The Lone Ranger
01-07-2007, 06:12 AM
An Introduction to Human Anatomy and Physiology
Chapter Five: Cellular (Aerobic) Respiration
Introduction: As you no-doubt recall, some organisms, known as autotrophs, can manufacture their own food. Most do this by using the molecule chlorophyll to absorb solar energy, which is then used to manufacture glucose from carbon dioxide and water. This is photosynthesis; almost all plants and algae, and many bacteria are capable of manufacturing food in this way.
Other organisms are heterotrophs, because they cannot manufacture their own food. Instead, they must eat other organisms.
As you also know, organisms need food for two reasons. Food provides the raw materials organisms need in order to build body tissues, and it provides the energy organisms need for growth, maintenance, and repair of body tissues. Mostly, this energy is provided by the chemical breakdown of carbohydrate molecules; as molecular bonds are broken, the energy they store is released, and ATP molecules are used to transport the energy to wherever it’s needed.
The process in which organisms break down high-energy molecules (glucose, in particular) to produce energy that is stored and transported by ATP molecules takes place inside cells and is known as cellular respiration. Because the complete breakdown of glucose into CO2 and H2O requires oxygen to proceed, cellular respiration is often referred to as aerobic respiration – though strictly speaking, aerobic respiration is the second half of cellular respiration.
Respiration, by the way, is one of those confusing words that’s used in several different ways. Most people think of “respiration” as the moving of air in and out of the lungs, but strictly speaking, that’s “breathing” – or more properly, ventilation. “Respiration” is the metabolic breakdown of organic molecules for energy. If oxygen is used in the breakdown process (as it usually is), then it’s aerobic respiration; otherwise, it’s anaerobic respiration.
Cellular Respiration: A Brief Overview:
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Cellular Respiration: (The Simplified Version)
The first stage, glycolysis, is anaerobic and takes place in the cytoplasm of the cell. The remaining stages require oxygen and take place inside of mitochondria.
The Stages of Cellular Respiration:
Glycolysis: Cellular respiration begins in the cytoplasm of a cell, when a glucose molecule is split into two molecules of pyruvate. (Strictly speaking, pyruvate is a negatively-charged ion, rather than a molecule.) This process is known as glycolysis, and since it does not require oxygen, it is a form of anaerobic respiration. (Many bacteria are capable of respiring anaerobically only, and are killed by exposure to oxygen.) The energy released during glycolysis is used by the cell to generate 2 ATP molecules by adding a single phosphate group to each of 2 ADP molecules. (Covalent chemical bonds, as you recall, store energy that is released when those bonds are broken.)
This splitting of glucose during glycolysis also produces high-energy hydrogen ions (H+). A pyruvate (C3H3O3-) ion is negatively-charged because it’s “missing” a hydrogen ion, and so it has an “extra” electron that isn’t paired with a proton. In addition, you may have noticed that two pyruvate ions, even if you take into account the “missing” hydrogen ions, don’t have as many hydrogen atoms as did the original glucose molecule. The uncharged hydrogen atoms that are liberated when glucose is split into pyruvate ultimately combine with oxygen to form water molecules.
Meanwhile, there’s still that H+ to account for. The highly energetic ions would surely do damage if left to themselves. They are gotten rid of by combining them with the coenzyme NAD (Nicotinamide Adenine Dinucleotide), to form NADH.
So, glycolysis produces 2 molecules of ATP that store and transport energy, plus two molecules of NADH, which get rid of the energetic and therefore potentially dangerous H+ ions.
Of course, the fact that the H+ ions are so energetic means that it is possible to extract energy from them, and the cell does exactly that. Energy stored in the NADH molecules is used to generate an additional 4 molecules of ATP. (Cardiac muscle cells and liver cells are more efficient at this, and can generate not 4 but 6 ATP molecules in this way.)
So, at the end of this first stage in cellular respiration, the cell will typically have generated a total of 6 molecules of ATP from the splitting of a glucose molecule – 2 ATP molecules from glycolysis and 4 more from processing of electrons temporarily stored in molecules of NADH. If the 2 pyruvates produced by the splitting of the original glucose molecule are to be broken down any further, however, oxygen is required. Otherwise, the pyruvate is processed into less-dangerous substances and excreted.
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Glycolysis: (The Simplified Version)
A single 6-carbon glucose molecule is ultimately broken down into two 3-carbon pyruvate ions. Note that the net production of ATP molecules is 2, because while 4 ATP molecules are produced by splitting of the glucose into pyruvate, 2 ATP molecules were used to generate the energy necessary to split the glucose molecule in the first place.
Complete breakdown of pyruvate into CO2 and H2O can only take place inside the mitochondria, so after glycolysis has been completed, the pyruvate is normally transported into mitochondria for further breakdown. A few cells in the body lack mitochondria (notably, red blood cells), and so are capable only of anaerobic respiration. (Perhaps that’s one of the reasons that RBCs don’t live very long.)
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A Typical Mitochondrion - Slightly Larger than Life-Size:
A single cell can contain several thousand mitochondria, and cells with especially high energy demands generally have the largest number of mitochondria.
During heavy exercise, skeletal muscle cells can find themselves in an anaerobic (lacking oxygen) condition, because their metabolic rates are so high that they use up all the oxygen that can be delivered to them by the blood. They can survive and continue to function for a time by producing energy through glycolysis. Because the end-products of glycolysis are poisonous to cells, however, there’s a sharp limit to how long skeletal muscles can function anaerobically. (One of the major effects of repeated exercise is that the circulatory system becomes more efficient at delivering oxygen to skeletal muscles, and so they can function for longer periods of time before running out of oxygen.)
The TCA Cycle: When oxygen is available, the 2 pyruvate ions produced during glycolysis are transported into a mitochondrion. Inside the mitochondrion, the pyruvates are completely broken down in the TCA Cycle – that is, the Tricarboxylic Acid Cycle. The TCA Cycle produces 2 more ATP molecules, plus 8 NADH and 2 FADH2 molecules. (FADH2, or flavin adenine dinucleotide, like NADH is a coenzyme.)
The TCA Cycle is also known as the Citric Acid Cycle or the Krebs Cycle. An acquaintance of mine has the last name of Krebs, and she is a quite proficient unicyclist. She refers to her unicycle as “The Krebs Cycle” – biologist humor at its finest.
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[B]The TCA Cycle:
The cycle begins when a 2-carbon acetyl-CoA molecule binds to a 4-carbon oxaloacetate molecule to form a 6-carbon molecule of citrate (citric acid). Two of the carbon atoms are stripped off the citrate, one at a time, and combined with oxygen to form CO2, which is excreted. Ultimately, the stripping of 2 carbons from citrate regenerates oxaloacetate, which can then combine with another molecule of acetyl-CoA. (The molecules named in blue but not shown are the enzymes that drive the chemical reactions.)
The Electron Transport System: The NADH and FADH2 molecules feed the energetic electrons they’ve captured into electron transport chains, which strip energy from the electrons. Ultimately, the energetic electrons and H+ ions are gotten rid of by combining them with oxygen to form water (H2O). The electron transport system produces an additional 28 ATP molecules.
The Advantages of Aerobic Respiration: Typically, anaerobic respiration can produce only 2 molecules of ATP for each glucose molecule broken down. Additionally, it produces highly toxic waste products. Even under the best of conditions, anaerobic respiration produces only 6 molecules of ATP for each molecule of glucose consumed.
By contrast, aerobic respiration can produce up to 38 molecules of ATP for each molecule of glucose consumed. Not only that, but its waste products – water and carbon dioxide – are much less toxic. So, organisms that can respire aerobically can generate far more energy per glucose molecule than can anaerobic organisms. It’s unsurprising, therefore, that virtually every organism on the planet is capable of aerobic respiration.
Cellular Respiration: A Slightly More Detailed Look: Glycolysis:
The Problems With Anaerobic Respiration:Some organisms are incapable of aerobic respiration, and to them, oxygen is a deadly poison. (Some disinfectants, such as hydrogen peroxide for instance, work by releasing oxygen that kills anaerobic bacteria.) What’s more, some of the cells in our body find themselves incapable of respiring aerobically from time to time. The problems with anaerobic respiration are twofold: first, it doesn’t generate nearly as much energy as does aerobic respiration, and second, the breakdown products of glucose can be quite toxic. Anaerobically-respiring cells therefore face real challenges.
Fermentation:
The process by which the pyruvate produced during anaerobic respiration is converted to other substances that are either less toxic or more easily excreted is known as fermentation. In yeasts, the pyruvate is converted to CO2 and ethyl alcohol (ethanol) (C2H6O).
In bread dough, yeast cells (yeast is a single-celled fungus) break down sugars for energy, and the CO2 they release causes the bread to “rise.” The ethanol evaporates as the bread bakes.
Even though ethanol is poisonous in sufficient concentration, some people seem to like to drink it. So, they place yeast, sugar (most fruits contain the sugars glucose and/or fructose), and other substances into closed containers and allow the yeast to ferment the sugars and produce ethanol. Of course, once the ethanol concentration rises to a certain level, it poisons the yeast, so if you want your alcohol to be more concentrated, you must distill it.
In animals, pyruvate is converted not to ethanol and CO2, but to lactic acid (also known as lactate). Lactic acid is toxic as well, so if it were allowed to build up in tissues, it would eventually prove fatal.
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[B]Anaerobic Respiration in Animal Cells
In animal cells, when oxygen is unavailable, the breakdown of a glucose molecule into pyruvate ultimately results in the production of 2 molecules of toxic lactic acid (lactate).
If skeletal muscle cells run out of oxygen during heavy exercise, they can continue to function for a time by generating energy anaerobically. But this produces lactic acid and the cells run up an “oxygen debt.”
The lactic acid can be combined with oxygen and then sent on to mitochondria for breakdown – but only if oxygen is available, of course. Otherwise, the lactic acid simply builds up in muscle tissues. Eventually, it would reach toxic levels, except that the body won’t allow this to happen. When lactic acid levels reach a certain point, skeletal muscle cells stop functioning, which prevents lactic acid poisoning during heavy exercise – otherwise, you could kill yourself by, for example, trying to run a marathon.
Once the skeletal muscles stop contracting (either because you voluntarily stopped or because of excessive lactic acid buildup), they begin to “repay” their “oxygen debt.” The “oxygen debt” is the amount of extra oxygen that must be delivered to the muscle tissues by the blood in order to oxidize the lactic acid that has built up so that it can then be sent into the mitochondria for complete breakdown. Once the oxygen debt is repaid, the muscles can function normally again.
Aerobic Respiration: Steps in Aerobic Respiration:
Step 1: Pyruvate molecules are transported to the inner compartments of mitochondria. After it enters a mitochondrion, each pyruvate molecule is stripped of one of its carbon atoms. This carbon atom is combined with an oxygen molecule to produce CO2, which is released.
Step 2: The remaining 2-carbon compound is known as acetate, and it combines with a coenzyme known as coenzyme-A to form acetyl-CoA. (Acetyl-CoA can also be produced from fats and from some amino acids. Carbohydrates aren’t the only molecules that can be aerobically broken down for energy.)
Step 3: Each acetyl-CoA molecule enters the TCA Cycle. At the beginning of the TCA Cycle, an acetyl-CoA combines with the 4-carbon molecule oxaloacetate to form a 6-carbon molecule of citric acid.
Step 4: Each citric acid molecule takes two “turns” through the TCA Cycle. During each “turn,” a carbon atom is stripped off the citric acid and combined with an O2 molecule to produce CO2, which is released. So, after 2 complete turns of the TCA Cycle, the original oxaloacetate is regenerated, and it’s free to combine with another molecule of acetyl-CoA so that the process can start over. The original glucose molecule has now been completely broken down.
Step 5: The breaking of all these chemical bonds has produced lots of H+ ions and energetic electrons. The electrons are now sent through an electron transport system. An ETS is a series of chemicals that receives energetic electrons and removes energy from them in the process. The first chemical in the ETS receives an electron, removes energy from it, then passes it on to the next chemical in the series, which removes more energy from the electron before passing it to the next chemical – and so on. Finally, the “spent” electrons are combined with oxygen atoms to produce OH- ions.
The energy taken from the electrons is used to actively transport H+ ions across mitochondrial membranes and into the outer compartments of the mitochondrion, against the concentration gradient. Meanwhile, the OH- ions are actively transported into the inner compartments (the matrix) of the mitochondrion.
Step 6: The mitochondrion now has large amounts of positively-charged H+ in its outer compartments and equal amounts of negatively-charged OH- ions in its matrix. This is a highly unstable situation. When channel proteins in the mitochondrial membranes open, the H+ and OH- ions rush across the membranes and combine to form water (H2O). The ions move both because they’re attracted to each other by their opposite charges and because they’re moving from high concentration to low concentration. This process is known as chemiosmosis.
You’ll remember that anything which is moving has kinetic energy, and as the ions move across the mitochondrial membranes, proteins in the membranes “steal” some of their energy to make ATP.
[B]Cellular Respiration: Alternate Sources of Energy: Catabolizing Molecules Other Than Carbohydrates for Energy: Man does not live by glucose alone. If, for some reason, adequate amounts of glucose aren’t available, body cells can break down other organic molecules for energy. Usually, this means first converting those molecules to glucose, but that’s not always the case.
Lipid Catabolism: The chemical bonds in lipid molecules typically contain about twice the energy of those in carbohydrate molecules of the same size, which makes fats and oils excellent energy-storage molecules. If glucose supplies are low, triglycerides can be broken down for energy.
A triglyceride, as you recall, consists of 3 fatty acid chains bound to a molecule of glycerol. When triglycerides are catabolized for energy, the first step is the breaking of the triglyceride into its components through hydrolysis. The glycerol is then converted into pyruvate and sent into the TCA Cycle.
The fatty acid chains are broken down into 2-carbon fragments in a series of chemical reactions known as beta-oxidation. This occurs inside mitochondria, and beta-oxidation ultimately produces acetyl-CoA (which can then take part in the TCA Cycle), plus NADH and FADH2. Ultimately, the breakdown of a single 18-carbon fatty acid chain yields up to 144 molecules of ATP – considerably more than does the maximum of 114 ATP molecules that can be produced during the complete breakdown of three 6-carbon glucose molecules.
Chemical breakdown of lipids may produce lots of energy, but it’s slower than is breakdown of glucose. This is apparently why cells usually catabolize glucose for energy, rather than lipids. Skeletal muscle cells well illustrate the tradeoff between speed of energy production and quantity of energy produced per molecule catabolized – when they’re at rest, skeletal muscle cells catabolize lipids, but when actively contracting and so rapidly consuming energy, they switch to catabolizing glucose.
Protein Metabolism and Amino Acid Catabolism: Proteins, as you recall, are made up of long chains of amino acids. The first step in protein metabolism is the splitting of protein molecules into their component amino acids. If insufficient glucose or lipids are available for the body’s energy demands, some amino acids can be converted to pyruvate and sent into the TCA Cycle.
Amino acids don’t provide as much energy when broken down as do lipids; they provide about the same amount of energy as do carbohydrates, on average. Because of all the molecular processing that’s necessary, protein catabolism isn’t as efficient as is catabolism of either carbohydrates or lipids for energy, and protein catabolism is pretty-much a “last-ditch” effort on your body’s part to produce energy when carbohydrate and lipid levels are very low.
The first step in amino acid catabolism is the removal of the amino group (see Chapter 2) from the amino acid. This process requires a coenzyme known as pyridoxine, which is derived from Vitamin B6. The amino group is removed in one of two ways: transamination or deamination.
Transamination basically strips the amino acid of its amino group, attaching the amino group to a keto acid. A keto acid is chemically very similar to an amino acid, so transamination produces a “new” amino acid that, with slight modification, can be used in protein synthesis. Meanwhile, the remains of the original amino acid can enter the TCA Cycle and be broken down for energy. Cells of the liver, skeletal muscles, heart, lungs, kidneys and brain – all cells that are particularly active in protein synthesis – tend to convert amino acids for energy generation through transamination.
Deamination involves removal of the amino group from an amino acid. The remains of the amino acid can then sent into the TCA Cycle, while the amino group is converted to an ammonia molecule (NH3) or an ammonium ion (NH4+). Ammonia is highly toxic, even in low concentrations, and the liver converts it to urea, which is much less toxic. The urea is removed from the blood by the kidneys, diluted with water, and excreted as urine.
Glucose Availability and Energy Demands: Regulation of Blood Glucose Levels: When carbohydrates in your food are digested in the intestine, they’re either broken down into glucose or into similar simple sugars that can ultimately be converted to glucose. Glucose and other simple sugars produced in carbohydrate digestion are absorbed from the intestine into the blood, and glucose circulating in the blood is absorbed by cells as needed for aerobic respiration.
As you might imagine, blood glucose levels tend to spike right after a meal, but too much glucose in the blood will cause the blood pressure to rise dangerously, since the blood will be hypertonic to body tissues and so will absorb water from them. (This is why people with Type 2 diabetes tend to develop very high blood pressures, which causes damage to tissues.) So it’s important for the body to regulate blood glucose levels fairly precisely. If there’s more glucose in the blood than is needed, the hormone insulin is released by beta cells in the pancreas. Insulin causes cells of the liver and skeletal muscles to absorb glucose from the blood and convert it to glycogen for storage.
If blood glucose levels fall too low, alpha cells in the pancreas release the hormone glucagon, which causes liver and skeletal muscle cells to reconvert glycogen to glucose and release it into the blood. By regulating insulin/glucagon levels in the blood, the pancreas controls blood sugar levels.
Starvation and Energy Generation:
If you haven’t eaten in awhile and blood glucose levels begin to fall, the liver and skeletal muscles convert glycogen to glucose to compensate. But if you go for some time without eating – perhaps a day or two – glycogen reserves will be largely exhausted. In that case, lipids will be released into the blood from fat-storing adipose tissue. But there’s only so much body fat available, and if you go long-enough without eating, body fat levels will become sufficiently low that the body – “in desperation” – will begin to break down proteins and metabolize amino acids for energy. This can be a serious problem.
Some athletes have such low levels of body fat that their bodies may begin breaking down proteins for energy during hard exercise sessions. Since proteins are especially concentrated in muscle cells, the irony is that hard exercise coupled with inadequate amounts of carbohydrates and fats in the diet can cause the loss of muscle tissue – including in the heart. This is one of the reasons that some athletes are prone to developing heart problems and even dying of heart attacks at surprisingly young ages.
[B]Some Concluding Thoughts: The process of cellular respiration is immensely complicated, and it’s not possible to describe it in any real detail in an article of this scope. Heck, I once saw a large poster on a biochemist’s wall covered in tiny print that showed some of the known chemical reactions in cellular respiration – and that was by no means a complete listing.
Still, the overall process is quite straightforward: the body breaks down the chemical bonds of glucose (and occasionally, other molecules) in order to release the energy stored in those chemical bonds. That energy is then used to drive cellular processes. Since glucose is C6H12O6 and because the process of aerobic respiration uses oxygen (O2) for the complete breakdown of glucose, the ultimate chemical products of cellular respiration are water (H2O) and carbon dioxide (CO2).
And that’s why you inhale oxygen and you exhale carbon dioxide (and water). Just as an aside, some desert-dwelling animals are so good at conserving the water generated during cellular respiration that they can literally go their entire lives without taking a drink. Too bad we can’t do that, eh?
Chapter Five: Cellular (Aerobic) Respiration
Introduction: As you no-doubt recall, some organisms, known as autotrophs, can manufacture their own food. Most do this by using the molecule chlorophyll to absorb solar energy, which is then used to manufacture glucose from carbon dioxide and water. This is photosynthesis; almost all plants and algae, and many bacteria are capable of manufacturing food in this way.
Other organisms are heterotrophs, because they cannot manufacture their own food. Instead, they must eat other organisms.
As you also know, organisms need food for two reasons. Food provides the raw materials organisms need in order to build body tissues, and it provides the energy organisms need for growth, maintenance, and repair of body tissues. Mostly, this energy is provided by the chemical breakdown of carbohydrate molecules; as molecular bonds are broken, the energy they store is released, and ATP molecules are used to transport the energy to wherever it’s needed.
The process in which organisms break down high-energy molecules (glucose, in particular) to produce energy that is stored and transported by ATP molecules takes place inside cells and is known as cellular respiration. Because the complete breakdown of glucose into CO2 and H2O requires oxygen to proceed, cellular respiration is often referred to as aerobic respiration – though strictly speaking, aerobic respiration is the second half of cellular respiration.
Respiration, by the way, is one of those confusing words that’s used in several different ways. Most people think of “respiration” as the moving of air in and out of the lungs, but strictly speaking, that’s “breathing” – or more properly, ventilation. “Respiration” is the metabolic breakdown of organic molecules for energy. If oxygen is used in the breakdown process (as it usually is), then it’s aerobic respiration; otherwise, it’s anaerobic respiration.
Cellular Respiration: A Brief Overview:
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Cellular Respiration: (The Simplified Version)
The first stage, glycolysis, is anaerobic and takes place in the cytoplasm of the cell. The remaining stages require oxygen and take place inside of mitochondria.
The Stages of Cellular Respiration:
Glycolysis: Cellular respiration begins in the cytoplasm of a cell, when a glucose molecule is split into two molecules of pyruvate. (Strictly speaking, pyruvate is a negatively-charged ion, rather than a molecule.) This process is known as glycolysis, and since it does not require oxygen, it is a form of anaerobic respiration. (Many bacteria are capable of respiring anaerobically only, and are killed by exposure to oxygen.) The energy released during glycolysis is used by the cell to generate 2 ATP molecules by adding a single phosphate group to each of 2 ADP molecules. (Covalent chemical bonds, as you recall, store energy that is released when those bonds are broken.)
This splitting of glucose during glycolysis also produces high-energy hydrogen ions (H+). A pyruvate (C3H3O3-) ion is negatively-charged because it’s “missing” a hydrogen ion, and so it has an “extra” electron that isn’t paired with a proton. In addition, you may have noticed that two pyruvate ions, even if you take into account the “missing” hydrogen ions, don’t have as many hydrogen atoms as did the original glucose molecule. The uncharged hydrogen atoms that are liberated when glucose is split into pyruvate ultimately combine with oxygen to form water molecules.
Meanwhile, there’s still that H+ to account for. The highly energetic ions would surely do damage if left to themselves. They are gotten rid of by combining them with the coenzyme NAD (Nicotinamide Adenine Dinucleotide), to form NADH.
So, glycolysis produces 2 molecules of ATP that store and transport energy, plus two molecules of NADH, which get rid of the energetic and therefore potentially dangerous H+ ions.
Of course, the fact that the H+ ions are so energetic means that it is possible to extract energy from them, and the cell does exactly that. Energy stored in the NADH molecules is used to generate an additional 4 molecules of ATP. (Cardiac muscle cells and liver cells are more efficient at this, and can generate not 4 but 6 ATP molecules in this way.)
So, at the end of this first stage in cellular respiration, the cell will typically have generated a total of 6 molecules of ATP from the splitting of a glucose molecule – 2 ATP molecules from glycolysis and 4 more from processing of electrons temporarily stored in molecules of NADH. If the 2 pyruvates produced by the splitting of the original glucose molecule are to be broken down any further, however, oxygen is required. Otherwise, the pyruvate is processed into less-dangerous substances and excreted.
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Glycolysis: (The Simplified Version)
A single 6-carbon glucose molecule is ultimately broken down into two 3-carbon pyruvate ions. Note that the net production of ATP molecules is 2, because while 4 ATP molecules are produced by splitting of the glucose into pyruvate, 2 ATP molecules were used to generate the energy necessary to split the glucose molecule in the first place.
Complete breakdown of pyruvate into CO2 and H2O can only take place inside the mitochondria, so after glycolysis has been completed, the pyruvate is normally transported into mitochondria for further breakdown. A few cells in the body lack mitochondria (notably, red blood cells), and so are capable only of anaerobic respiration. (Perhaps that’s one of the reasons that RBCs don’t live very long.)
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A Typical Mitochondrion - Slightly Larger than Life-Size:
A single cell can contain several thousand mitochondria, and cells with especially high energy demands generally have the largest number of mitochondria.
During heavy exercise, skeletal muscle cells can find themselves in an anaerobic (lacking oxygen) condition, because their metabolic rates are so high that they use up all the oxygen that can be delivered to them by the blood. They can survive and continue to function for a time by producing energy through glycolysis. Because the end-products of glycolysis are poisonous to cells, however, there’s a sharp limit to how long skeletal muscles can function anaerobically. (One of the major effects of repeated exercise is that the circulatory system becomes more efficient at delivering oxygen to skeletal muscles, and so they can function for longer periods of time before running out of oxygen.)
The TCA Cycle: When oxygen is available, the 2 pyruvate ions produced during glycolysis are transported into a mitochondrion. Inside the mitochondrion, the pyruvates are completely broken down in the TCA Cycle – that is, the Tricarboxylic Acid Cycle. The TCA Cycle produces 2 more ATP molecules, plus 8 NADH and 2 FADH2 molecules. (FADH2, or flavin adenine dinucleotide, like NADH is a coenzyme.)
The TCA Cycle is also known as the Citric Acid Cycle or the Krebs Cycle. An acquaintance of mine has the last name of Krebs, and she is a quite proficient unicyclist. She refers to her unicycle as “The Krebs Cycle” – biologist humor at its finest.
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[B]The TCA Cycle:
The cycle begins when a 2-carbon acetyl-CoA molecule binds to a 4-carbon oxaloacetate molecule to form a 6-carbon molecule of citrate (citric acid). Two of the carbon atoms are stripped off the citrate, one at a time, and combined with oxygen to form CO2, which is excreted. Ultimately, the stripping of 2 carbons from citrate regenerates oxaloacetate, which can then combine with another molecule of acetyl-CoA. (The molecules named in blue but not shown are the enzymes that drive the chemical reactions.)
The Electron Transport System: The NADH and FADH2 molecules feed the energetic electrons they’ve captured into electron transport chains, which strip energy from the electrons. Ultimately, the energetic electrons and H+ ions are gotten rid of by combining them with oxygen to form water (H2O). The electron transport system produces an additional 28 ATP molecules.
The Advantages of Aerobic Respiration: Typically, anaerobic respiration can produce only 2 molecules of ATP for each glucose molecule broken down. Additionally, it produces highly toxic waste products. Even under the best of conditions, anaerobic respiration produces only 6 molecules of ATP for each molecule of glucose consumed.
By contrast, aerobic respiration can produce up to 38 molecules of ATP for each molecule of glucose consumed. Not only that, but its waste products – water and carbon dioxide – are much less toxic. So, organisms that can respire aerobically can generate far more energy per glucose molecule than can anaerobic organisms. It’s unsurprising, therefore, that virtually every organism on the planet is capable of aerobic respiration.
Cellular Respiration: A Slightly More Detailed Look: Glycolysis:
The Problems With Anaerobic Respiration:Some organisms are incapable of aerobic respiration, and to them, oxygen is a deadly poison. (Some disinfectants, such as hydrogen peroxide for instance, work by releasing oxygen that kills anaerobic bacteria.) What’s more, some of the cells in our body find themselves incapable of respiring aerobically from time to time. The problems with anaerobic respiration are twofold: first, it doesn’t generate nearly as much energy as does aerobic respiration, and second, the breakdown products of glucose can be quite toxic. Anaerobically-respiring cells therefore face real challenges.
Fermentation:
The process by which the pyruvate produced during anaerobic respiration is converted to other substances that are either less toxic or more easily excreted is known as fermentation. In yeasts, the pyruvate is converted to CO2 and ethyl alcohol (ethanol) (C2H6O).
In bread dough, yeast cells (yeast is a single-celled fungus) break down sugars for energy, and the CO2 they release causes the bread to “rise.” The ethanol evaporates as the bread bakes.
Even though ethanol is poisonous in sufficient concentration, some people seem to like to drink it. So, they place yeast, sugar (most fruits contain the sugars glucose and/or fructose), and other substances into closed containers and allow the yeast to ferment the sugars and produce ethanol. Of course, once the ethanol concentration rises to a certain level, it poisons the yeast, so if you want your alcohol to be more concentrated, you must distill it.
In animals, pyruvate is converted not to ethanol and CO2, but to lactic acid (also known as lactate). Lactic acid is toxic as well, so if it were allowed to build up in tissues, it would eventually prove fatal.
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[B]Anaerobic Respiration in Animal Cells
In animal cells, when oxygen is unavailable, the breakdown of a glucose molecule into pyruvate ultimately results in the production of 2 molecules of toxic lactic acid (lactate).
If skeletal muscle cells run out of oxygen during heavy exercise, they can continue to function for a time by generating energy anaerobically. But this produces lactic acid and the cells run up an “oxygen debt.”
The lactic acid can be combined with oxygen and then sent on to mitochondria for breakdown – but only if oxygen is available, of course. Otherwise, the lactic acid simply builds up in muscle tissues. Eventually, it would reach toxic levels, except that the body won’t allow this to happen. When lactic acid levels reach a certain point, skeletal muscle cells stop functioning, which prevents lactic acid poisoning during heavy exercise – otherwise, you could kill yourself by, for example, trying to run a marathon.
Once the skeletal muscles stop contracting (either because you voluntarily stopped or because of excessive lactic acid buildup), they begin to “repay” their “oxygen debt.” The “oxygen debt” is the amount of extra oxygen that must be delivered to the muscle tissues by the blood in order to oxidize the lactic acid that has built up so that it can then be sent into the mitochondria for complete breakdown. Once the oxygen debt is repaid, the muscles can function normally again.
Aerobic Respiration: Steps in Aerobic Respiration:
Step 1: Pyruvate molecules are transported to the inner compartments of mitochondria. After it enters a mitochondrion, each pyruvate molecule is stripped of one of its carbon atoms. This carbon atom is combined with an oxygen molecule to produce CO2, which is released.
Step 2: The remaining 2-carbon compound is known as acetate, and it combines with a coenzyme known as coenzyme-A to form acetyl-CoA. (Acetyl-CoA can also be produced from fats and from some amino acids. Carbohydrates aren’t the only molecules that can be aerobically broken down for energy.)
Step 3: Each acetyl-CoA molecule enters the TCA Cycle. At the beginning of the TCA Cycle, an acetyl-CoA combines with the 4-carbon molecule oxaloacetate to form a 6-carbon molecule of citric acid.
Step 4: Each citric acid molecule takes two “turns” through the TCA Cycle. During each “turn,” a carbon atom is stripped off the citric acid and combined with an O2 molecule to produce CO2, which is released. So, after 2 complete turns of the TCA Cycle, the original oxaloacetate is regenerated, and it’s free to combine with another molecule of acetyl-CoA so that the process can start over. The original glucose molecule has now been completely broken down.
Step 5: The breaking of all these chemical bonds has produced lots of H+ ions and energetic electrons. The electrons are now sent through an electron transport system. An ETS is a series of chemicals that receives energetic electrons and removes energy from them in the process. The first chemical in the ETS receives an electron, removes energy from it, then passes it on to the next chemical in the series, which removes more energy from the electron before passing it to the next chemical – and so on. Finally, the “spent” electrons are combined with oxygen atoms to produce OH- ions.
The energy taken from the electrons is used to actively transport H+ ions across mitochondrial membranes and into the outer compartments of the mitochondrion, against the concentration gradient. Meanwhile, the OH- ions are actively transported into the inner compartments (the matrix) of the mitochondrion.
Step 6: The mitochondrion now has large amounts of positively-charged H+ in its outer compartments and equal amounts of negatively-charged OH- ions in its matrix. This is a highly unstable situation. When channel proteins in the mitochondrial membranes open, the H+ and OH- ions rush across the membranes and combine to form water (H2O). The ions move both because they’re attracted to each other by their opposite charges and because they’re moving from high concentration to low concentration. This process is known as chemiosmosis.
You’ll remember that anything which is moving has kinetic energy, and as the ions move across the mitochondrial membranes, proteins in the membranes “steal” some of their energy to make ATP.
[B]Cellular Respiration: Alternate Sources of Energy: Catabolizing Molecules Other Than Carbohydrates for Energy: Man does not live by glucose alone. If, for some reason, adequate amounts of glucose aren’t available, body cells can break down other organic molecules for energy. Usually, this means first converting those molecules to glucose, but that’s not always the case.
Lipid Catabolism: The chemical bonds in lipid molecules typically contain about twice the energy of those in carbohydrate molecules of the same size, which makes fats and oils excellent energy-storage molecules. If glucose supplies are low, triglycerides can be broken down for energy.
A triglyceride, as you recall, consists of 3 fatty acid chains bound to a molecule of glycerol. When triglycerides are catabolized for energy, the first step is the breaking of the triglyceride into its components through hydrolysis. The glycerol is then converted into pyruvate and sent into the TCA Cycle.
The fatty acid chains are broken down into 2-carbon fragments in a series of chemical reactions known as beta-oxidation. This occurs inside mitochondria, and beta-oxidation ultimately produces acetyl-CoA (which can then take part in the TCA Cycle), plus NADH and FADH2. Ultimately, the breakdown of a single 18-carbon fatty acid chain yields up to 144 molecules of ATP – considerably more than does the maximum of 114 ATP molecules that can be produced during the complete breakdown of three 6-carbon glucose molecules.
Chemical breakdown of lipids may produce lots of energy, but it’s slower than is breakdown of glucose. This is apparently why cells usually catabolize glucose for energy, rather than lipids. Skeletal muscle cells well illustrate the tradeoff between speed of energy production and quantity of energy produced per molecule catabolized – when they’re at rest, skeletal muscle cells catabolize lipids, but when actively contracting and so rapidly consuming energy, they switch to catabolizing glucose.
Protein Metabolism and Amino Acid Catabolism: Proteins, as you recall, are made up of long chains of amino acids. The first step in protein metabolism is the splitting of protein molecules into their component amino acids. If insufficient glucose or lipids are available for the body’s energy demands, some amino acids can be converted to pyruvate and sent into the TCA Cycle.
Amino acids don’t provide as much energy when broken down as do lipids; they provide about the same amount of energy as do carbohydrates, on average. Because of all the molecular processing that’s necessary, protein catabolism isn’t as efficient as is catabolism of either carbohydrates or lipids for energy, and protein catabolism is pretty-much a “last-ditch” effort on your body’s part to produce energy when carbohydrate and lipid levels are very low.
The first step in amino acid catabolism is the removal of the amino group (see Chapter 2) from the amino acid. This process requires a coenzyme known as pyridoxine, which is derived from Vitamin B6. The amino group is removed in one of two ways: transamination or deamination.
Transamination basically strips the amino acid of its amino group, attaching the amino group to a keto acid. A keto acid is chemically very similar to an amino acid, so transamination produces a “new” amino acid that, with slight modification, can be used in protein synthesis. Meanwhile, the remains of the original amino acid can enter the TCA Cycle and be broken down for energy. Cells of the liver, skeletal muscles, heart, lungs, kidneys and brain – all cells that are particularly active in protein synthesis – tend to convert amino acids for energy generation through transamination.
Deamination involves removal of the amino group from an amino acid. The remains of the amino acid can then sent into the TCA Cycle, while the amino group is converted to an ammonia molecule (NH3) or an ammonium ion (NH4+). Ammonia is highly toxic, even in low concentrations, and the liver converts it to urea, which is much less toxic. The urea is removed from the blood by the kidneys, diluted with water, and excreted as urine.
Glucose Availability and Energy Demands: Regulation of Blood Glucose Levels: When carbohydrates in your food are digested in the intestine, they’re either broken down into glucose or into similar simple sugars that can ultimately be converted to glucose. Glucose and other simple sugars produced in carbohydrate digestion are absorbed from the intestine into the blood, and glucose circulating in the blood is absorbed by cells as needed for aerobic respiration.
As you might imagine, blood glucose levels tend to spike right after a meal, but too much glucose in the blood will cause the blood pressure to rise dangerously, since the blood will be hypertonic to body tissues and so will absorb water from them. (This is why people with Type 2 diabetes tend to develop very high blood pressures, which causes damage to tissues.) So it’s important for the body to regulate blood glucose levels fairly precisely. If there’s more glucose in the blood than is needed, the hormone insulin is released by beta cells in the pancreas. Insulin causes cells of the liver and skeletal muscles to absorb glucose from the blood and convert it to glycogen for storage.
If blood glucose levels fall too low, alpha cells in the pancreas release the hormone glucagon, which causes liver and skeletal muscle cells to reconvert glycogen to glucose and release it into the blood. By regulating insulin/glucagon levels in the blood, the pancreas controls blood sugar levels.
Starvation and Energy Generation:
If you haven’t eaten in awhile and blood glucose levels begin to fall, the liver and skeletal muscles convert glycogen to glucose to compensate. But if you go for some time without eating – perhaps a day or two – glycogen reserves will be largely exhausted. In that case, lipids will be released into the blood from fat-storing adipose tissue. But there’s only so much body fat available, and if you go long-enough without eating, body fat levels will become sufficiently low that the body – “in desperation” – will begin to break down proteins and metabolize amino acids for energy. This can be a serious problem.
Some athletes have such low levels of body fat that their bodies may begin breaking down proteins for energy during hard exercise sessions. Since proteins are especially concentrated in muscle cells, the irony is that hard exercise coupled with inadequate amounts of carbohydrates and fats in the diet can cause the loss of muscle tissue – including in the heart. This is one of the reasons that some athletes are prone to developing heart problems and even dying of heart attacks at surprisingly young ages.
[B]Some Concluding Thoughts: The process of cellular respiration is immensely complicated, and it’s not possible to describe it in any real detail in an article of this scope. Heck, I once saw a large poster on a biochemist’s wall covered in tiny print that showed some of the known chemical reactions in cellular respiration – and that was by no means a complete listing.
Still, the overall process is quite straightforward: the body breaks down the chemical bonds of glucose (and occasionally, other molecules) in order to release the energy stored in those chemical bonds. That energy is then used to drive cellular processes. Since glucose is C6H12O6 and because the process of aerobic respiration uses oxygen (O2) for the complete breakdown of glucose, the ultimate chemical products of cellular respiration are water (H2O) and carbon dioxide (CO2).
And that’s why you inhale oxygen and you exhale carbon dioxide (and water). Just as an aside, some desert-dwelling animals are so good at conserving the water generated during cellular respiration that they can literally go their entire lives without taking a drink. Too bad we can’t do that, eh?