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Chapter 5: capturing and releasing energy. Part 4. Fermentation. Fermentation is an anaerobic pathway that can release energy from carbohydrates. It occurs in the absence of oxygen, thus anaerobic.
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Fermentation • Fermentation is an anaerobic pathway that can release energy from carbohydrates. • It occurs in the absence of oxygen, thus anaerobic. • Bacteria and single-celled protists that live under anaerobic conditions such as those in the human gut, in sea sediments, mud, sewage treatment plants, etc. use this pathway. • Most of these anaerobic cells cannot tolerate oxygen and die when exposed to it. • Other single-celled organisms, such as yeast, can perform either fermentation or aerobic respiration. • Animal muscle cells can also use both pathways. • Generally, however, multicellular organisms cannot rely on this pathway alone to produce the ATP needed for survival since this process yields only the 2 ATP from glycolysis.
fermentation • Both aerobic respiration and fermentation begin with the same reaction, glycolysis. • After glycolysis, fermentation yields two pyruvate molecules, two NADH, and 2 ATP, just like in aerobic respiration. • However, after this, fermentation continues to occur in the cytoplasm, while aerobic respiration occurs in the mitochondrion. • Also, in fermentation, pyruvate is not cleaved to produce carbon dioxide like it is aerobic respiration. • In addition, electrons don’t flow through an electron transport chain during fermentation and this is the reason why only 2 ATP form. • Instead electrons are moved away from the the NADH to regenerate NAD+ so that glycolysis can continue to happen over and over (because glycolysis requires NAD+ to continue).
Alcoholic fermentation • During this kind of fermentation, the pyruvate from glycolysis is converted to ethyl alcohol (or ethanol). • This happens when the 3-carbon pyruvate is cleaved into a 2-carbon molecule called acetaldehyde and the carbon that is cut off leaves as carbon dioxide. • This process is taken advantage of when bakers use yeast to make bread. • The carbon dioxide released by the yeast as they break down the carbohydrates in the dough during alcoholic fermentation is what causes cakes and breads to rise. • This process is also used to produce wine. • Crushed grapes are left in vats with large populations of yeast cells, which convert the sugars in the grapes to ethanol. • However, once the concentration of alcohol in the vat reaches 12%, it kills the yeast. • Therefore, wines naturally will never have more than 12% alcohol.
Lactic acid fermentation • In this kind of fermentation, the electrons/H+ carried by NADH are transferred directly to pyruvate. • This converts 3-carbon pyruvate to 3-carbon lactate (lactic acid). • It also converts NADH back to NAD+, which is needed for glycolysis to continue. • A special bacterium used in milk performs lactic acid fermentation, producing buttermilk, cheese, and yogurt. • Some yeast that do this reaction are used to preserve pickles and sauerkraut, among other foods.
Lactic acid fermentation • Animal skeletal muscles contain two types of fibers: slow fibers and fast fibers. • These fibers differ in how they make ATP. • Slow fibers have many mitochondria and use aerobic respiration. • They are used by your muscles during extended periods of activity, such as a long run or swim. • They also have an abundance of myoglobin, an oxygen storing protein much like the hemoglobin in your blood which causes them to appear red and allows them to do aerobic respiration.
Lactic acid fermentation • Fast muscle fibers contain few mitochondria and no myoglobin, so they cannot carry out aerobic respiration and appear white instead of red. • They make most of their ATP using lactic acid fermentation. • This pathway makes ATP quickly but not for very long, since the lactic acid actually causes the muscle to cramp and fatigue quickly. • Therefore, these fibers are used for quick, strenuous activities such as sprinting or weightlifting, rather than sustained activities. • This is the main reason chickens can’t fly very far; their breast and wing muscle contain almost all white, fast fibers. • Chickens usually walk or run with their legs. • This is because their leg muscles contain mostly slow, red fibers. • This is why chicken breast and wing are white and the leg and thigh are dark.
Lactic acid fermentation • Unlike chickens, most human muscles contain a mixture of both fast and slow fibers, but the ratio of fast to slow varies in different muscles and in different individuals. • For example, great sprinters have a higher proportion of fast fibers, while great marathon runners have a higher proportion of slow fibers.
The Fate of glucose • When a cell takes in more than enough glucose, the rate of ATP production increases dramatically. • If this ATP is not used quickly, its concentration increases in the cytoplasm. • This high concentration of ATP causes glucose to be diverted away from glycolysis, to stop respiration and ATP formation. • Instead, glucose is diverted to another pathway, one that makes glycogen, which is a polysaccharide used for longer term storage of energy. • Also, if you eat too many carbohydrates, your blood level of glucose gets too high and the Acetyl C0-A that is used during the Kreb’s cycle is diverted away toward a pathway that makes fatty acids. • This is why excess carbohydrates in your diet end up as fat.
The fate of glucose • Hormones control how the glucose in your diet is used. • When the amount of glucose in your bloodstream rises, your pancreas releases a hormone called insulin to help your cells to take up that glucose to be used for energy production or storage as glycogen (a long-term storage carbohydrate) or fat. • When the amount of glucose in your bloodstream declines, the pancreas releases a different hormone called glucagon. • Glucagon initiates the conversion of stored carbohydrates, such as glycogen, to glucose, which is a carbohydrate that can be used very quickly by a cell for energy. • Glycogen makes up about 1% of an average adult’s total stored energy, which is about the equivalent of 2 cups of pasta. • If you don’t eat regularly, you will deplete your body’s glycogen stores in less than twelve hours.
Energy from fats • While glycogen makes up only 1% of the stored energy reserves in an adult, 78% of that reserve if made up of fat and 21% is made up of protein. • How does the body tap into these reserves in order to retrieve their stored energy? • Special fats called triglycerides are the main fat storage molecule in your body. • Triglycerides consist of a glycerol molecule with three fatty acids attached. • The glycerol molecules can be converted to an intermediate of glycolysis by liver enzymes. • The fatty acids can be cut by enzymes into fragments, which can then be converted to Acetyl Co-A so that they can be used in the Kreb’s cycle. • Fats provide more energy per molecule than carbohydrates and this is why, during prolonged exercise, fatty acid breakdown provides about half of the ATP that is needed by muscle, liver, and kidney cells.
Energy from fats • Diets that are extremely low in carbohydrates force the body to use fats for energy. • The breakdown of fats produces ketones, which cells can use for energy instead of glucose. • The state produced in the body as a result of the use of ketones for energy is associated with starvation. • It causes the level of LDL (“bad” fats) to increase in the blood and can damage the kidneys and liver.
Energy from proteins • Enzymes in your digestive system can break proteins down into their amino acid subunits. • These amino acids are then further broken down by removing their amino group, which becomes ammonia and is eliminated from the body as urine. • The carbon portion of the protein is then split to form Acetyl Co-A, pyruvate, or other intermediates used in the Kreb’s cycle.
homework • Compare and Contrast alcoholic fermentation with lactic acid fermentation. • Explain why chicken breast and wing are white, while their leg and thigh are dark. • Both fats and proteins can enter what reaction of respiration after being broken down?
