>- they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an [pb_glossary id=\"5511\"]endergonic reaction[\/pb_glossary].\u00a0 Using an exothermic reaction to power an endothermic reaction is known as [pb_glossary id=\"5981\"]energy coupling[\/pb_glossary].<\/span>\r\n\r\n
Cellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1820\" align=\"alignright\" width=\"489\"] Figure 4.10.7. Reactants and products of the electron transport chain.<\/em>[\/caption]\r\n\r\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.\r\nTransporting Electrons<\/h2>\r\nThe [pb_glossary id=\"5473\"]Electron transport chain[\/pb_glossary]<\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0[pb_glossary id=\"1817\"]electrochemical gradient[\/pb_glossary]\u00a0that drives the synthesis of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_1819\" align=\"aligncenter\" width=\"811\"] Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\n\r\n\r\nMaking ATP<\/span>\r\n\r\n<\/div>\r\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
Results of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\r\nAfter glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:\r\n\r\n \t- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\r\n \t
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\r\n \t
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\r\n<\/ul>\r\nThe events of cellular respiration up to this point are\u00a0[pb_glossary id=\"993\"]exergonic reactions[\/pb_glossary]<\/span>- they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an [pb_glossary id=\"5511\"]endergonic reaction[\/pb_glossary].\u00a0 Using an exothermic reaction to power an endothermic reaction is known as [pb_glossary id=\"5981\"]energy coupling[\/pb_glossary].<\/span>\r\n\r\n
Cellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1820\" align=\"alignright\" width=\"489\"] Figure 4.10.7. Reactants and products of the electron transport chain.<\/em>[\/caption]\r\n\r\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.\r\nTransporting Electrons<\/h2>\r\nThe [pb_glossary id=\"5473\"]Electron transport chain[\/pb_glossary]<\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0[pb_glossary id=\"1817\"]electrochemical gradient[\/pb_glossary]\u00a0that drives the synthesis of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_1819\" align=\"aligncenter\" width=\"811\"] Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\n\r\n\r\nMaking ATP<\/span>\r\n\r\n<\/div>\r\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
Cellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1820\" align=\"alignright\" width=\"489\"] Figure 4.10.7. Reactants and products of the electron transport chain.<\/em>[\/caption]\r\n\r\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.\r\nTransporting Electrons<\/h2>\r\nThe [pb_glossary id=\"5473\"]Electron transport chain[\/pb_glossary]<\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0[pb_glossary id=\"1817\"]electrochemical gradient[\/pb_glossary]\u00a0that drives the synthesis of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_1819\" align=\"aligncenter\" width=\"811\"] Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\n\r\n\r\nMaking ATP<\/span>\r\n\r\n<\/div>\r\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em>[\/caption]\r\n\r\nHow Much ATP?<\/h1>\r\n<\/div>\r\nYou have seen how the three stages of [pb_glossary id=\"5877\"]aerobic respiration[\/pb_glossary] use the energy in glucose to make [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.\r\n\r\n\r\n4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
4.10 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\nReferences<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\nBetts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\nThe Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\nFigure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n\nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\nC6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\nCellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\nFigure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\nSplitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\nFigure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\nResults of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\nTransition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\nFigure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n\nStructure of the Mitochondrion<\/h2>\n<\/div>\nFigure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\nBefore you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\nFigure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n\nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\nThe Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\nThe final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\nResults of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
\n- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\nFigure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n\u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\nTransporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n <\/p>\nFigure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n\nMaking ATP<\/span><\/p>\n<\/div>\nAs shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n\n\n
- \r\n \t
- [pb_glossary id=\"5725\"]Cellular respiration[\/pb_glossary] is the [pb_glossary id=\"1796\"]aerobic[\/pb_glossary] process by which living cells break down [pb_glossary id=\"5451\"]glucose[\/pb_glossary] molecules, release energy, and form molecules of [pb_glossary id=\"5549\"]ATP[\/pb_glossary].\u00a0Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and\u00a0water.<\/li>\r\n \t
- The first stage of cellular respiration, called [pb_glossary id=\"1798\"]glycolysis[\/pb_glossary], takes place in the cytoplasm. In this step,\u00a0enzymes\u00a0split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.\u00a0 Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.<\/li>\r\n \t
- The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.<\/li>\r\n \t
- The second stage of cellular respiration, called the [pb_glossary id=\"1805\"]Krebs cycle[\/pb_glossary], takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their\u00a0chemical bonds\u00a0being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).<\/li>\r\n \t
- The third and final stage of cellular respiration, called [pb_glossary id=\"5473\"]electron transport[\/pb_glossary], takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions\u00a0across the membrane, creating an\u00a0electrochemical gradient\u00a0that drives the synthesis of many more molecules of ATP.<\/li>\r\n \t
- In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n 4.10 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- What is the purpose of cellular respiration? Provide a concise summary of the process.<\/li>\r\n \t
- State what happens during glycolysis.<\/li>\r\n \t
- Describe the structure of a mitochondrion.<\/li>\r\n \t
- What molecule is present at both the beginning and end of the Krebs cycle?<\/li>\r\n \t
- What happens during the electron transport stage of cellular respiration?<\/li>\r\n \t
- How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?<\/li>\r\n \t
- Do plants undergo cellular respiration? Why or why not?<\/li>\r\n \t
- Explain why the process of cellular respiration described in this section is considered aerobic.<\/li>\r\n \t
- Name three energy-carrying molecules involved in cellular respiration.<\/li>\r\n \t
- [h5p id=\"478\"]<\/li>\r\n \t
- Which stage of aerobic cellular respiration produces the most ATP?<\/li>\r\n \t
- [h5p id=\"479\"]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 4.10 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo\r\nATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s\r\n
Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.10.1<\/strong>\r\n\r\nSmores<\/a> by Jessica Ruscello<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.10.2<\/strong>\r\n\r\nCarbohydrate_Metabolism<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 4.10.3<\/strong>\r\n\r\nGlycolysis by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.4<\/strong>\r\n\r\nTransition Reaction by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.5<\/strong>\r\n\r\nMitochondrion<\/a>\u00a0by Mariana Ruiz Villarreal [LadyofHats<\/a>] on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.10.6<\/strong>\r\n\r\nKrebs cycle by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.7<\/strong>\r\n\r\nElectron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0\/) license.\r\n\r\nFigure 4.10.8<\/strong>\r\n\r\nThe_Electron_Transport_Chain<\/a> by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\n<\/div>\r\n
References<\/h2>\r\n
CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https:\/\/www.youtube.com\/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo<\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). <\/span>Figure\u00a0<\/span><\/span>24.8<\/span><\/span>\u00a0<\/span>Electron Transport Chain [digital image]<\/span><\/span>. In Anatomy & Physiology, Connexions<\/em> (Section ). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/span><\/span><\/p>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure <\/span><\/span>24.9<\/span><\/span>\u00a0<\/span>Carbohydrate Metabolism [digital image]<\/span><\/span>. <\/span><\/span>In Anatomy & Physiology, Connexions<\/em> (Section 24.2). OpenStax.\u00a0 https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/24-2-carbohydrate-metabolism<\/p>\r\n
The Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https:\/\/www.youtube.com\/watch?v=4Eo7JtRA7lg&t=3s<\/p>\r\n ","rendered":"
<\/p>\n
Figure 4.10.1 Ready to make s’mores!<\/em><\/figcaption><\/figure>\n \nBring on the S’mores!<\/h1>\n<\/div>\n
This inviting camp fire can be used for both\u00a0heat\u00a0and light. Heat and light are two\u00a0forms of energy<\/a>\u00a0that are released when a fuel like wood is burned. The\u00a0cells\u00a0<\/a>of living things also get\u00a0energy\u00a0by “burning.” They “burn” glucose<\/a> in\u00a0a\u00a0process called cellular respiration<\/a>.<\/p>\n
\nWhat Is Cellular Respiration?<\/h1>\n<\/div>\n
Cellular respiration<\/a><\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down glucose<\/a> molecules and release\u00a0energy<\/a>. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0ATP<\/a><\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:<\/p>\n
C6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\n
In words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0aerobic<\/strong><\/a> process.<\/p>\n
Cellular respiration occurs in the cells<\/a> of all living things, both autotrophs and heterotrophs<\/a>. All of them burn glucose<\/a> to form ATP<\/a>. The reactions of cellular respiration<\/a> can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.<\/p>\n
Figure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em><\/figcaption><\/figure>\n \n<\/p>\n<\/div>\n
\nCellular Respiration Stage I:\u00a0Glycolysis<\/h1>\n<\/div>\n
The first stage of cellular respiration is\u00a0glycolysis<\/strong>,\u00a0which happens\u00a0in the cytosol<\/a> of the cytoplasm<\/a>.<\/p>\n
Splitting Glucose<\/h2>\n
The word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0Enzymes<\/a>\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.<\/p>\n
Figure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em><\/figcaption><\/figure>\n Results of Glycolysis<\/h2>\n
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.<\/p>\n
Transition Reaction<\/h2>\n
Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.\u00a0 The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).<\/p>\n
Figure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em><\/figcaption><\/figure>\n \nStructure of the Mitochondrion<\/h2>\n<\/div>\n
Figure 4.10.5 Labelled mitochondrion structure. <\/em><\/figcaption><\/figure>\n Before you read about the last two stages of cellular respiration, you need to know more about the mitochondrion<\/a>, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.<\/p>\n
\nThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.<\/p>\n<\/div>\n
As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the\u00a0intermembrane space<\/a><\/strong>. The space enclosed by the inner membrane is called the\u00a0matrix<\/a><\/strong>. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron\u00a0transport)\u00a0happens on\u00a0the inner membrane.<\/p>\n
\nCellular Respiration Stage II: The\u00a0Krebs Cycle<\/h1>\n<\/div>\n
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the\u00a0Krebs cycle<\/a> <\/strong>(also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.<\/p>\n
Figure 4.10.6 Reactants and products of the Krebs Cycle.<\/em><\/figcaption><\/figure>\n \nSteps of the Krebs Cycle<\/span><\/p>\n<\/div>\n
The Krebs cycle<\/a> itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.<\/p>\n
After citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.<\/p>\n
The final step of the Krebs cycle regenerates<\/em> OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.<\/p>\n
Results of the Glycolysis, Transition Reaction and Krebs Cycle<\/h2>\n
After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its\u00a0chemical bonds\u00a0has been stored in a total of 16 energy-carrier molecules. These molecules are:<\/p>\n
- \n
- 4 ATP (2 from glycolysis, 2 from Krebs Cycle)<\/li>\n
- 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)<\/li>\n
- 2 FADH2 <\/sub>(both from the Krebs cycle)<\/li>\n<\/ul>\n
The events of cellular respiration up to this point are\u00a0exergonic reactions<\/a><\/span>– they are releasing energy that had been stored in the bonds of the glucose molecule.\u00a0 This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction<\/a>.\u00a0 Using an exothermic reaction to power an endothermic reaction is known as energy coupling<\/a>.<\/span><\/p>\n
\nCellular Respiration Stage III:\u00a0Electron Transport Chain<\/h1>\n<\/div>\n
Figure 4.10.7. Reactants and products of the electron transport chain.<\/em><\/figcaption><\/figure>\n \u00a0ETC, the final stage in cellular respiration produces 32 ATP.\u00a0\u00a0<\/span><\/span>The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2<\/sub>\u00a0is transferred to ATP.\u00a0 In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2<\/sub>, forming water.\u00a0 Figure 4.10.8 shows the reactants and products of the ETC.<\/p>\n
Transporting Electrons<\/h2>\n
The Electron transport chain<\/a><\/strong> is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2<\/sub>, and they move along electron-transport chains on the inner membrane of the mitochondrion.\u00a0An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by\u00a0chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This\u00a0ion\u00a0transfer creates an\u00a0electrochemical gradient<\/a>\u00a0that drives the synthesis of ATP<\/a>.<\/p>\n
<\/p>\n
Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.<\/em><\/figcaption><\/figure>\n \nMaking ATP<\/span><\/p>\n<\/div>\n
As shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the \u201cspent\u201d electrons combine with oxygen to form water.<\/p>\n
\nHow Much ATP?<\/h1>\n<\/div>\n
You have seen how the three stages of aerobic respiration<\/a> use the energy in glucose to make ATP<\/a>. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2<\/sub>\u00a0from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.<\/p>\n
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