Created by: CK-12/Adapted by Christine Miller

 

A Deceptively Simple Model

This simple model sums up one of the most important ideas in biology, which is called the central dogma of molecular biology (you'll read more about it below). You probably recognize the spiral-shaped structure in the nucleus. It represents a molecule of DNA, the biochemical molecule that stores genetic information in most living cells. The yellow chain represents a newly formed polypeptide — the beginning stage of creating a protein. Proteins are the class of biochemical molecules that carry out virtually all life processes. What is the structure in the center of the model? It appears to resemble DNA, but it is smaller and simpler. This molecule is the key to the central dogma, and it may have been the first type of biochemical molecule to evolve.

DNA is found in chromosomes. In eukaryotic cells, chromosomes always remain in the nucleus, but proteins are made at ribosomes in the cytoplasm. How do the instructions in DNA get to the site of protein synthesis outside the nucleus?

Another type of nucleic acid is responsible. This nucleic acid is RNA, or ribonucleic acid. RNA is a small molecule that can squeeze through pores in the nuclear membrane. It carries the information from DNA in the nucleus to a ribosome in the cytoplasm and then helps assemble the protein. In short:

DNA → RNA → Protein

This expresses in words what the diagram in Figure 5.5.1 shows. The genetic instructions encoded in DNA in the nucleus are transcribed to RNA. Then, RNA carries the instructions to a ribosome in the cytoplasm, where they are translated into a protein. Discovering this sequence of events was a major milestone in molecular biology. It's called the central dogma of molecular biology.

DNA alone cannot “tell” your cells how to make proteins. It needs the help of RNA, the other main player in the central dogma of molecular biology. Like DNA, RNA is a nucleic acid, so it consists of repeating nucleotides bonded together to form a polynucleotide chain. RNA differs from DNA in several ways: it exists as a single stranded molecule, contains the sugar ribose (as opposed to deoxyribose) and uses the base uracil instead of thymine.

The main function of RNA is to help make proteins. There are three main types of RNA involved in protein synthesis:

1. 

   Messenger RNA (mRNA) copies (or transcribes) the genetic instructions from DNA in the nucleus and carries them to the cytoplasm.

2. Ribosomal RNA (rRNA) helps form ribosomes, where proteins are assembled. Ribosomes also contain proteins.
3. Transfer RNA (tRNA) brings amino acids to ribosomes, where rRNA catalyzes the formation of chemical bonds between them to form a protein.

In section 5.7 Protein Synthesis, you can read in detail about how these three types of RNA build primary structure of proteins.

RNA is a very versatile molecule which plays multiple roles in living things. In addition to helping to make proteins, for example, there are RNA molecules that regulate the expression of genes, and RNA molecules that catalyze other biochemical reactions needed to sustain life. Because of the diversity of roles that RNA molecules play, they have been called the Swiss Army knives of the cellular world.

The function of DNA is to store genetic information inside cells. It does this job well, but that's about all it can do. DNA can't act as an enzyme, for example, to catalyze biochemical reactions that are needed to keep us alive. Proteins are needed for this and many other life functions. Proteins work exceptionally well to keep us alive, but they are unable to store genetic information. Proteins need DNA for that. Without DNA, proteins could not exist. On the other hand, without proteins, DNA could not survive. This poses a chicken-and-egg sort of problem: Which evolved first? DNA or proteins?

Some scientists think that the answer is neither. They speculate instead that RNA was the first biochemical to evolve. The reason? RNA can do more than one job. It can store information as DNA does, but it can also perform various jobs (such as catalysis) to keep cells alive, as proteins do. The idea that RNA was the first biochemical to evolve, predating both DNA and proteins, is called the RNA world hypothesis. According to this hypothesis, billions of years ago, RNA molecules evolved that could both survive and make copies of themselves. According to the hypothesis, early RNA molecules eventually evolved the ability to make proteins, and at some point RNA mutated to form DNA.

The RNA world hypothesis has not gained enough support in the scientific community to be accepted as a scientific theory. In fact, there are probably as many detractors as supporters of the hypothesis. Do a web search to learn more about the RNA world hypothesis and the evidence and arguments for and against it. When weighing the information you gather, consider the likely reliability of the different websites you visit. Based on what you determine are the most reliable sources and the most convincing arguments, form your own opinion about the hypothesis. You may decide to accept or reject the hypothesis. Alternatively, you may decide to reserve judgement until — or if — more evidence or arguments are forthcoming.

Attributions

Figure 5.5.1

From DNA to Protein: Transcription through Translation by OpenStax College on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 5.5.2

Molbio-Header by Squidonius  on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.5.2

ARNm-Rasmol by Corentin Le Reun on Wikimedia Commons is is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).ublic domain.

References

Amoeba Sisters. (2019, August 29). DNA vs RNA (Updated). YouTube. https://www.youtube.com/watch?v=JQByjprj_mA&feature=youtu.be

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 3.29 From DNA to Protein: Transcription through Translation [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-4-protein-synthesis#fig-ch03_04_05

NOVA PBS Official. (2014, April 23). The RNA origin of life. YouTube. https://www.youtube.com/watch?v=VYQQD0KNOis&feature=youtu.be

Wikipedia contributors. (2020, June 28). RNA world. In Wikipedia. https://en.wikipedia.org/w/index.php?title=RNA_world&oldid=964998696

 

Image shows a section of DNA code - a specific sequence of the bases A, C, G, and T.

Refers to the body system consisting of the heart, blood vessels and the blood. Blood contains oxygen and other nutrients which your body needs to survive. The body takes these essential nutrients from the blood.

Created by: CK-12/Adapted by Christine Miller

 

If you look at Figure 3.11.2, you will see where Earth’s water is found. The term water generally refers to its liquid state, and water is a liquid over a wide range of temperatures on Earth. Water, however, also occurs on Earth as a solid (ice) and as a gas (water vapor).

You are likely already aware of some of the properties of water. For example, you know that water is tasteless and odorless. You also probably know that water is transparent, which means that light can pass through it. This is important for organisms that live in the water, because some of them need sunlight to make food by photosynthesis.

Chemical Structure of Water

To understand some of water’s properties, you need to know more about its chemical structure. Each molecule of water consists of one atom of oxygen and two atoms of hydrogen. The oxygen atom in a water molecule attracts electrons more strongly than the hydrogen atoms do. As a result, the oxygen atom has a slightly negative charge, and the hydrogen atoms have a slightly positive charge. A difference in electrical charge between different parts of the same molecule is called polarity. The diagram in Figure 3.11.3 shows water’s polarity.

 

When it comes to charged molecules, opposites attract. In the case of water, the positive (hydrogen) end of one water molecule is attracted to the negative (oxygen) end of a nearby water molecule. Because of this attraction, weak bonds form between adjacent water molecules, as shown in Figure 3.11.4. The type of bond that forms between water molecules is called a hydrogen bond. Bonds between molecules are not as strong as bonds within molecules, but in water, they are strong enough to hold together nearby molecules.

Properties of Water

Hydrogen bonds between water molecules explain some of water’s properties — for example, why water molecules tend to "stick" together. Did you ever watch water drip from a leaky faucet or from a melting icicle? If you did, then you know that water always falls in drops, rather than as separate molecules. The dew drops pictured to the left are another example of water molecules sticking together.

Hydrogen bonds cause water to have a relatively high boiling point of 100°C (212°F). Extra energy is needed to break these bonds and separate water molecules so they can escape into the air as water vapor. Because of its high boiling point, most water on Earth is in a liquid state, rather than a gaseous state. Water in its liquid state is needed by all living things. Hydrogen bonds also cause water to expand when it freezes. This, in turn, causes ice to have a lower density (that is, less mass per unit volume) than liquid water. The lower density of ice means that it floats on water. In cold climates, ice floats on top of the water in lakes. This allows lake animals like fish to survive the winter by staying in the liquid water under the ice.

Watch the video below to hear more about hydrogen bonding and it's effects on the properties of water:

https://www.youtube.com/watch?v=UukRgqzk-KE

The human body is about 70 per cent water (not counting the water in body fat, which varies from person to person). The body needs all this water to function normally. Just why is so much water required by human beings and other organisms? Water can dissolve many substances that organisms need. Water's polarity helps it dissolve other polar substances. Water is also necessary for many biochemical reactions. The examples below are among the most important biochemical processes that occur in living things, but they are just two of the many ways that water is involved in biochemical reactions.

• Photosynthesis: In this process, cells use the energy in sunlight to change carbon dioxide and water to glucose and oxygen. The reactions of photosynthesis can be represented by the chemical equation:

6CO₂ + 6H₂O + Energy → C₆H₁₂O₆ + 6O₂

• Cellular respiration: In this process, cells break down glucose in the presence of oxygen and release carbon dioxide, water, and energy. The reactions of cellular respiration can be represented by the chemical equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Water is involved in many other biochemical reactions and almost all life processes depend on water.

Are you a marathon runner or other endurance athlete? Do you live and work in a hot, humid climate? If you answered "yes" to either question, you may be at risk of water intoxication.

Water is considered the least toxic chemical compound, so it may surprise you to learn that drinking too much water can cause serious illness and even death. Water intoxication is a potentially fatal disturbance in brain functions. It results when the normal balance of sodium and other electrolytes in the body is pushed outside safe limits by overhydration, or taking in too much water. The condition is also called hyponatremia, which refers to a lower-than-normal level of sodium in the blood that occurs when more water is entering than leaving the body.

As excessive water is consumed, fluid outside the cells decreases in its concentration of sodium and other electrolytes relative to the concentration inside the cells. This causes fluid to enter the cells by osmosis to balance the electrolyte concentration. The extra fluid in the cells causes them to swell. In the brain, this swelling increases the pressure inside the skull. It is this increase in pressure that leads to the first observable symptoms of water intoxication, which typically include headache, confusion, irritability, and drowsiness. As the condition worsens, additional symptoms may occur, such as difficulty breathing during exertion, muscle weakness and pain, or nausea and vomiting. If the condition persists, the cells in the brain may swell to the point where blood flow is interrupted or pressure is applied to the brain stem. This is extremely dangerous and may lead to seizures, brain damage, coma, or even death.

Under normal circumstances, it is very rare to accidentally consume too much water. However, it is relatively common in athletes who participate in endurance activities, such as marathon running. A study conducted on participants of the 2002 Boston Marathon, for example, found that 13 per cent of the runners finished the race with water intoxication (Almond, et al., 2005). The study also found that water intoxication was just as likely to occur in runners who drank sports drinks containing electrolytes as those who drank plain water. Water intoxication is so common at marathon events that medical personnel who work at such events are trained to suspect water intoxication when runners collapse or show signs of confusion.

Because of the publicity water intoxication has received lately, sports experts have lowered their recommendations for water intake during endurance events. They now advise drinking only when thirsty rather than drinking to "stay ahead of thirst," which they recommended previously. Keeping water intake in line with water loss is the best way to prevent water intoxication. Mild water intoxication can be treated by restricting fluid intake. In more severe cases, treatment may require the use of diuretic drugs (which increase urination) or other types of drugs to reduce blood volume. Serious water intoxication should be considered a true medical emergency.

Attributions

Figure 3.11.1

Planet Earth by NASA (photo taken by either Harrison Schmitt  or Ron Evans (of the Apollo 17 crew), on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.11.2

Total water on earth by LadyofHats at CK12, is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license. 

Figure 3.11.3

Polarity of water by Christine Miller is released into the Public Domain (https://creativecommons.org/publicdomain/mark/1.0/).

Figure 3.11.4

Hydrogen bonds, translated by Michal Maňas (User:snek01) is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original uploader was Qwerter at Czech Wikipedia.)

Figure 3.11.5

Dew by Pascal Chanel on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.11.6

Woman in a wheelchair marathon by Kevin André on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Almond, C.S., Shin, A.Y., Fortescue, E.B. et al. (2005, April). Hyponatremia among runners in the Boston Marathon. The New England Journal of Medicine, 352 (15), 1550–1624. doi:10.1056/NEJMoa043901. PMID 15829535.

Amoeba Sisters. (2016, July 26). Properties of Water. YouTube. https://www.youtube.com/watch?v=3jwAGWky98c&feature=youtu.be

Ruiz Villarreal, M. (LadyofHats). (2016, August 15). Figure 2. Total water on earth [digital image]. In Brainard, J., Henderson, R., CK-12's College Human Biology FlexBook® (section 3.11). CK12 Foundation. https://www.ck12.org/book/ck-12-college-human-biology/

TED-Ed. (2013, February 4). How polarity makes water behave strangely - Christina Kleinberg. YouTube.  https://www.youtube.com/watch?v=ASLUY2U1M-8&feature=youtu.be

TED-Ed. (2013, October 22). Why does ice float in water? - George Zaidan and Charles Morton. YouTube. https://www.youtube.com/watch?v=UukRgqzk-KE&feature=youtu.be

 

 

 

Created by CK-12 Foundation/Adapted by Christine Miller

As you learned in the beginning of this chapter, botulinum toxin — one form of which is sold under the brand name Botox — does much more than smooth out wrinkles. It can be used to treat a number of disorders involving excessive muscle contraction, including cervical dystonia. You also learned that cervical dystonia, which Edward suffers from, causes abnormal, involuntary muscle contractions of the neck. This results in jerky movements of the head and neck, and/or a sustained abnormal tilt to the head. It is often painful and can significantly interfere with a person’s life.

How could a toxin actually help treat a muscular disorder? The botulinum toxin is produced by the soil bacterium, Clostridium botulinum, and it is the cause of the potentially deadly disease called botulism. Botulism is often a foodborne illness, commonly caused by foods that are improperly canned. Other forms of botulism are caused by wound infections, or occur when infants consume spores of the bacteria from soil or honey.

Botulism can be life-threatening, because it paralyzes muscles throughout the body, including those involved in breathing. When a very small amount of botulinum toxin is injected carefully into specific muscles by a trained medical professional, however, it can be useful in inhibiting unwanted muscle contractions.

For cosmetic purposes, botulinum toxin injected into the facial muscles relaxes them to reduce the appearance of wrinkles. When used to treat cervical dystonia, it is injected into the muscles of the neck to inhibit excessive muscle contractions. For many patients, this helps relieve the abnormal positioning, movements, and pain associated with the disorder. The effect is temporary, so the injections must be repeated every three to four months to keep the symptoms under control.

How does botulinum toxin inhibit muscle contraction? First, recall how skeletal muscle contraction works. A motor neuron instructs skeletal muscle fibres to contract at a synapse between them called the neuromuscular junction. A nerve impulse called an action potential travels down to the axon terminal of the motor neuron, where it causes the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles. The ACh travels across the synaptic cleft and binds to ACh receptors on the muscle fibre, signaling the muscle fibre to contract. According to the sliding filament theory, the contraction of the muscle fibre occurs due to the sliding of myosin and actin filaments across each other. This causes the Z discs of the sacromeres to move closer together, shortening the sacromeres and causing the muscle fibre to contract.

If you wanted to inhibit muscle contraction, at what points could you theoretically interfere with this process? Inhibiting the action potential in the motor neuron, the release of ACh, the activity of ACh receptors, or the sliding filament process in the muscle fibre would all theoretically impair this process and inhibit muscle contraction. For example, in the disease myasthenia gravis, the function of the ACh receptors is impaired, causing a lack of sufficient muscle contraction. As you have learned, this results in muscle weakness that can eventually become life-threatening. Botulinum toxin works by inhibiting the release of ACh from the motor neurons, thereby removing the signal instructing the muscles to contract.

Fortunately, Edward’s excessive muscle contractions and associated pain improved significantly thanks to botulinum toxin injections. Although cervical dystonia cannot currently be cured, botulinum toxin injections have improved the quality of life for many patients with this and other disorders involving excessive involuntary muscle contractions.

As you have learned in this chapter, our muscular system allows us to do things like make voluntary movements, digest our food, and pump blood through our bodies. Whether they are in your arm, heart, stomach, or blood vessels, muscle tissue works by contracting. But as you have seen here, too much contraction can be a very bad thing. Fortunately, scientists and physicians have found a way to put a potentially deadly toxin — and wrinkle-reducing treatment — to excellent use as a medical treatment for some muscular system disorders.

Attributions

Figure 12.7.1

Botox, he whispered by Michael Reuter on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 12.7.2

botulism by jason wilson on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Reference

Pearson Scott Foresman. (2020, April 14). File:Biceps (PSF).jpg [digital image]. Wikimedia Commons. https://commons.wikimedia.org/w/index.php?title=File:Biceps_(PSF).jpg&oldid=411251538. [Public Domain (https://en.wikipedia.org/wiki/Public_domain)]

 

Involuntary, striated muscle found only in the walls of the heart; also called myocardium.

Structures containing neuronal cell bodies in the peripheral nervous system.

A hollow, tube-like structure through which blood flows in the cardiovascular system; vein, artery, or capillary.

Created by CK-12 Foundation/Adapted by Christine Miller

Is this woman double jointed? No, there is actually no such thing — at least as far as humans are concerned. However, some people, like the woman pictured in Figure 11.6.1, are much more flexible than others, generally because they have looser ligaments. Physicians call this condition joint hypermobility. Regardless of what it’s called, the feats of people with highly mobile joints can be quite impressive.

Joints are locations at which bones of the skeleton connect with one another. A joint is also called an articulation. The majority of joints are structured in such a way that they allow movement. However, not all joints allow movement. Of joints that do allow movement, the extent and direction of the movements they allow also vary.

Joints can be classified structurally or functionally. The structural classification of joints depends on the manner in which the bones connect to each other. The functional classification of joints depends on the nature of the movement the joints allow. There is significant overlap between the two types of classifications, because function depends largely on structure.

Structural Classification of Joints

The structural classification of joints is based on the type of tissue that binds the bones to each other at the joint. There are three types of joints in the structural classification: fibrous, cartilaginous, and synovial joints.

1. Fibrous joints are joints in which bones are joined by dense connective tissue that is rich in collagen fibres. These joints are also called sutures. The joints between bones of the cranium are fibrous joints.
2. Cartilaginous joints are joints in which bones are joined by cartilage. The joints between most of the vertebrae in the spine are cartilaginous joints.
3. Synovial joints are characterized by a fluid-filled space (called a synovial cavity) between the bones of the joints. You can see a drawing of a typical synovial joint in Figure 11.6.2. The cavity is enclosed by a membrane and filled with a fluid (called synovial fluid) that provides extra cushioning to the ends of the bones. Cartilage covers the articulating surfaces of the two bones, but the bones are actually held together by ligaments. The knee is a synovial joint.

Functional Classification of Joints

The functional classification of joints is based on the type and degree of movement that they allow. There are three types of joints in the functional classification: immovable, partly movable, and movable joints.

1. Immovable joints allow little or no movement at the joint. Most immovable joints are fibrous joints. Besides the bones of the cranium, immovable joints include joints between the tibia and fibula in the lower leg, and between the radius and ulna in the lower arm.
2. Partly movable joints permit slight movement. Most partly movable joints are cartilaginous joints. Besides the joints between vertebrae, they include the joints between the ribs and sternum (breast bone).
3. Movable joints allow bones to move freely. All movable joints are synovial joints. Besides the knee, they include the shoulder, hip, and elbow. Movable joints are the most common type of joints in the body.

Types of Movable Joints

Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: pivot, hinge, saddle, plane, condyloid, and ball-and-socket joints. An example of each class — as well as the type of movement it allows — is shown in Figure 11.6.3.

• A ball-and-socket joint allows the greatest range of movement of any movable joint. It allows forward and backward motion, as well as upward and downward movement. It also allows rotation in a circle. The hip and shoulder are the only two ball-and-socket joints in the human body.
• A pivot joint allows one bone to rotate around another. An example of a pivot joint is the joint between the first two vertebrae in the spine. This joint allows the head to rotate from left to right and back again.
• A hinge joint allows back and forth movement like the hinge of a door. An example of a hinge joint is the elbow. This joint allows the arm to bend back and forth.
• A saddle joint allows two different types of movement. An example of a saddle joint is the joint between the first metacarpal bone in the hand and one of the carpal bones in the wrist. This joint allows the thumb to move toward and away from the index finger, and also to cross over the palm toward the little finger.
• A plane joint (also called a gliding joint) allows two bones to glide over one another. The joints between the tarsals in the ankles and between the carpals in the wrists are mainly gliding joints. In the wrist, this type of joint allows the hand to bend upward at the wrist, and also to wave from side to side while the lower arm is held steady.
• A condyloid joint is one in which an oval-shaped head on one bone moves in an elliptical cavity in another bone, allowing movement in all directions, except rotation around an axis. The joint between the radius in the lower arm and carpal bones of the wrist is a condyloid joint, as is the joint at the base of the index finger.

Of all the parts of the skeletal system, the joints are generally the most fragile and subject to damage. If the cartilage that cushions bones at joints wears away, it does not grow back. Eventually, all of the cartilage may wear away. This causes osteoarthritis, which can be both painful and debilitating. In serious cases of osteoarthritis, people may lose the ability to climb stairs, walk long distances, perform routine daily activities, or participate in activities they love, such as gardening or playing sports. If you protect your joints, you can reduce your chances of joint damage, pain, and disability. If you already have joint damage, it is equally important to protect your joints and limit further damage. Follow these five tips:

1. Maintain a normal, healthy weight. The more you weigh, the more force you exert on your joints. When you walk, each knee has to bear a force equal to as much as six times your body weight. If a person weighs 200 pounds, each knee bears more than half a ton of weight with every step. Seven in ten knee replacement surgeries for osteoarthritis can be attributed to obesity.
2. Avoid too much high-impact exercise. Examples of high-impact activities include volleyball, basketball, and tennis. These activities generally involve running or jumping on hard surfaces, which puts tremendous stress on weight-bearing joints, especially the knees. Replace some or all of your high-impact activities with low-impact activities, such as biking, swimming, yoga, or lifting light weights.
3. Reduce your risk of injury. Don’t be a weekend warrior, sitting at a desk all week and then crowding all your physical activity into two days. Get involved in a regular, daily exercise routine that keeps your body fit and your muscles toned. Building up muscles will make your joints more stable, allowing stress to spread across them. Be sure to do some stretching every day to keep the muscles around joints flexible and less prone to injury.
4. Distribute work over your body, and use your largest, strongest joints. Use your shoulder, elbow, and wrist to lift heavy objects — not just your fingers. Hold small items in the palm of your hand, rather than by the fingers. Carry heavy items in a backpack, rather than in your hands. Hold weighty objects close to your body, instead of at arms’ length. Lift with your hips and knees, not your back.
5. Respect pain. If it hurts, stop doing it. Take a break from the activity — at least until the pain stops. Try to use joints only to the point of mild fatigue, not pain.

Attributions

Figure 11.6.1

Tags: Sports Gymnastics Fitness Woman Preparation by nastya_gepp on Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).

Figure 11.6.2

Synovial_Joints by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 11.6.3

Types_of_Synovial_Joints by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References

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, June 19). Figure 9.8 Synovial joints [digital image].  In Anatomy and Physiology (Section 9.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/9-4-synovial-joints

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, June 19). Figure 9.10 Types of synovial joints [digital image].  In Anatomy and Physiology (Section 9.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/9-4-synovial-joints

Mayo Clinic Staff. (n.d.). Osteoarthritis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/osteoarthritis/symptoms-causes/syc-20351925

TED-Ed. (2015, May 5). Why do your knuckles pop? - Eleanor Nelsen. YouTube. https://www.youtube.com/watch?v=IjiKUmfaZr4

TED-Ed. (2019, November 7). Why haven’t we cured arthritis? - Kaitlyn Sadtler and Heather J. Faust. YouTube. https://www.youtube.com/watch?v=FWsBm3hr3B0

 

 

We all get tired sometimes, especially if we have been doing a lot of physical activity, like the athletes pictured in Figure 4.1.1. But for Jasmin (Figure 4.1.2), a 34-year-old former high school track star who is now a recreational runner, her tiredness was going far beyond what she thought should be normal for someone in generally good physical shape.

She was experiencing extreme fatigue after her runs, as well as muscle cramping, spasms, and an unusual sense of heaviness in her legs. At first, she just chalked it up to getting older, but her exhaustion and pain worsened to the point where the former athlete could no longer run for more than a few minutes at a time. She began to experience other unusual symptoms, such as blurry vision and vomiting for no apparent reason.

Concerned, Jasmin went to her doctor, who ran many tests and consulted with several specialists. After several months, she was finally diagnosed with a mitochondrial disease. Jasmin is surprised. She has an 8-year-old niece with a mitochondrial disease, but her niece’s symptoms started when she was very young, and they included seizures and learning disabilities. How can Jasmin have the same disease, but different symptoms? Why didn't she have problems until adulthood, while her niece experienced symptoms at an early age? And what are mitochondria, anyway?

 

Attributes

Figure 4.1.1

Difficult competition by Massimo Sartirana on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 4.1.2

Exhausted by Kevin Grieve on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Created by: CK-12/Adapted by Christine Miller

We all get tired sometimes, especially if we have been doing a lot of physical activity, like the athletes pictured in Figure 4.1.1. But for Jasmin (Figure 4.1.2), a 34-year-old former high school track star who is now a recreational runner, her tiredness was going far beyond what she thought should be normal for someone in generally good physical shape.

She was experiencing extreme fatigue after her runs, as well as muscle cramping, spasms, and an unusual sense of heaviness in her legs. At first, she just chalked it up to getting older, but her exhaustion and pain worsened to the point where the former athlete could no longer run for more than a few minutes at a time. She began to experience other unusual symptoms, such as blurry vision and vomiting for no apparent reason.

Concerned, Jasmin went to her doctor, who ran many tests and consulted with several specialists. After several months, she was finally diagnosed with a mitochondrial disease. Jasmin is surprised. She has an 8-year-old niece with a mitochondrial disease, but her niece’s symptoms started when she was very young, and they included seizures and learning disabilities. How can Jasmin have the same disease, but different symptoms? Why didn't she have problems until adulthood, while her niece experienced symptoms at an early age? And what are mitochondria, anyway?

 

Attributes

Figure 4.1.1

Difficult competition by Massimo Sartirana on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 4.1.2

Exhausted by Kevin Grieve on Unsplash is used under the Unsplash License (https://unsplash.com/license).

 

Figure 4.2.1 Human cells viewed with a very powerful tool called a scanning electron microscope.

What are these incredible objects? Would it surprise you to learn that they are all human cells? Cells are actually too small to see with the unaided eye. It is visible here in such detail because it is being viewed with a very powerful tool called a scanning electron microscope. Cells may be small in size, but they are extremely important to life. Like all other living things, you are made of cells. Cells are the basis of life, and without cells, life as we know it would not exist. You will learn more about these amazing building blocks of life in this section.

If you look at living matter with a microscope — even a simple light microscope — you will see that it consists of cells. Cells are the basic units of the structure and function of living things. They are the smallest units that can carry out the processes of life. All organisms are made up of one or more cells, and all cells have many of the same structures and carry out the same basic life processes. Knowing the structure of cells and the processes they carry out is necessary to an understanding of life itself.

The first time the word cell was used to refer to these tiny units of life was in 1665 by a British scientist named Robert Hooke. Hooke was one of the earliest scientists to study living things under a microscope. The microscopes of his day were not very strong, but Hooke was still able to make an important discovery. When he looked at a thin slice of cork under his microscope, he was surprised to see what looked like a honeycomb. Hooke made the drawing in the figure to the right to show what he saw. As you can see, the cork was made up of many tiny units. Hooke called these units cells because they resembled cells in a monastery.

By the early 1800s, scientists had observed cells of many different organisms. These observations led two German scientists named Theodor Schwann and Matthias Jakob Schleiden to propose cells as the basic building blocks of all living things. Around 1850, a German doctor named Rudolf Virchow was studying cells under a microscope, when he happened to see them dividing and forming new cells. He realized that living cells produce new cells through division. Based on this realization, Virchow proposed that living cells arise only from other living cells.

The ideas of all three scientists — Schwann, Schleiden, and Virchow — led to cell theory, which is one of the fundamental theories unifying all of biology.

Cell theory states that:

• All organisms are made of one or more cells.
• All the life functions of organisms occur within cells.
• All cells come from existing cells.

Starting with Robert Hooke in the 1600s, the microscope opened up an amazing new world — a world of life at the level of the cell. As microscopes continued to improve, more discoveries were made about the cells of living things, but by the late 1800s, light microscopes had reached their limit. Objects much smaller than cells (including the structures inside cells) were too small to be seen with even the strongest light microscope.

Then, in the 1950s, a new type of microscope was invented. Called the electron microscope, it used a beam of electrons instead of light to observe extremely small objects. With an electron microscope, scientists could finally see the tiny structures inside cells. They could even see individual molecules and atoms. The electron microscope had a huge impact on biology. It allowed scientists to study organisms at the level of their molecules, and it led to the emergence of the molecular biology field. With the electron microscope, many more cell discoveries were made.

Although cells are diverse, all cells have certain parts in common. These parts include a plasma membrane, cytoplasm, ribosomes, and DNA.

1. The plasma membrane (a type of cell membrane) is a thin coat of lipids that surrounds a cell. It forms the physical boundary between the cell and its environment. You can think of it as the “skin” of the cell.
2. Cytoplasm refers to all of the cellular material inside of the plasma membrane. Cytoplasm is made up of a watery substance called cytosol, and it contains other cell structures, such as ribosomes.
3. Ribosomes are the structures in the cytoplasm in which proteins are made.
4. DNA is a nucleic acid found in cells. It contains the genetic instructions that cells need to make proteins.

These four parts are common to all cells, from organisms as different as bacteria and human beings. How did all known organisms come to have such similar cells? The similarities show that all life on Earth has a common evolutionary history.

Created by: CK-12/Adapted by Christine Miller

Look at the big windows in this house (Figure 4.7.1). Imagine all the light they must let in on a sunny day. Now imagine living in a house that has walls without any windows or doors. Nothing could enter or leave. Or imagine living in a house with holes in the walls instead of windows and doors. Things could enter or leave, but you couldn’t control what came in or went out. Only when a house has walls with windows and doors that can be opened or closed, can you control what enters or leaves. Windows and doors allow you to let in light and the family dog and keep out rain and bugs, for example.

If a cell were a house, the plasma membrane would be walls with windows and doors. Moving things in and out of the cell is an important function of the plasma membrane. It controls everything that enters and leaves the cell. There are two basic ways that substances can cross the plasma membrane: passive transport — which requires no energy expenditure by the cell — and active transport — which requires energy from the cell.

Passive transport occurs when substances cross the plasma membrane without any input of energy from the cell. No energy is required because the substances are moving from an area where they have a higher concentration to an area where they have a lower concentration. Concentration refers to the number of particles of a substance per unit of volume. The more particles of a substance in a given volume, the higher the concentration. A substance always moves from an area where it is more concentrated to an area where it is less concentrated.

There are several different types of passive transport, including simple diffusion, osmosis, and facilitated diffusion. Each type is described below.

Simple Diffusion

Diffusion is the movement of a substance due to a difference in concentration. It happens without any help from other molecules. The substance simply moves from the area where it is more concentrated to the area where it is less concentrated. Picture someone spraying perfume in the corner of a room.  Do the perfume molecules stay in the corner?  No, they spread out, or diffuse throughout the room until they are evenly spread out.  Figure 4.7.2 shows how diffusion works across a cell membrane. Substances that can squeeze between the lipid molecules in the plasma membrane by simple diffusion are generally very small, hydrophobic molecules, such as molecules of oxygen and carbon dioxide.

Osmosis is a special type of diffusion — the diffusion of water molecules across a membrane. Like other molecules, water moves from an area of higher concentration to an area of lower concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane.  In Figure 4.7.3, the dotted red line shows a semi-permeable membrane.  In the first beaker, there is an uneven concentration of solutes on either side of the membrane, but the solute cannot cross — diffusion of the solute can't occur.  In this case, water will move to even out the concentration as has happened on the beaker on the right side.  The water levels are uneven, but the process of osmosis has evened out the concentration gradient.

Facilitated Diffusion

Water and many other substances cannot simply diffuse across a membrane. Hydrophilic molecules, charged ions, and relatively large molecules (such as glucose) all need help with diffusion. This help comes from special proteins in the membrane known as transport proteins. Diffusion with the help of transport proteins is called facilitated diffusion. There are several types of transport proteins, including channel proteins and carrier proteins. Both are shown in Figure 4.7.4.

• Channel proteins form pores (or tiny holes) in the membrane. This allows water molecules and small ions to pass through the membrane without coming into contact with the hydrophobic tails of the lipid molecules in the interior of the membrane.
• Carrier proteins bind with specific ions or molecules. In doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane.

For a cell to function normally, the inside of it must maintain a stable state. The concentrations of salts, nutrients, and other substances must be kept within certain ranges. The state in which stable conditions are maintained inside a cell (or an entire organism) is called homeostasis. Homeostasis requires constant adjustments, because conditions are always changing both inside and outside the cell. The transport of substances into and out of cells as described in this section plays an important role in homeostasis. By allowing the movement of substances into and out of cells, transport keeps conditions within normal ranges inside the cells and throughout the organism as a whole.

Watch this video "Cell Transport," by the Amoeba Sisters:

https://www.youtube.com/watch?v=Ptmlvtei8hw

Cell Transport with the Amoeba Sisters, 2016.

Attributions

Figure 4.7.1

Windows/ The Oyster Suite in Eureka, CA by Drew Coffman on Unsplash is used under the Unsplash License https://unsplash.com/license).

Figure 4.7.2

Diffusion/ Scheme simple diffusion in cell membrane  by Mariana Ruiz Villarreal [LadyofHats] is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.7.3

Osmosis by OpenStax on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 4.7.4

Scheme facilitated diffusion in cell membrane by Mariana Ruiz Villarreal [LadyofHats] is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2016, June 24). Cell transport. YouTube. https://www.youtube.com/watch?v=Ptmlvtei8hw&feature=youtu.be

Amoeba Sisters. (2018, June 27). Osmosis and water potential. YouTube.  https://www.youtube.com/watch?v=L-osEc07vMs&feature=youtu.be

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 3.7 Osmosis [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Professor Dave Explains. (2016, September 5). Structure of the cell membrane - Active and passive transport. https://www.youtube.com/watch?v=AcrqIxt8am8&feature=youtu.be

 

 

Created by CK-12 Foundation/Adapted by Christine Miller

Imagine the man in Figure 12.3.1 turns his eyes in your direction. This is a very small movement, considering the conspicuously large and strong external eye muscles that control eyeball movements. These muscles have been called the strongest muscles in the human body relative to the work they do. However, the external eye muscles actually do a surprising amount of work. Eye movements occur almost constantly during waking hours, especially when we are scanning faces or reading. Eye muscles are also exercised nightly during the phase of sleep called rapid eye movement sleep. External eye muscles can move the eyes because they are made mainly of muscle tissue.

Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles are connective tissues, such as tendons that attach skeletal muscles to bones and sheaths of connective tissues that cover or line muscle tissues. Only muscle tissue per se, has cells with the ability to contract.

There are three major types of muscle tissues in the human body: skeletal, smooth, and cardiac muscle tissues. Figure 12.3.2 shows how the three types of muscle tissues appear under magnification. When you read about each type below, you will learn why the three types appear as they do.

Skeletal muscle is muscle tissue that is attached to bones by tendons, which are bundles of collagen fibres. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are voluntary, or under conscious control of the central nervous system via the somatic nervous system. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42% skeletal muscles, and the average adult female is about 36% skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figure 12.3.3 below.

Skeletal Muscle Pairs

To move bones in opposite directions, skeletal muscles often consist of muscle pairs that work in opposition to one another, also called antagonistic muscle pairs.  For example, when the biceps muscle (on the front of the upper arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in Figure 12.3.4. When the triceps muscle (on the back of the upper arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles, also shown in Figure 12.3.4, are an example of a muscle pair where the muscles work in opposition to each other.

Skeletal Muscle Structure

Each skeletal muscle consists of hundreds — or even thousands — of skeletal muscle fibres, which are long, string-like cells. As shown in Figure 12.3.5 below, skeletal muscle fibres are individually wrapped in connective tissue called endomysium. The skeletal muscle fibres are bundled together in units called muscle fascicles, which are surrounded by sheaths of connective tissue called perimysium. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibres. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called epimysium. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibres, allowing them to withstand the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors the muscles to tendons.

The same bundles-within-bundles structure is replicated within each muscle fibre. As shown in Figure 12.3.6, a muscle fibre consists of a bundle of myofibrils, which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein actin, which are anchored to structures called Z discs, and thick filaments of the protein myosin. The filaments are arranged together within a myofibril in repeating units called sarcomeres, which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal and cardiac muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated, because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibres. Other components of a skeletal muscle fibre include multiple nuclei and mitochondria.

Slow- and Fast-Twitch Skeletal Muscle Fibres

Skeletal muscle fibres can be divided into two types, called slow-twitch (or type I) muscle fibres and fast-twitch (or type II) muscle fibres.

• Slow-twitch muscle fibres are dense with capillaries and rich in mitochondria and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibres, slow-twitch fibres can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibres can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.
• Fast-twitch muscle fibres contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.

Proportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.

Smooth muscle is muscle tissue in the walls of internal organs and other internal structures such as blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are involuntary, so they are not under conscious control. Instead, they are controlled by the autonomic nervous system, hormones, neurotransmitters, and other physiological factors.

Structure of Smooth Muscle

The cells that make up smooth muscle are generally called myocytes. Unlike the muscle fibres of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in sarcomeres. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain myofibrils, which in turn contain bundles of myosin and actin filaments. The filaments cause contractions when they slide over each other, as shown in Figure 12.3.7.

Unlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of elastin, collagen, and other stretchy fibres. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (see Figures 12.3.8 and 12.3.9), both of which must stretch considerably as they perform their normal functions.

The following list indicates where many smooth muscles are found, along with some of their specific functions.

• Walls of organs of the gastrointestinal tract (such as the esophagus, stomach, and intestines), moving food through the tract by peristalsis
• Walls of air passages of the respiratory tract (such as the bronchi), controlling the diameter of the passages and the volume of air that can pass through them
• Walls of organs of the male and female reproductive tracts; in the uterus, for example, pushing a baby out of the uterus and into the birth canal
• Walls of structures of the urinary system, including the urinary bladder, allowing the bladder to expand so it can hold more urine, and then contract as urine is released
• Walls of blood vessels, controlling the diameter of the vessels and thereby affecting blood flow and blood pressure
• Walls of lymphatic vessels, squeezing the fluid called lymph through the vessels
• Iris of the eyes, controlling the size of the pupils and thereby the amount of light entering the eyes
• Arrector pili in the skin, raising hairs in hair follicles in the dermis

Cardiac muscle is found only in the wall of the heart. It is also called myocardium. As shown in Figure 12.3.10, myocardium is enclosed within connective tissues, including the endocardium on the inside of the heart and pericardium on the outside of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac muscle cells in an area of the heart muscle called the sinoatrial node.

Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres inside the muscle fibres. However, in cardiac muscle, the myofibrils are branched at irregular angles rather than arranged in parallel rows (as they are in skeletal muscle). This explains why cardiac and skeletal muscle tissues look different from one another.

The cells of cardiac muscle tissue are arranged in interconnected networks. This arrangement allows rapid transmission of electrical impulses, which stimulate virtually simultaneous contractions of the cells. This enables the cells to coordinate contractions of the heart muscle.

The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime. Although the power output of the heart is much less than the maximum power output of some other muscles in the human body, the heart does its work continuously over an entire lifetime without rest. Cardiac muscle contains a great many mitochondria, which produce ATP for energy and help the heart resist fatigue.

Cardiomyopathy is a disease in which the muscles of the heart are no longer able to effectively pump blood to the body — extreme forms of this disease can lead to heart failure.  There are four main types of cardiomyopathy (also illustrated in Figure 12.3.11):

• Dilated (congestive) cardiomyopathy: the left ventricle (the chamber itself) of the heart becomes enlarged and can't pump blood our to the body.  This is normally related to coronary artery disease and/or heart attack
• Hypertrophic cardiomyopathy: abnormal thickening of the muscular walls of the left ventricle make the chamber less able to work properly.  This condition is more common in patients with a family history of the disease.
• Restrictive cardiomyopathy: the myocardium becomes abnormally rigid and inelastic and is unable to expand in between heartbeats to refill with blood.  Restrictive cardiomyopathy typically affects older people.
• Arrhythmogenic right ventricular cardiomyopathy: the right ventricular muscle is replaced by adipose or scar tissue, reducing elasticity and interfering with normal heartbeat and rhythm.  This disease is often caused by genetic mutations.

Cardiomyopathy is typically diagnosed with a physical exam supplemented by medical and family history, an angiogram, blood tests, chest x-rays and electrocardiograms.  In some cases your doctor would also requisition a CT scan and/or genetic testing.

When treating cardiomyopathy, the goal is to reduce symptoms that affect everyday life.  Certain medications can help regularize and slow heart rate, decrease chances of blood clots and cause vasodilation in the coronary arteries.  If medication is not sufficient to manage symptoms, a pacemaker or even a heart transplant may be the best option.  Lifestyle can also help manage the symptoms of cardiomyopathy — people living with this disease are encouraged to avoid drug and alcohol use, control high blood pressure, eat a healthy diet, get ample rest and exercise, as well as reduce stress levels.

 

Attributions

Figure 12.3.1

Look by ali-yahya-155huuQwGvA [photo] by Ali Yahya on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 12.3.2

Skeletal_Smooth_Cardiac by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.3

Anterior_and_Posterior_Views_of_Muscles by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.4

Antagonistic Muscle Pair by Laura Guerin at CK-12 Foundation on Wikimedia Commons is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license. 

Figure 12.3.5

Muscle_Fibes_(large) by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.6

Muscle_Fibers_(small) by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.7

Smooth_Muscle_Contraction by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.8

Blausen_0747_Pregnancy by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.9

Size_of_Uterus_Throughout_Pregnancy-02 by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.10

1024px-Blausen_0470_HeartWall by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.11

Tipet_e_kardiomiopative by Npatchett at English Wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license. (Work derived from Blausen 0165 Cardiomyopathy Dilated by BruceBlaus)

References

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, June 19). Figure 4.18 Muscle tissue [digital image].  In Anatomy and Physiology (Section 4.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/4-4-muscle-tissue-and-motion

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, June 19). Figure 28.18 Size of uterus throughout pregnancy [digital image].  In Anatomy and Physiology (Section 28.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/28-4-maternal-changes-during-pregnancy-labor-and-birth

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. (2016, May 18). Figure 10.3 The three connective tissue layers [digital image].  In Anatomy and Physiology (Section 10.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-2-skeletal-muscle

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. (2016, May 18). Figure 10.4 Muscle fiber [digital image].  In Anatomy and Physiology (Section 10.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-2-skeletal-muscle

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. (2016, May 18). Figure 10.24 Muscle contraction [digital image].  In Anatomy and Physiology (Section 10.8). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-8-smooth-muscle

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. (2016, May 18). Figure 11.5 Overview of the muscular system [digital image].  In Anatomy and Physiology (Section 11.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/11-2-naming-skeletal-muscles

Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Brainard, J/ CK-12 Foundation. (2012). Figure 5 Triceps and biceps muscles in the upper arm are opposing muscles. [digital image]. In CK-12 Biology (Section 21.3) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-biology/section/21.3/ (Last modified August 11, 2017.)

khanacademymedicine. (2012, October 19). Three types of muscle | Circulatory system physiology | NCLEX-RN | Khan Academy. YouTube.

TED-Ed. (2017, February 14).  What happens during a heart attack? - Krishna Sudhir. YouTube. https://www.youtube.com/watch?v=3_PYnWVoUzM&feature=youtu.be

division of the peripheral nervous system that controls involuntary activities

A hormone is a signaling molecule produced by glands in multicellular organisms that target distant organs to regulate physiology and behavior.

The body system which acts as a chemical messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In humans, the major endocrine glands are the thyroid gland and the adrenal glands.

We stay alive because millions of different chemical reactions are taking place inside our bodies all the time. Each of our cells is like the busy auto assembly line pictured in Figure 3.10.1. Raw materials, half-finished products, and waste materials are constantly being used, produced, transported, and excreted. The "workers" on the cellular assembly line are mainly enzymes. These are the proteins that make biochemical reactions happen.

Chemical reactions that take place inside living things are called biochemical reactions. The sum of all the biochemical reactions in an organism is called metabolism. Metabolism includes both exothermic (energy-releasing) chemical reactions and endothermic (energy-absorbing) chemical reactions.

Catabolic Reactions

Exothermic reactions in organisms are called catabolic reactions. These reactions break down molecules into smaller units and release energy. An example of a catabolic reaction is the breakdown of glucose during cellular respiration, which releases energy that cells need to carry out life processes.

Anabolic Reactions

Endothermic reactions in organisms are called anabolic reactions. These reactions build up bigger molecules from smaller ones and absorb energy. An example of an anabolic reaction is the joining of amino acids to form a protein. Which type of reactions — catabolic or anabolic — do you think occur when your body digests food?

Most of the biochemical reactions that happen inside of living organisms require help. Why is this the case? For one thing, temperatures inside living things are usually too low for biochemical reactions to occur quickly enough to maintain life. The concentrations of reactants may also be too low for them to come together and react. Where do the biochemical reactions get the help they need to proceed? From the enzymes.

An enzyme is a protein that speeds up a biochemical reaction. It is a biological catalyst. An enzyme generally works by reducing the amount of activation energy needed to start the reaction. The graph in Figure 3.10.2 shows the activation energy needed for glucose to combine with oxygen. Less activation energy is needed when the correct enzyme is present than when it is not present.

How Well Enzymes Work

Enzymes are involved in most biochemical reactions, and they do their jobs extremely well. A typical biochemical reaction that would take several days or even several centuries to happen without an enzyme is likely to occur in just a split second with the proper enzyme! Without enzymes to speed up biochemical reactions, most organisms could not survive.

Enzymes are substrate-specific. The substrate of an enzyme is the specific substance it affects. Each enzyme works only with a particular substrate, which explains why there are so many different enzymes. In addition, for an enzyme to work, it requires specific conditions, such as the right temperature and pH. Some enzymes work best under acidic conditions, for example, while others work best in neutral environments.

Enzyme-Deficiency Disorders

There are hundreds of known inherited metabolic disorders in humans. In most of them, a single enzyme is either not produced by the body at all, or is otherwise produced in a form that doesn't work. The missing or defective enzyme is like an absentee worker on the cell's assembly line. Imagine the auto assembly line from the image at the start of this section.  What if the worker who installed the steering wheel was absent?  How would this impact the overall functioning of the vehicle?  When an enzyme is missing, toxic chemicals build up, or an essential product isn't made. Generally, the normal enzyme is missing because the individual with the disorder inherited two copies of a gene mutation, which may have originated many generations previously.

Any given inherited metabolic disorder is generally quite rare in the general population. However, there are so many different metabolic disorders that a total of one in 1,000 to 2,500 newborns can be expected to have one.

Figure 3.10.1

Auto Assembly line by Brian Snelson on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 3.10.2

Enzyme_activation_energy by G. Andruk [IMeowbot at the English language Wikipedia], is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

References

Amoeba Sisters. (2016, August 28). Enzymes (updated). YouTube. https://www.youtube.com/watch?v=qgVFkRn8f10&feature=youtu.be

TED-Ed. (2015, January 15). What triggers a chemical reaction? - Kareem Jarrah. YouTube. https://www.youtube.com/watch?v=8m6RtOpqvtU&feature=youtu.be

The smallest type of blood vessel that connects arterioles and venules and that transfers substances between blood and tissues.

Created by CK-12 Foundation/Adapted by Christine Miller

Imagine the man in Figure 12.3.1 turns his eyes in your direction. This is a very small movement, considering the conspicuously large and strong external eye muscles that control eyeball movements. These muscles have been called the strongest muscles in the human body relative to the work they do. However, the external eye muscles actually do a surprising amount of work. Eye movements occur almost constantly during waking hours, especially when we are scanning faces or reading. Eye muscles are also exercised nightly during the phase of sleep called rapid eye movement sleep. External eye muscles can move the eyes because they are made mainly of muscle tissue.

Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles are connective tissues, such as tendons that attach skeletal muscles to bones and sheaths of connective tissues that cover or line muscle tissues. Only muscle tissue per se, has cells with the ability to contract.

There are three major types of muscle tissues in the human body: skeletal, smooth, and cardiac muscle tissues. Figure 12.3.2 shows how the three types of muscle tissues appear under magnification. When you read about each type below, you will learn why the three types appear as they do.

Skeletal muscle is muscle tissue that is attached to bones by tendons, which are bundles of collagen fibres. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are voluntary, or under conscious control of the central nervous system via the somatic nervous system. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42% skeletal muscles, and the average adult female is about 36% skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figure 12.3.3 below.

Skeletal Muscle Pairs

To move bones in opposite directions, skeletal muscles often consist of muscle pairs that work in opposition to one another, also called antagonistic muscle pairs.  For example, when the biceps muscle (on the front of the upper arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in Figure 12.3.4. When the triceps muscle (on the back of the upper arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles, also shown in Figure 12.3.4, are an example of a muscle pair where the muscles work in opposition to each other.

Skeletal Muscle Structure

Each skeletal muscle consists of hundreds — or even thousands — of skeletal muscle fibres, which are long, string-like cells. As shown in Figure 12.3.5 below, skeletal muscle fibres are individually wrapped in connective tissue called endomysium. The skeletal muscle fibres are bundled together in units called muscle fascicles, which are surrounded by sheaths of connective tissue called perimysium. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibres. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called epimysium. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibres, allowing them to withstand the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors the muscles to tendons.

The same bundles-within-bundles structure is replicated within each muscle fibre. As shown in Figure 12.3.6, a muscle fibre consists of a bundle of myofibrils, which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein actin, which are anchored to structures called Z discs, and thick filaments of the protein myosin. The filaments are arranged together within a myofibril in repeating units called sarcomeres, which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal and cardiac muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated, because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibres. Other components of a skeletal muscle fibre include multiple nuclei and mitochondria.

Slow- and Fast-Twitch Skeletal Muscle Fibres

Skeletal muscle fibres can be divided into two types, called slow-twitch (or type I) muscle fibres and fast-twitch (or type II) muscle fibres.

• Slow-twitch muscle fibres are dense with capillaries and rich in mitochondria and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibres, slow-twitch fibres can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibres can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.
• Fast-twitch muscle fibres contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.

Proportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.

Smooth muscle is muscle tissue in the walls of internal organs and other internal structures such as blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are involuntary, so they are not under conscious control. Instead, they are controlled by the autonomic nervous system, hormones, neurotransmitters, and other physiological factors.

Structure of Smooth Muscle

The cells that make up smooth muscle are generally called myocytes. Unlike the muscle fibres of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in sarcomeres. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain myofibrils, which in turn contain bundles of myosin and actin filaments. The filaments cause contractions when they slide over each other, as shown in Figure 12.3.7.

Unlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of elastin, collagen, and other stretchy fibres. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (see Figures 12.3.8 and 12.3.9), both of which must stretch considerably as they perform their normal functions.

The following list indicates where many smooth muscles are found, along with some of their specific functions.

• Walls of organs of the gastrointestinal tract (such as the esophagus, stomach, and intestines), moving food through the tract by peristalsis
• Walls of air passages of the respiratory tract (such as the bronchi), controlling the diameter of the passages and the volume of air that can pass through them
• Walls of organs of the male and female reproductive tracts; in the uterus, for example, pushing a baby out of the uterus and into the birth canal
• Walls of structures of the urinary system, including the urinary bladder, allowing the bladder to expand so it can hold more urine, and then contract as urine is released
• Walls of blood vessels, controlling the diameter of the vessels and thereby affecting blood flow and blood pressure
• Walls of lymphatic vessels, squeezing the fluid called lymph through the vessels
• Iris of the eyes, controlling the size of the pupils and thereby the amount of light entering the eyes
• Arrector pili in the skin, raising hairs in hair follicles in the dermis

Cardiac muscle is found only in the wall of the heart. It is also called myocardium. As shown in Figure 12.3.10, myocardium is enclosed within connective tissues, including the endocardium on the inside of the heart and pericardium on the outside of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac muscle cells in an area of the heart muscle called the sinoatrial node.

Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres inside the muscle fibres. However, in cardiac muscle, the myofibrils are branched at irregular angles rather than arranged in parallel rows (as they are in skeletal muscle). This explains why cardiac and skeletal muscle tissues look different from one another.

The cells of cardiac muscle tissue are arranged in interconnected networks. This arrangement allows rapid transmission of electrical impulses, which stimulate virtually simultaneous contractions of the cells. This enables the cells to coordinate contractions of the heart muscle.

The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime. Although the power output of the heart is much less than the maximum power output of some other muscles in the human body, the heart does its work continuously over an entire lifetime without rest. Cardiac muscle contains a great many mitochondria, which produce ATP for energy and help the heart resist fatigue.

Cardiomyopathy is a disease in which the muscles of the heart are no longer able to effectively pump blood to the body — extreme forms of this disease can lead to heart failure.  There are four main types of cardiomyopathy (also illustrated in Figure 12.3.11):

• Dilated (congestive) cardiomyopathy: the left ventricle (the chamber itself) of the heart becomes enlarged and can't pump blood our to the body.  This is normally related to coronary artery disease and/or heart attack
• Hypertrophic cardiomyopathy: abnormal thickening of the muscular walls of the left ventricle make the chamber less able to work properly.  This condition is more common in patients with a family history of the disease.
• Restrictive cardiomyopathy: the myocardium becomes abnormally rigid and inelastic and is unable to expand in between heartbeats to refill with blood.  Restrictive cardiomyopathy typically affects older people.
• Arrhythmogenic right ventricular cardiomyopathy: the right ventricular muscle is replaced by adipose or scar tissue, reducing elasticity and interfering with normal heartbeat and rhythm.  This disease is often caused by genetic mutations.

Cardiomyopathy is typically diagnosed with a physical exam supplemented by medical and family history, an angiogram, blood tests, chest x-rays and electrocardiograms.  In some cases your doctor would also requisition a CT scan and/or genetic testing.

When treating cardiomyopathy, the goal is to reduce symptoms that affect everyday life.  Certain medications can help regularize and slow heart rate, decrease chances of blood clots and cause vasodilation in the coronary arteries.  If medication is not sufficient to manage symptoms, a pacemaker or even a heart transplant may be the best option.  Lifestyle can also help manage the symptoms of cardiomyopathy — people living with this disease are encouraged to avoid drug and alcohol use, control high blood pressure, eat a healthy diet, get ample rest and exercise, as well as reduce stress levels.

 

Attributions

Figure 12.3.1

Look by ali-yahya-155huuQwGvA [photo] by Ali Yahya on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 12.3.2

Skeletal_Smooth_Cardiac by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.3

Anterior_and_Posterior_Views_of_Muscles by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.4

Antagonistic Muscle Pair by Laura Guerin at CK-12 Foundation on Wikimedia Commons is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license. 

Figure 12.3.5

Muscle_Fibes_(large) by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.6

Muscle_Fibers_(small) by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.7

Smooth_Muscle_Contraction by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 12.3.8

Blausen_0747_Pregnancy by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.9

Size_of_Uterus_Throughout_Pregnancy-02 by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.10

1024px-Blausen_0470_HeartWall by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 12.3.11

Tipet_e_kardiomiopative by Npatchett at English Wikipedia on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license. (Work derived from Blausen 0165 Cardiomyopathy Dilated by BruceBlaus)

References

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, June 19). Figure 4.18 Muscle tissue [digital image].  In Anatomy and Physiology (Section 4.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/4-4-muscle-tissue-and-motion

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, June 19). Figure 28.18 Size of uterus throughout pregnancy [digital image].  In Anatomy and Physiology (Section 28.4). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/28-4-maternal-changes-during-pregnancy-labor-and-birth

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. (2016, May 18). Figure 10.3 The three connective tissue layers [digital image].  In Anatomy and Physiology (Section 10.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-2-skeletal-muscle

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. (2016, May 18). Figure 10.4 Muscle fiber [digital image].  In Anatomy and Physiology (Section 10.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-2-skeletal-muscle

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. (2016, May 18). Figure 10.24 Muscle contraction [digital image].  In Anatomy and Physiology (Section 10.8). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/10-8-smooth-muscle

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. (2016, May 18). Figure 11.5 Overview of the muscular system [digital image].  In Anatomy and Physiology (Section 11.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/11-2-naming-skeletal-muscles

Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Brainard, J/ CK-12 Foundation. (2012). Figure 5 Triceps and biceps muscles in the upper arm are opposing muscles. [digital image]. In CK-12 Biology (Section 21.3) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-biology/section/21.3/ (Last modified August 11, 2017.)

khanacademymedicine. (2012, October 19). Three types of muscle | Circulatory system physiology | NCLEX-RN | Khan Academy. YouTube.

TED-Ed. (2017, February 14).  What happens during a heart attack? - Krishna Sudhir. YouTube. https://www.youtube.com/watch?v=3_PYnWVoUzM&feature=youtu.be

The transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent allele frequencies and therefore effectively be a single population.

Created by: CK-12/Adapted by Christine Miller

This inviting camp fire can be used for both heat and light. Heat and light are two forms of energy that are released when a fuel like wood is burned. The cells of living things also get energy by "burning." They "burn" glucose in a process called cellular respiration.

Cellular respiration is the process by which living cells break down glucose molecules and release energy. The process is similar to burning, although it doesn’t produce light or intense heat as a campfire does. This is because cellular respiration releases the energy in glucose slowly and in many small steps. It uses the energy released to form molecules of ATP, the energy-carrying molecules that cells use 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.  Cellular respiration involves many chemical reactions, but they can all be summed up with this chemical equation:

C₆H₁₂O₆  6O₂ → 6CO₂  6H₂O Chemical Energy (in ATP)

In words, the equation shows that glucose (C₆H₁₂O₆) and oxygen (O₂) react to form carbon dioxide (CO₂) and water (H₂O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an aerobic process.

Cellular respiration occurs in the cells of all living things, both autotrophs and heterotrophs. All of them burn glucose to form ATP. The reactions of cellular respiration 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.

The first stage of cellular respiration is glycolysisno post, which happens in the cytosol of the cytoplasm.

Splitting Glucose

The word glycolysis literally means “glucose splitting,” which is exactly what happens in this stage. Enzymes split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the following diagram.

Results of Glycolysis

Energy is needed at the start of glycolysisno post 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 of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD  to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.

Transition Reaction

Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.  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.  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).

Before you read about the last two stages of cellular respiration, you need to know more about the mitochondrion, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.

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 intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron transport) happens on the inner membrane.

Recall that glycolysisno post 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 Krebs cycle (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.  The reactions that occur next are shown in Figure 4.10.6.

The Krebs cycle 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.

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, another energy-carrying coenzyme. Carbon dioxide is also released as a waste product of these reactions.

The final step of the Krebs cycle regenerates 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 pyruvic acid molecules when it splits glucose.

Results of the Glycolysis, Transition Reaction and Krebs Cycle

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 chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:

• 4 ATP (2 from glycolysis, 2 from Krebs Cycle)
• 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)
• 2 FADH₂ (both from the Krebs cycle)

The events of cellular respiration up to this point are exergonic reactions- they are releasing energy that had been stored in the bonds of the glucose molecule.  This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction.  Using an exothermic reaction to power an endothermic reaction is known as energy coupling.

 ETC, the final stage in cellular respiration produces 32 ATP.  The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH₂ is transferred to ATP.  In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH₂, forming water.  Figure 4.10.8 shows the reactants and products of the ETC.

Transporting Electrons

The Electron transport chain 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, and they move along electron-transport chains on the inner membrane of the mitochondrion. An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by chemical 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 ion transfer creates an electrochemical gradient that drives the synthesis of ATP.

 

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 “spent” electrons combine with oxygen to form water.

You have seen how the three stages of aerobic respiration use the energy in glucose to make ATP. 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 from 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.

References

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

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 24.8 Electron Transport Chain [digital image]. In Anatomy & Physiology, Connexions (Section ). OpenStax.  https://openstax.org/books/anatomy-and-physiology/pages/24-2-carbohydrate-metabolism

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 24.9 Carbohydrate Metabolism [digital image]. In Anatomy & Physiology, Connexions (Section 24.2). OpenStax.  https://openstax.org/books/anatomy-and-physiology/pages/24-2-carbohydrate-metabolism

The Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https://www.youtube.com/watch?v=4Eo7JtRA7lg&t=3s

 

An involuntary human body response mediated by the nervous and endocrine systems that prepares the body to fight or flee from perceived danger.

You probably know that  batteries contain dangerous chemicals, including strong acids. Strong acids can hurt you if they come into contact with your skin or eyes. Therefore, it may surprise you to learn that your life depends on acids. There are many acids inside your body, and some of them are as strong as battery acid. Acids are needed for digestion and some forms of energy production. Genes are made of nucleic acids, proteins of amino acids, and lipids of fatty acids.

Acids (such as battery acid) are solutions. A solution is a mixture of two or more substances that has the same composition throughout. Many solutions are a mixture of water and some other substance. Not all solutions are acids. Some are bases and some are neither acids nor bases. To understand acids and bases, you need to know more about pure water.

In pure water (such as distilled water), a tiny fraction of water molecules naturally breaks down to form ions. An ion is an electrically charged atom or molecule. The breakdown of water is represented by the chemical equation:

2 H₂O → H₃O⁺ + OH^-

The products of this reaction are a hydronium ion (H3O⁺) and a hydroxide ion (OH^-). The hydroxide ion, which has a negative charge, forms when a water molecule gives up a positively charged hydrogen ion (H⁺). The hydronium ion, which has a positive charge, forms when another water molecule accepts the hydrogen ion.

The concentration of hydronium ions in a solution is known as acidity. In pure water, the concentration of hydronium ions is very low; only about one in ten million water molecules naturally breaks down to form a hydronium ion. As a result, pure water is essentially neutral. Acidity is measured on a scale called pH, as shown in Figure 3.12.2. Pure water has a pH of 7, so the point of neutrality on the pH scale is 7.

Acids

If a solution has a higher concentration of hydronium ions than pure water, it has a pH lower than 7. A solution with a pH lower than 7 is called an acid. As the hydronium ion concentration increases, the pH value decreases. Therefore, the more acidic a solution is, the lower its pH value is.

Did you ever taste vinegar? Like other acids, it tastes sour. Stronger acids can be harmful to organisms. Even stomach acid would eat through the stomach if it were not lined with a layer of mucus. Strong acids can also damage materials, even hard materials such as glass.

Bases

If a solution has a lower concentration of hydronium ions than pure water, it has a pH higher than 7. A solution with a pH higher than 7 is called a base. Bases, such as baking soda, have a bitter taste. Like strong acids, strong bases can harm organisms and damage materials. For example, lye can burn the skin, and bleach can remove the colour from clothing.

Many acids and bases in living things provide the pH that enzymes need. Enzymes are biological catalysts that must work effectively for biochemical reactions to occur. Most enzymes can do their job only at a certain level of acidity. Cells secrete acids and bases to maintain the proper pH for enzymes to do their work.

Every time you digest food, acids and bases are at work in your digestive system. Consider the enzyme pepsin, which helps break down proteins in the stomach. Pepsin needs an acidic environment to do its job. The stomach secretes a strong acid called hydrochloric acid that allows pepsin to work. When stomach contents enter the small intestine, the acid must be neutralized, because enzymes in the small intestine need a basic environment in order to work. An organ called the pancreas secretes a base named bicarbonate into the small intestine, and this base neutralizes the acid.

Do you ever have heartburn? The answer is probably "yes." More than 60 million Americans have heartburn at least once a month, and more than 15 million suffer from it on a daily basis. Knowing more about heartburn may help you prevent it or know when it's time to seek medical treatment.

Heartburn doesn't have anything to do with the heart, but it does cause a burning sensation in the vicinity of the chest. Normally, the acid secreted into the stomach remains in the stomach where it is needed to allow pepsin to do its job of digesting proteins. A long tube called the esophagus carries food from the mouth to the stomach. A sphincter, or valve, between the esophagus and stomach opens to allow swallowed food to enter the stomach and then closes to prevent stomach contents from backflowing into the esophagus. If this sphincter is weak or relaxes inappropriately, stomach contents flow into the esophagus. Because stomach contents are usually acidic, this causes the burning sensation known as heartburn. People who are prone to heartburn and suffer from it often may be diagnosed with GERD, which stands for gastroesophageal reflux disease.

GERD — as well as occasional heartburn — often can be improved by dietary and other lifestyle changes that decrease the amount and acidity of reflux from the stomach into the esophagus.

• Some foods and beverages seem to contribute to GERD, so these should be avoided. Problematic foods include chocolate, fatty foods, peppermint, coffee, and alcoholic beverages.
• Decreasing portion size and eating the last meal of the day at least a couple of hours before bedtime may reduce the risk of reflux occurring.
• Smoking tends to weaken the lower esophageal sphincter, so quitting the habit may help control reflux.
• GERD is often associated with being overweight. Losing weight often brings improvement.
• Some people are helped by sleeping with the head of the bed elevated. This allows gravity to help control the backflow of acids into the esophagus from the stomach.

If you have frequent heartburn and lifestyle changes don't help, you may need medication to control the condition. Over-the-counter (OTC) antacids may be all that you need to control the occasional heartburn attack. OTC medications are usually bases that neutralize stomach acids. They may also create bubbles that help block stomach contents from entering the esophagus. For some people, OTC medications are not enough, and prescription medications are instead required for the control of GERD. These prescription medications generally work by inhibiting acid secretion in the stomach.

Be sure to see a doctor if you can't control your heartburn, or you have it often. Untreated GERD not only interferes with quality of life, it may also lead to more serious complications, ranging from esophageal bleeding to esophageal cancer.

 

a colorless cell that circulates in the blood and body fluids and is involved in counteracting foreign substances and disease; a white (blood) cell. There are several types, all amoeboid cells with a nucleus, including lymphocytes, granulocytes, monocytes, and macrophages.

You probably know that  batteries contain dangerous chemicals, including strong acids. Strong acids can hurt you if they come into contact with your skin or eyes. Therefore, it may surprise you to learn that your life depends on acids. There are many acids inside your body, and some of them are as strong as battery acid. Acids are needed for digestion and some forms of energy production. Genes are made of nucleic acids, proteins of amino acids, and lipids of fatty acids.

Acids (such as battery acid) are solutions. A solution is a mixture of two or more substances that has the same composition throughout. Many solutions are a mixture of water and some other substance. Not all solutions are acids. Some are bases and some are neither acids nor bases. To understand acids and bases, you need to know more about pure water.

In pure water (such as distilled water), a tiny fraction of water molecules naturally breaks down to form ions. An ion is an electrically charged atom or molecule. The breakdown of water is represented by the chemical equation:

2 H₂O → H₃O⁺ + OH^-

The products of this reaction are a hydronium ion (H3O⁺) and a hydroxide ion (OH^-). The hydroxide ion, which has a negative charge, forms when a water molecule gives up a positively charged hydrogen ion (H⁺). The hydronium ion, which has a positive charge, forms when another water molecule accepts the hydrogen ion.

The concentration of hydronium ions in a solution is known as acidity. In pure water, the concentration of hydronium ions is very low; only about one in ten million water molecules naturally breaks down to form a hydronium ion. As a result, pure water is essentially neutral. Acidity is measured on a scale called pH, as shown in Figure 3.12.2. Pure water has a pH of 7, so the point of neutrality on the pH scale is 7.

Acids

If a solution has a higher concentration of hydronium ions than pure water, it has a pH lower than 7. A solution with a pH lower than 7 is called an acid. As the hydronium ion concentration increases, the pH value decreases. Therefore, the more acidic a solution is, the lower its pH value is.

Did you ever taste vinegar? Like other acids, it tastes sour. Stronger acids can be harmful to organisms. Even stomach acid would eat through the stomach if it were not lined with a layer of mucus. Strong acids can also damage materials, even hard materials such as glass.

Bases

If a solution has a lower concentration of hydronium ions than pure water, it has a pH higher than 7. A solution with a pH higher than 7 is called a base. Bases, such as baking soda, have a bitter taste. Like strong acids, strong bases can harm organisms and damage materials. For example, lye can burn the skin, and bleach can remove the colour from clothing.

Many acids and bases in living things provide the pH that enzymes need. Enzymes are biological catalysts that must work effectively for biochemical reactions to occur. Most enzymes can do their job only at a certain level of acidity. Cells secrete acids and bases to maintain the proper pH for enzymes to do their work.

Every time you digest food, acids and bases are at work in your digestive system. Consider the enzyme pepsin, which helps break down proteins in the stomach. Pepsin needs an acidic environment to do its job. The stomach secretes a strong acid called hydrochloric acid that allows pepsin to work. When stomach contents enter the small intestine, the acid must be neutralized, because enzymes in the small intestine need a basic environment in order to work. An organ called the pancreas secretes a base named bicarbonate into the small intestine, and this base neutralizes the acid.

Do you ever have heartburn? The answer is probably "yes." More than 60 million Americans have heartburn at least once a month, and more than 15 million suffer from it on a daily basis. Knowing more about heartburn may help you prevent it or know when it's time to seek medical treatment.

Heartburn doesn't have anything to do with the heart, but it does cause a burning sensation in the vicinity of the chest. Normally, the acid secreted into the stomach remains in the stomach where it is needed to allow pepsin to do its job of digesting proteins. A long tube called the esophagus carries food from the mouth to the stomach. A sphincter, or valve, between the esophagus and stomach opens to allow swallowed food to enter the stomach and then closes to prevent stomach contents from backflowing into the esophagus. If this sphincter is weak or relaxes inappropriately, stomach contents flow into the esophagus. Because stomach contents are usually acidic, this causes the burning sensation known as heartburn. People who are prone to heartburn and suffer from it often may be diagnosed with GERD, which stands for gastroesophageal reflux disease.

GERD — as well as occasional heartburn — often can be improved by dietary and other lifestyle changes that decrease the amount and acidity of reflux from the stomach into the esophagus.

• Some foods and beverages seem to contribute to GERD, so these should be avoided. Problematic foods include chocolate, fatty foods, peppermint, coffee, and alcoholic beverages.
• Decreasing portion size and eating the last meal of the day at least a couple of hours before bedtime may reduce the risk of reflux occurring.
• Smoking tends to weaken the lower esophageal sphincter, so quitting the habit may help control reflux.
• GERD is often associated with being overweight. Losing weight often brings improvement.
• Some people are helped by sleeping with the head of the bed elevated. This allows gravity to help control the backflow of acids into the esophagus from the stomach.

If you have frequent heartburn and lifestyle changes don't help, you may need medication to control the condition. Over-the-counter (OTC) antacids may be all that you need to control the occasional heartburn attack. OTC medications are usually bases that neutralize stomach acids. They may also create bubbles that help block stomach contents from entering the esophagus. For some people, OTC medications are not enough, and prescription medications are instead required for the control of GERD. These prescription medications generally work by inhibiting acid secretion in the stomach.

Be sure to see a doctor if you can't control your heartburn, or you have it often. Untreated GERD not only interferes with quality of life, it may also lead to more serious complications, ranging from esophageal bleeding to esophageal cancer.

 

Created by CK-12/Adapted by Christine Miller

So Many Species!

The collage shows a single species in each of the six kingdoms into which all of Earth's living things are commonly classified. How many species are there in each kingdom? In a word: millions. A total of almost two million living species have already been identified, and new species are being discovered all the time. Scientists estimate that there may be as many as 30 million unique species alive on Earth today! Clearly, there is a tremendous variety of life on Earth.

What Is Biodiversity?

Biological diversity, or biodiversity, refers to all of the variety of life that exists on Earth. Biodiversity can be described and measured at three different levels: species diversity, genetic diversity, and ecosystem diversity.

• Species diversity refers to the number of different species in an ecosystem or on Earth as a whole. This is the most common way to measure biodiversity. Current estimates for Earth's total number of living species range from 5 to 30 million species.
• Genetic diversity refers to the variation in genes within all of these species.
• Ecosystem diversity refers to the variety of ecosystems on Earth. An ecosystem is a system formed by populations of many different species interacting with each other and their environment.

Biodiversity is most often measured by counting species, but what is a species? The answer to that question is not as straightforward as you might think. Formally, a species is defined as a group of actually or potentially interbreeding organisms. This means that members of the same species are similar enough to each other to produce fertile offspring together. By this definition of species, all human beings alive today belong to one species, Homo sapiens. All humans can potentially interbreed with each other, but not with members of any other species.

In the real world, it isn't always possible to make the observations necessary to determine whether or not different organisms can interbreed. For one thing, many species reproduce asexually, so individuals never interbreed — even with members of their own species. When studying extinct species represented by fossils, it is usually impossible to know if different organisms could interbreed. Keep in mind that 99 per cent of all species that have ever existed are now extinct! In practice, many biologists and virtually all paleontologists generally define species on the basis of morphology, rather than breeding behavior. Morphology refers to the form and structure of organisms. For classification purposes, it generally refers to relatively obvious physical traits. Typically, the more similar to one another different organisms appear, the greater the chance that they will be classified in the same species.

People have been trying to classify the tremendous diversity of life on Earth for more than two thousand years. The science of classifying organisms is called taxonomy. Classification is an important step in understanding the present diversity and past evolutionary history of life on Earth. It helps us make sense of the overwhelming diversity of living things.

Linnaean Classification

All modern classification systems have their roots in the Linnaean classification system, which was developed by Swedish botanist Carolus Linnaeus in the 1700s. He tried to classify all living things known in his time by grouping together organisms that s

hared obvious morphological traits, such as number of legs or shape of leaves. For his contribution, Linnaeus is known as the “father of taxonomy.”

The Linnaean system of classification consists of a hierarchy of groupings, called taxa (singular, taxon).  In the original system, taxa ranged from the kingdom to the species. The kingdom (ex. plant kingdom, animal kingdom) is the largest and most inclusive grouping. It consists of organisms that share just a few basic similarities. The species is the smallest and most exclusive grouping. Ideally, it consists of organisms that are similar enough to interbreed, as discussed above. Similar species are classified together in the same genus (plural, genera), then similar genera are classified together in the same family, and so on, all the way up to the kingdom.

A phrase to help you remember the order of the groupings is shown below.  The first letter of each word is the first letter of the level of classification.

Dad Keeps Pots Clean Or Family Gets Sick

Binomial Nomenclature

Perhaps the single greatest contribution Linnaeus made to science was his method of naming species. This method, called binomial nomenclature, gives each species a unique, two-word Latin name consisting of the genus name followed by a specific species identifier. An example is Homo sapiens, the two-word Latin name for humans. It literally means “wise human.” This is a reference to our big brains.

Why is having two names so important? It is similar to people having a first and a last name. You may know several people with the first name Michael, but adding Michael’s last name usually pins down exactly which Michael you mean. In the same way, having two names for a species helps to uniquely identify it.

Revisions in the Linnaean Classification

Linnaeus published his classification system in the 1700s. Since then, many new species have been discovered. Scientists can also now classify organisms on the basis of their biochemical and genetic similarities and differences, and not just their outward morphology. These changes have led to revisions in the original Linnaean system of classification.

A major change to the Linnaean system is the addition of a new taxon called the domain. The domain is a taxon that is larger and more inclusive than the kingdom, as shown in the figure above. Most biologists agree that there are three domains of life on Earth: Bacteria, Archaea, and Eukarya . Both the Bacteria and the Archaea domains consist of single-celled organisms that lack a nucleus. This means that their genetic material is not enclosed within a membrane inside the cell. The Eukarya domain, in contrast, consists of all organisms whose cells do have a nucleus, so that their genetic material is enclosed within a membrane inside the cell. The Eukarya domain is made up of both single-celled and multicellular organisms. This domain includes several kingdoms, including the animal, plant, fungus, and protist kingdoms.

Linnaeus classified organisms based on morphology. Basically, organisms were grouped together if they looked alike. After Darwin published his theory of evolution in the 1800s, scientists looked for a way to classify organisms that accounted for phylogeny. Phylogeny is the evolutionary history of a group of related organisms. It is represented by a phylogenetic tree, or some other tree-like diagram, like the one shown above to illustrate the three domains. A phylogenetic tree shows how closely related different groups of organisms are to one another.  Each branching point represents a common ancestor of the branching groups.

Attributions

Figure 2.4.1 (6 Kingdoms collage)

• Salmonella, by unknown/ NIAID on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
• Fern from pxhere, is used under a CC0 1.0 universal public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
• Photo [squirrel] , by Radoslaw Prekurat on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Blood Milk Mushroom by Hans on Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).
• Fungi by Ste Wright on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• EscherichiaColi NIAID [adapted], by Rocky Mountain Laboratories, ca:NIAID, ca:NIH on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 2.4.2

Biological classification, by Pengo [Peter Halasz] on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 2.4.3

The three domains of life and major groups within, by C. Miller, 2019, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2017, March 8). Classification. YouTube. https://www.youtube.com/watch?v=DVouQRAKxYo&feature=youtu.be

A-Z Animals. (2008, December 1). Animal classification. https://a-z-animals.com/reference/animal-classification/

TED-Ed. (2015, April 20). Why is biodiversity so important? - Kim Preshoff. YouTube. https://www.youtube.com/watch?v=GK_vRtHJZu4

Wikipedia contributors. (2020, June 21). Carl Linnaeus. Wikipedia. https://en.wikipedia.org/w/index.php?title=Carl_Linnaeus&oldid=963767022

A group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body.

 

Divide and Split

Can you guess what the colourful image in Figure 4.13.1 represents? It shows a eukaryotic cell during the process of cell division. In particular, the image shows the cell in a part of cell division called anaphase, where the DNA is being pulled to opposite ends of the cell. Normally, DNA is located in the nucleus of most human cells. The nucleus divides before the cell itself splits in two, and before the nucleus divides, the cell’s DNA is replicated (or copied). There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To answer that question, you first need to know more about DNA and the forms it takes.

Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. Only once a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a chromosome, like the one shown below.

Most cells in the human body have two pairs of 23 different chromosomes, for a total of 46 chromosomes. Cells that have two pairs of chromosomes are called diploid. Because DNA has already replicated when it coils into a chromosome, each chromosome actually consists of two identical structures called sister chromatids. Sister chromatids are joined together at a region called a centromere.

 

The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. This is shown in the figure below.

Mitosis actually occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase.

Prophase

The first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells. They help ensure that the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The spindle consists of fibres made of microtubules.

During metaphase, spindle fibres attach to the centromere of each pair of sister chromatids. As you can see in Figure 4.13.7, the sister chromatids line up at the equator (or center) of the cell. The spindle fibres ensure that sister chromatids will separate and go to different daughter cells when the cell divides.

Anaphase

During anaphase, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibres. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole. At the end of anaphase, each pole of the cell has a complete set of chromosomes.

Telophase

During telophase, the chromosomes begin to uncoil and form chromatin. This prepares the genetic material for directing the metabolic activities of the new cells. The spindle also breaks down, and new nuclear envelopes form.

Cytokinesis is the final stage of cell division. During cytokinesis, the cytoplasm splits in two and the cell divides, as shown below. In animal cells, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form. Thus, the goal of mitosis and cytokinesis is now complete, because one parent cell has given rise to two daughter cells. The daughter cells have the same chromosomes as the parent cell.

Acquired Immunodeficiency Syndrome - a chronic, potentially life-threatening condition caused by the human immunodeficiency virus (HIV). By damaging your immune system, HIV interferes with your body's ability to fight infection and disease.

A condition in which you don't have enough healthy red blood cells to carry adequate oxygen to the body's tissues resulting in symptoms including weakness and fatigue.

Jasmin discovered that her extreme fatigue, muscle pain, vision problems, and vomiting were due to problems in her mitochondria, like the damaged mitochondria shown in red in Figure 4.14.1. Mitochondria are small, membrane-bound organelles found in eukaryotic cells that provide energy for the cells of the body. They do this by carrying out the final two steps of aerobic cellular respiration: the Krebs cycle and electron transport. This is the major way that the human body breaks down the sugar glucose from food into a form of energy cells can use, namely the molecule ATP.

Because mitochondria provide energy for cells, you can understand why Jasmin was experiencing extreme fatigue, particularly after running. Her damaged mitochondria could not keep up with her need for energy, particularly after intense exercise, which requires a lot of additional energy. What is perhaps not so obvious are the reasons for her other symptoms, such as blurry vision, muscle spasms, and vomiting. All of the cells in the body require energy in order to function properly. Mitochondrial diseases can cause problems in mitochondria in any cell of the body, including muscle cells and cells of the nervous system, which includes the brain and nerves. The nervous system and muscles work together to control vision and digestive system functions, such as vomiting, so when they are not functioning properly, a variety of symptoms can emerge. This also explains why Jasmin’s niece, who has a similar mitochondrial disease, has symptoms related to brain function, such as seizures and learning disabilities. Our cells are microscopic, and mitochondria are even tinier — but they are essential for the proper functioning of our bodies. When they are damaged, serious health effects can occur.

 

One seemingly confusing aspect of mitochondrial diseases is that the type of symptoms, severity of symptoms, and age of onset can vary wildly between people — even within the same family! In Jasmin’s case, she did not notice symptoms until adulthood, while her niece had more severe symptoms starting at a much younger age. This makes sense when you know more about how mitochondrial diseases work.

Inherited mitochondrial diseases can be due to damage in either the DNA in the nucleus of cells or in the DNA in the mitochondria themselves. Recall that mitochondria are thought to have evolved from prokaryotic organisms that were once free-living, but were then infected or engulfed by larger cells. One of the pieces of evidence that supports this endosymbiotic theory is that mitochondria have their own, separate DNA. When the mitochondrial DNA is damaged (or mutated) it can result in some types of mitochondrial diseases. However, these mutations do not typically affect all of the mitochondria in a cell. During cell division, organelles such as mitochondria are replicated and passed down to the new daughter cells. If some of the mitochondria are damaged, and others are not, the daughter cells can have different amounts of damaged mitochondria. This helps explain the wide range of symptoms in people with mitochondrial diseases — even ones in the same family — because different cells in their bodies are affected in varying degrees. Jasmin’s niece was affected strongly and her symptoms were noticed early, while Jasmin’s symptoms were more mild and did not become apparent until adulthood.

There is still much more that needs to be discovered about the different types of mitochondrial diseases. But by learning about cells, their organelles, how they obtain energy, and how they divide, you should now have a better understanding of the biology behind these diseases.

Apply your understanding of cells to your own life. Can you think of other diseases that affect cellular structures or functions. Do they affect people you know? Since your entire body is made of cells, when cells are damaged or not functioning properly, it can cause a wide variety of health problems.

Attributions

Figure 4.14.1

Mitochondrial Disease muscle sample by Nephron is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 4.14.2

Aunt and Niece by Tatiana Rodriguez on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Reference

Wikipedia contributors. (2020, June 6). Mitochondrial disease. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Mitochondrial_disease&oldid=961126371

Created by CK-12 Foundation/Adapted by Christine Miller

Fashion Statement

This colourful hairstyle makes quite a fashion statement. Many people spend a lot of time and money on their hair, even if they don’t have an exceptional hairstyle like this one. Besides its display value, hair actually has important physiological functions.

What is Hair?

Hair is a filament that grows from a hair follicle in the dermis of the skin. It consists mainly of tightly packed, keratin-filled cells called keratinocytes. The human body is covered with hair follicles, with the exception of a few areas, including the mucous membranes, lips, palms of the hands, and soles of the feet.

Structure of Hair

The part of the hair located within the follicle is called the hair root. The root is the only living part of the hair. The part of the hair that is visible above the surface of the skin is the hair shaft. The shaft of the hair has no biochemical activity and is considered dead.

Follicle and Root

Hair growth begins inside a follicle (see Figure 10.5.2 below). Each hair follicle contains stem cells that can keep dividing, which allows hair to grow. The stem cells can also regrow a new hair after one falls out. Another structure associated with a hair follicle is a sebaceous gland that produces oily sebum. The sebum lubricates and helps to waterproof the hair. A tiny arrector pili muscle is also attached to the follicle. When it contracts, the follicle moves, and the hair in the follicle stands up.

 

Shaft

The hair shaft is a hard filament that may grow very long. Hair normally grows in length by about half an inch a month. In cross-section, a hair shaft can be divided into three zones, called the cuticle, cortex, and medulla.

• The cuticle (or outer coat) is the outermost zone of the hair shaft. It consists of several layers of flat, thin keratinocytes that overlap one another like shingles on a roof. This arrangement helps the cuticle repel water. The cuticle is also covered with a layer of lipids, just one molecule thick, which increases its ability to repel water. This is the zone of the hair shaft that is visible to the eye.
• The cortex is the middle zone of the hair shaft, and it is also the widest part. The cortex is highly structured and organized, consisting of keratin bundles in rod-like structures. These structures give hair its mechanical strength. The cortex also contains melanin, which gives hair its colour.
• The medulla is the innermost zone of the hair shaft. This is a small, disorganized, and more open area at the center of the hair shaft. The medulla is not always present. When it is present, it contains highly pigmented cells full of keratin.

Characteristics of Hair

Two visible characteristics of hair are its colour and texture. In adult males, the extent of balding is another visible characteristic. All three characteristics are genetically controlled.

Hair Colour

All natural hair colours are the result of melanin, which is produced in hair follicles and packed into granules in the hair. Two forms of melanin are found in human hair: eumelanin and pheomelanin. Eumelanin is the dominant pigment in brown hair and black hair, and pheomelanin is the dominant pigment in red hair. Blond hair results when you have only a small amount of melanin in the hair. Gray and white hair occur when melanin production slows down, and eventually stops.

Figure 10.5.3 Variation in hair colouration. Which types of melanin are present for each hair colour shown?

Hair Texture

Hair exists in a variety of textures. The main aspects of hair texture are the curl pattern, thickness, and consistency.

• The shape of the hair follicle determines the shape of the hair shaft. The shape of the hair shaft, in turn, determines the curl pattern of the hair. Round hair shafts produce straight hair. Hair shafts that are oval or have other shapes produce wavy or curly hair .
• The size of the hair follicle determines the thickness of hair. Thicker hair has greater volume than thinner hair.
• The consistency of hair is determined by the hair follicle volume and the condition of the hair shaft. The consistency of hair is generally classified as fine, medium, or coarse. Fine hair has the smallest circumference, and coarse hair has the largest circumference. Medium hair falls in between these two extremes. Coarse hair also has a more open cuticle than thin or medium hair does, which causes it to be more porous.
  

  

Functions of Hair

In humans, one function of head hair is to provide insulation and help the head retain heat. Head hair also protects the skin on the head from damage by UV light.

The function of hair in other locations on the body is debated. One idea is that body hair helps keep us warm in cold weather. When the body is too cold, arrector pili muscles contract and cause hairs to stand up (shown in Figure 10.5.5), trapping a layer of warm air above the epidermis. However, this is more effective in mammals that have thick hair or fur than it is in relatively hairless human beings.

Human hair has an important sensory function, as well. Sensory receptors in the hair follicles can sense when the hair moves, whether it moves because of a breeze, or because of the touch of a physical object. The receptors may also provide sensory awareness of the presence of parasites on the skin.

 

Some hairs, such as the eyelashes, are especially sensitive to the presence of potentially harmful matter. The eyelashes grow at the edge of the eyelid and can sense when dirt, dust, or another potentially harmful object is too close to the eye. The eye reflexively closes as a result of this sensation. The eyebrows also provide some protection to the eyes. They protect the eyes from dirt, sweat, and rain. In addition, the eyebrows play a key role in nonverbal communication (see Figure 10.5.6). They help express emotions such as sadness, anger, surprise, and excitement.

Hair in Human Evolution

Among mammals, humans are nearly unique in having undergone significant loss of body hair during their evolution. Humans are also unlike most other mammals in having curly hair as one variation in hair texture. Even non-human primates (see Figure 10.5.7) all have straight hair. This suggests that curly hair evolved at some point during human evolution.

Loss of Body Hair

One hypothesis for the loss of body hair in the human lineage is that it would have facilitated cooling of the body by the evaporation of sweat. Humans also evolved far more sweat glands than other mammals, which is consistent with this hypothesis, because sweat evaporates more quickly from less hairy skin. Another hypothesis for human hair loss is that it would have led to fewer parasites on the skin. This might have been especially important when humans started living together in larger, more crowded social groups.

These hypotheses may explain why we lost body hair, but they can’t explain why we didn’t also lose head hair and hair in the pubic region and armpits. It is possible that head hair was retained because it protected the scalp from UV light. As our bipedal ancestors walked on the open savannas of equatorial Africa, the skin on the head would have been an area exposed to the most direct rays of sunlight in an upright hominid. Pubic and armpit hair may have been retained because they served as signs of sexual maturity, which would have been important for successful mating and reproduction.

Evolution of Curly Hair

Greater protection from UV light has also been posited as a possible selective agent favoring the evolution of curly hair. Researchers have found that straight hair allows more light to pass into the body through the hair shaft via the follicle than does curly hair. In this way, human hair is like a fibre optic cable. It allows light to pass through easily when it is straight, but it impedes the passage of light when it is kinked or coiled. This is indirect evidence that UV light may have been a selective agent leading to the evolution of curly hair.

Social and Cultural Significance of Hair

Hair has great social significance for human beings. Body hair is an indicator of biological sex, because hair distribution is sexually dimorphic. Adult males are generally hairier than adult females, and facial hair in particular is a notable secondary male sex characteristic. Hair may also be an indicator of age. White hair is a sign of older age in both males and females, and male pattern baldness is a sign of older age in males. In addition, hair colour and texture can be a sign of ethnic ancestry.

Hair also has great cultural significance. Hairstyle and colour may be an indicator of social group membership and for better or worse can be associated with specific stereotypes. Head shaving has been used in many times and places as a punishment, especially for women. On the other hand, in some cultures, cutting off one’s hair symbolizes liberation from one’s past. In other cultures, it is a sign of mourning. There are also many religious-based practices involving hair. For example, the majority of Muslim women hide their hair with a headscarf. Sikh men grow their hair long and cover it with a turban. Amish men (like the one pictured in Figure 10.5.8) grow facial hair only after they marry — but just a beard, and not a mustache.

Unfortunately, sometimes hairstyle, colour and characteristics are used to apply stereotypes, particularly with respect to women.  "Blonde jokes" are a good example of how negative stereotypes are maintained despite having no actual truth behind them.  Many stereotypes related to hair are hidden, even from persons perpetrating the stereotype.  Often a hairstyle is judged by another as having ties to gender, sexuality, worldview and/or socioeconomic status; even when these inferences are woefully inaccurate.  It is important to be aware of our own biases and determine if these biases are appropriate - take a look at the collage in Figure 10.5.9.  What are your initial reactions?  Are these reactions founded in fact?  Do you harbor an unfair bias?

Figure 10.5.9 What are your biases?  Are they fair?

 

Attributions

Figure 10.5.1

Hair by jessica-dabrowski-TETR8YLSqt4 [photo] by Jessica Dabrowski on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.5.2

Blausen_0438_HairFollicleAnatomy_02 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 10.5.3

• Standing tall by Ilaya Raja on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Blond-haired woman smiling by Carlos Lindner on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Smith Mountain Lake redhead by Chris Ross Harris on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Through the look of experience by Laura Margarita Cedeño Peralta on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.5.4

Curly hair by chris-benson-clvEami9RN4 [photo] by Chris Benson on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.5.5

1024px-PilioerectionAnimation by AnthonyCaccese on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 10.5.6

Pout by alexander-dummer-Em8I8Z_DwA4 [photo] by Alexander Dummer on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.5.7

Cotton_top_tamarin_monkey._(12046035746) by Bernard Spragg. NZ, from Christchurch, New Zealand on Wikimedia Commons is used under a CC0 1.0 Universal
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).

Figure 10.5.8

Amish hairstyle by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under  • Terms of Use • Attribution
Figure 10.5.9

• Rainbow Hair Bubble Man by Behrouz Jafarnezhad on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Pink hair in Atlanta, United States by Tammie Allen on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Magdalena 2 by Valerie Elash on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Perfect Style by Daria Volkova on Unsplash is used under the Unsplash License (https://unsplash.com/license)
• Stay Classy by Fayiz Musthafa on Unsplash is used under the Unsplash License (https://unsplash.com/license)
• Take your time by Jan Tinneberg on Unsplash is used under the Unsplash License (https://unsplash.com/license)

References

Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Brainard, J/ CK-12 Foundation. (2016). Figure 7 This style of facial hair is adopted by most Amish men after they marry [digital image]. In CK-12 College Human Biology (Section 12.5) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/12.5/

Sony Pictures Animation. (2019, December 5). Hair love | Oscar®-winning short film (Full) | Sony Pictures Animation. YouTube. https://www.youtube.com/watch?v=kNw8V_Fkw28

TED-Ed. (2015, August 25). Why do some people go bald? – Sarthak Sinha. YouTube. https://www.youtube.com/watch?v=8diYLhl8bWU

TEDx Talks. (2015, February 4). Why do we care about hair | Naomi Abigail | TEDxBaDinh. YouTube. https://www.youtube.com/watch?v=hDW5e3NR1Cw

Created by CK-12 Foundation/Adapted by Christine Miller

A tiny sperm from dad breaks through the surface of a huge egg from mom. Voilà! In nine months, a new son or daughter will be born. Like most other multicellular organisms, human beings reproduce sexually. In human sexual reproduction, males produce sperm and females produce eggs, and a new offspring forms when a sperm unites with an egg. How do sperm and eggs form? And how do they arrive together at the right place and time so they can unite to form a new offspring? These are functions of the reproductive system.

The reproductive system is the human organ system responsible for the production and fertilization of gametes (sperm or eggs) and, in females, the carrying of a fetus. Both male and female reproductive systems have organs called gonads that produce gametes. A gamete is a haploid cell that combines with another haploid gamete during fertilization, forming a single diploid cell called a zygote. Besides producing gametes, the gonads also produce sex hormones. Sex hormones are endocrine hormones that control the development of sex organs before birth, sexual maturation at puberty, and reproduction once sexual maturation has occurred. Other reproductive system organs have various functions, such as maturing gametes, delivering gametes to the site of fertilization, and providing an environment for the development and growth of an offspring.

The reproductive system is the only human organ system that is significantly different between males and females. Embryonic structures that will develop into the reproductive system start out the same in males and females, but by birth, the reproductive systems have differentiated. How does this happen?

Sex Differentiation

Starting around the seventh week after conception in genetically male (XY) embryos, a gene called SRY on the Y chromosome (shown in Figure 18.2.2) initiates the production of multiple proteins. These proteins cause undifferentiated gonadal tissue to develop into male gonads (testes). The male gonads then secrete hormones — including the male sex hormone testosterone — that trigger other changes in the developing offspring (now called a fetus), causing it to develop a complete male reproductive system. Without a Y chromosome, an embryo will develop female gonads (ovaries) that will produce the female sex hormone estrogen. Estrogen, in turn, will lead to the formation of the other organs of a normal female reproductive system.

Homologous Structures

Undifferentiated embryonic tissues develop into different structures in male and female fetuses. Structures that arise from the same tissues in males and females are called homologous structures. The male testes and female ovaries, for example, are homologous structures that develop from the undifferentiated gonads of the embryo. Likewise, the male penis and female clitoris are homologous structures that develop from the same embryonic tissues.

Sex Hormones and Maturation

Male and female reproductive systems are different at birth, but they are immature and incapable of producing gametes or sex hormones. Maturation of the reproductive system occurs during puberty, when hormones from the hypothalamus and pituitary gland stimulate the testes or ovaries to start producing sex hormones again. The main sex hormones are testosterone in males and estrogen in females. Sex hormones, in turn, lead to the growth and maturation of the reproductive organs, rapid body growth, and the development of secondary sex characteristics. Secondary sex characteristics are traits that are different in mature males and females, but are not directly involved in reproduction. They include facial hair in males and breasts in females.

The main structures of the male reproductive system are external to the body and illustrated in Figure 18.2.3. The two testes (singular, testis) hang between the thighs in a sac of skin called the scrotum. The testes produce both sperm and testosterone. Resting atop each testis is a coiled structure called the epididymis (plural, epididymes). The function of the epididymes is to mature and store sperm. The penis is a tubular organ that contains the urethra and has the ability to stiffen during sexual arousal. Sperm passes out of the body through the urethra during a sexual climax (orgasm). This release of sperm is called ejaculation.

In addition to these organs, the male reproductive system consists of several ducts and glands that are internal to the body. The ducts, which include the vas deferens (also called the ductus deferens), transport sperm from the epididymis to the urethra. The glands, which include the prostate gland and seminal vesicles, produce fluids that become part of semen. Semen is the fluid that carries sperm through the urethra and out of the body. It contains substances that control pH and provide sperm with nutrients for energy.

The main structures of the female reproductive system are internal to the body and shown in the following figure. They include the paired ovaries, which are small, ovoid structures that produce ova and secrete estrogen. The two oviducts (sometimes called Fallopian tubes or uterine tubes) start near the ovaries and end at the uterus. Their function is to transport ova from the ovaries to the uterus. If an egg is fertilized, it usually occurs while it is traveling through an oviduct. The uterus is a pear-shaped muscular organ that functions to carry a fetus until birth. It can expand greatly to accommodate a growing fetus, and its muscular walls can contract forcefully during labour to push the baby out of the uterus and into the vagina. The vagina is a tubular tract connecting the uterus to the outside of the body. The vagina is where sperm are usually deposited during sexual intercourse and ejaculation. The vagina is also called the birth canal because a baby travels through the vagina to leave the body during birth.

The external structures of the female reproductive system are referred to collectively as the vulva. They include the clitoris, which is homologous to the male penis. They also include two pairs of labia (singular, labium), which surround and protect the openings of the urethra and vagina.

Attributions

Figure 18.2.1

Sperm-egg by Unknown author on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).  

Figure 18.2.2

Y Chromosome by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license. 

Figure 18.2.3

3D_Medical_Animation_Vas_Deferens by https://www.scientificanimations.com on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 18.2.4

Blausen_0399_FemaleReproSystem_01 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References 

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Reactions. (2015, June 8). Hormones and gender transition. YouTube. https://www.youtube.com/watch?v=l5knvmy1Z3s&feature=youtu.be

TED-Ed. (2012, April 23). Sex determination: More complicated than you thought. YouTube. https://www.youtube.com/watch?v=kMWxuF9YW38&feature=youtu.be

TED-Ed. (2017, April 24). The evolution of animal genitalia - Menno Schilthuizen. YouTube. https://www.youtube.com/watch?v=vcPJkz-D5II&feature=youtu.be

 

 

Who's Who?

Figure 3.7.1 Identical twins show clearly the importance of genes in making us who we are. Genes would not be possible without nucleic acids.

Nucleic acids are the class of biochemical compounds that includes DNA and RNA. These molecules are built of small monomers called nucleotides. Many nucleotides bind together to form a chain called a polynucleotide. The nucleic acid DNA (deoxyribonucleic acid) consists of two polynucleotide chains or strands. Thus, DNA is sometimes called double-stranded. The nucleic acid RNA (ribonucleic acid) consists of just one polynucleotide chain or strand, so RNA is sometimes called single-stranded.

Each nucleotide consists of three smaller molecules:

1. A sugar molecule (the sugar deoxyribose in DNA and the sugar ribose in RNA)
2. A phosphate group
3. A nitrogen base

The nitrogen bases in a nucleic acid stick out from the backbone. There are four different nitrogen bases: cytosine, adenine, guanine, and either thymine (in DNA) or uracil (in RNA). In DNA, bonds form between bases on the two nucleotide chains and hold the chains together. Each type of base binds with just one other type of base: cytosine always binds with guanine, and adenine always binds with thymine. These pairs of bases are called complementary base  pairs.

The binding of complementary bases causes DNA molecules automatically to take their well-known double helix shape, which is shown in the animation in Figure 3.7.3. A double helix is like a spiral staircase. It forms naturally and is very strong, making the two polynucleotide chains difficult to break apart.

DNA Molecule. Hydrogen bonds between complementary bases help form the double helix of a DNA molecule. The letters A, T, G, and C stand for the bases adenine, thymine, guanine, and cytosine. The sequence of these four bases in DNA is a code that carries instructions for making proteins. Shown is a representation of how the double helix folds into a chromosome.

 

DNA makes up genes, and the sequence of bases in DNA makes up the genetic code. Between “starts” and “stops,” the code carries instructions for the correct sequence of amino acids in a protein. RNA uses the information in DNA to assemble the correct amino acids and help make the protein. The information in DNA is passed from parent cells to daughter cells whenever cells divide, and it is also passed from parents to offspring when organisms reproduce. This is how inherited characteristics are passed from one generation to the next.

ATP is Energy

There is one type of specialized nucleic acid that exists only as a monomer.  It stands apart from the other nucleic acids because it does not code for, or help create, proteins.   This molecule is ATP, which stands for adenosine triphosphate.  It consists of a sugar, adenosine, and three phosphate groups.  It's primary role is as the basic energy currency in the cell.  The way ATP works is all based on the phosphates.  As shown in Figure 3.7.4, a large amount of energy is stored in the bond between the second and third phosphate group.  When this bond is broken, it functions as an exothermic reaction and this energy can be used to power other processes taking place in the cell.

 

Reference

TED-Ed. (2012, November 26). DNA: The book of you - Joe Hanson. YouTube, 2012. https://www.youtube.com/watch?v=aeAL6xThfL8&feature=youtu.be

 

 

This young woman has naturally red hair (Figure 5.3.1). Why is her hair red instead of some other colour? In general, what gives her the specific traits she has? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.

Today, it is commonly known that DNA is the genetic material that is passed from parents to offspring and determines our traits. For a long time, scientists knew such molecules existed — that is, they were aware that genetic information is contained within biochemical molecules. What they didn’t know was which specific molecules play this role. In fact, for many decades, scientists thought that proteinsno post were the molecules that contain genetic information.

Determining that DNA is the genetic material was an important milestone in biology. It took many scientists undertaking creative experiments over several decades to show with certainty that DNA is the molecule that determines the traits of organisms. This research began in the early part of the 20th century.

Griffith's Experiments with Mice

One of the first important discoveries was made in the 1920s by an American scientist named Frederick Griffith.  Griffith was studying mice and two different strains of a bacterium, called R (rough)-strain and S (smooth)-strain. He injected the two bacterial strains into mice. The S-strain was virulent and killed the mice, whereas the R-strain was not virulent and did not kill the mice. You can see these details in Figure 5.3.2. Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the dead bacteria did not harm the mice. However, when the dead S-strain bacteria were mixed with live R-strain bacteria and injected, the mice died.

Based on his observations, Griffith deduced that something in the dead S-strain was transferred to the previously harmless R-strain, making the R-strain deadly. What was this "something?" What type of substance could change the characteristics of the organism that received it?

Avery and His Colleagues Make a Major Contribution

In the early 1940s, a team of scientists led by Canadian-American Oswald Avery tried to answer the question raised by Griffith’s research results. First, they inactivated various substances in the S-strain bacteria. Then they killed the S-strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind that the R-strain bacteria normally did not harm the mice.) When they inactivated proteinsno post, the R-strain was deadly to the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain proteins, the R-strain was changed (or transformed) into the deadly strain. However, when the researchers inactivated DNA in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA — and not protein — is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material.

Hershey and Chase Confirm the Results

The conclusion that DNA is the genetic material was not widely accepted until it was confirmed by additional research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. Instead, they are basically DNA (or RNA) inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then, it uses the cell’s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in DNA viruses. This allowed them to identify which molecule the viruses inserted into bacterial cells. DNA was the molecule they identified. This confirmed that DNA is the genetic material.

Other important discoveries about DNA were made in the mid-1900s by Erwin Chargaff. He studied DNA from many different species and was especially interested in the four different nitrogen bases of DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). Chargaff found that concentrations of the four bases differed between species. Within any given species, however, the concentration of adenine was always the same as the concentration of thymine, and the concentration of guanine was always the same as the concentration of cytosine. These observations came to be known as Chargaff’s rules. The significance of the rules would not be revealed until the double-helix structure of DNA was discovered.

After DNA was shown to be the genetic material, scientists wanted to learn more about its structure and function. James Watson and Francis Crick are usually given credit for discovering that DNA has a double helix shape, as shown in Figure 5.3.3. In fact, Watson and Crick's discovery of the double helix depended heavily on the prior work of Rosalind Franklin and other scientists, who had used X-rays to learn more about DNA’s structure. Unfortunately, Franklin and these others have not always been given credit for their important contributions to the discovery of the double helix.

The double helix shape of DNA, along with Chargaff’s rules, led to a better understanding of DNA. As a nucleic acid, DNA is made from nucleotide monomers. Long chains of nucleotides form polynucleotides, and the DNA double helix consists of two polynucleotide chains. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of the four bases (adenine, cytosine, guanine, or thymine). The sugar and phosphate molecules in adjacent nucleotides bond together and form the "backbone" of each polynucleotide chain.

Scientists concluded that bonds between the bases hold together the two polynucleotide chains of DNA. Moreover, adenine always bonds with thymine, and cytosine always bonds with guanine. That's why these pairs of bases are called complementary base pairs.  Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If adenine were to bond with guanine, as well as thymine, for example, the distance between the two DNA chains would vary. When a one-ring molecule (like thymine) always bonds with a two-ring molecule (like adenine), however, the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix. The bonded base pairs (A-T and G-C) stick into the middle of the double helix, forming the "steps" of the spiral staircase.

 

Attributions

Figure 5.3.1

Redhead [photo] by Hichem Dahmani on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.3.2

Griffith’s mice by Mariana Ruiz Villarreal [LadyofHats] for CK-12 Foundation is used under a
CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under  • Terms of Use • Attribution

Figure 5.3.3

DNA_Overview by Michael Ströck [mstroeck] on Wikimedia Commons is used under a CC BY SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

References

Brainard, J/ CK-12. (2012). Concentration. In Physical Science [website]. CK12.org. https://www.ck12.org/c/physical-science/concentration/?referrer=crossref

OpenMind. (2017, September 11). The discovery of the structure of DNA. YouTube. https://www.youtube.com/watch?v=V6bKn34nSbk&feature=youtu.be

SciShow. (2013, July 9). Rosalind Franklin: Great minds. YouTube. https://www.youtube.com/watch?v=JiME-W58KpU&feature=youtu.be

Wikipedia contributors. (2020, June 27). Alfred Hershey. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Alfred_Hershey&oldid=964789559

Wikipedia contributors. (2020, June 5). Erwin Chargaff. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Erwin_Chargaff&oldid=960942873

Wikipedia contributors. (2020, June 29). Francis Crick. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Francis_Crick&oldid=965135362

Wikipedia contributors. (2020, July 6). Frederick Griffith. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Frederick_Griffith&oldid=966352134

Wikipedia contributors. (2020, July 5). James Watson. In Wikipedia. https://en.wikipedia.org/w/index.php?title=James_Watson&oldid=966111944

Wikipedia contributors. (2020, March 31). Martha Chase. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Martha_Chase&oldid=948408219

Wikipedia contributors. (2020, July 2). Oswald Avery. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Oswald_Avery&oldid=965632585

Wikipedia contributors. (2020, June 30). Rosalind Franklin. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Rosalind_Franklin&oldid=965334881

 

Image shows a pictomicrograph of a protozoan parasite of the Giardia lamblia species. It is roughly cone-shaped, with several flagella trailing from the narrow end of it.