Anaerobic Glycolysis: Generating ATP without Oxygen
Anaerobic Glycolysis:
Generating ATP without
Oxygen
Up to this point we have been
describing aerobic cellular respiration.
We will now consider how animals
generate ATP without oxygen, that is,
anaerobically.
Under anaerobic conditions, glucose and other 6-carbon sugars are first broken down stepwise to a pair of 3- carbon pyruvic acid molecules, yielding two molecules of ATP and four atoms of hydrogen (four reducing equivalents, represented by 2 NADH+ H+). In the absence of molecular oxygen, further oxidation of pyruvic acid cannot occur because the Krebs cycle and electron transport chain cannot operate and cannot, therefore, provide a mechanism for reoxidizing the NADH produced in glycolysis. The problem is neatly solved in most animal cells by reducing pyruvic acid to lactic acid (Figure 4-16). Pyruvic acid becomes the final electron acceptor and lactic acid the end product of anaerobic glycolysis. This frees the hydrogen-bound carrier to recycle and pick up more H+. In alcoholic fermentation (as in yeast, for example) the steps are identical to glycolysis down to pyruvic acid. One of its carbons is then released as carbon dioxide, and the resulting 2-carbon compound is reduced to ethanol, thus regenerating the NAD.
Anaerobic glycolysis is only oneeighteenth as efficient as complete oxidation of glucose to carbon dioxide and water, but its key virtue is that it provides some high-energy phosphate in situations in which oxygen is absent or in short supply. Many microorganisms live in places where oxygen is severely depleted, such as waterlogged soil, in mud of lake or sea bottom, or within a decaying carcass. Vertebrate skeletal muscle may rely heavily on glycolysis during short bursts of activity when contraction is so rapid and powerful that oxygen delivery to tissues is not sufficient to supply energy demands by oxidative phosphorylation alone. At such times an animal has no choice but to supplement oxidative phosphorylation with anaerobic glycolysis. Intense activity is followed by a period of increased oxygen consumption as lactic acid diffuses from muscle to the liver where it is metabolized. Because oxygen consumption increases following heavy activity, the animal is said to have acquired an oxygen debt during activity, which is repaid when activity, ceases and accumulated lactic acid is metabolized.
Some animals rely heavily on anaerobic glycolysis during normal activities. For example, diving birds and mammals fall back on glycolysis almost entirely to give them the energy needed to sustain long dives. Salmon would never reach their spawning grounds were it not for anaerobic glycolysis providing almost all of the ATP used in the powerful muscular bursts needed to carry them up rapids and falls. Many parasitic animals have dispensed with oxidative phosphorylation entirely at some stages of their life cycles. They secrete relatively reduced end products of their energy metabolism, such as succinic acid, acetic acid, and propionic acid. These compounds are produced in mitochondrial reactions that derive several more molecules of ATP than does the cycle from glycolysis to lactic acid, although such sequences are still far less efficient than the classical electron transport chain.
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| Figure 4-16 Anaerobic glycolysis, a process that proceeds in the absence of oxygen. Glucose is broken down to two molecules of pyruvic acid, generating four molecules of ATP and yielding two, since two molecules of ATP are used to produce fructose-1,6- diphosphate. Pyruvic acid, the final electron acceptor for the hydrogen ions and electrons released during pyruvic acid formation, is converted to lactic acid. |
Under anaerobic conditions, glucose and other 6-carbon sugars are first broken down stepwise to a pair of 3- carbon pyruvic acid molecules, yielding two molecules of ATP and four atoms of hydrogen (four reducing equivalents, represented by 2 NADH+ H+). In the absence of molecular oxygen, further oxidation of pyruvic acid cannot occur because the Krebs cycle and electron transport chain cannot operate and cannot, therefore, provide a mechanism for reoxidizing the NADH produced in glycolysis. The problem is neatly solved in most animal cells by reducing pyruvic acid to lactic acid (Figure 4-16). Pyruvic acid becomes the final electron acceptor and lactic acid the end product of anaerobic glycolysis. This frees the hydrogen-bound carrier to recycle and pick up more H+. In alcoholic fermentation (as in yeast, for example) the steps are identical to glycolysis down to pyruvic acid. One of its carbons is then released as carbon dioxide, and the resulting 2-carbon compound is reduced to ethanol, thus regenerating the NAD.
Anaerobic glycolysis is only oneeighteenth as efficient as complete oxidation of glucose to carbon dioxide and water, but its key virtue is that it provides some high-energy phosphate in situations in which oxygen is absent or in short supply. Many microorganisms live in places where oxygen is severely depleted, such as waterlogged soil, in mud of lake or sea bottom, or within a decaying carcass. Vertebrate skeletal muscle may rely heavily on glycolysis during short bursts of activity when contraction is so rapid and powerful that oxygen delivery to tissues is not sufficient to supply energy demands by oxidative phosphorylation alone. At such times an animal has no choice but to supplement oxidative phosphorylation with anaerobic glycolysis. Intense activity is followed by a period of increased oxygen consumption as lactic acid diffuses from muscle to the liver where it is metabolized. Because oxygen consumption increases following heavy activity, the animal is said to have acquired an oxygen debt during activity, which is repaid when activity, ceases and accumulated lactic acid is metabolized.
Some animals rely heavily on anaerobic glycolysis during normal activities. For example, diving birds and mammals fall back on glycolysis almost entirely to give them the energy needed to sustain long dives. Salmon would never reach their spawning grounds were it not for anaerobic glycolysis providing almost all of the ATP used in the powerful muscular bursts needed to carry them up rapids and falls. Many parasitic animals have dispensed with oxidative phosphorylation entirely at some stages of their life cycles. They secrete relatively reduced end products of their energy metabolism, such as succinic acid, acetic acid, and propionic acid. These compounds are produced in mitochondrial reactions that derive several more molecules of ATP than does the cycle from glycolysis to lactic acid, although such sequences are still far less efficient than the classical electron transport chain.
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