Anaerobic Release of Energy- “Anaerobic Glycolysis”
Occasionally, oxygen becomes either unavailable or insufficient, so that oxidative phosphorylation cannot take place. Yet even under these conditions, a small amount of energy can still be released to the cells by the glycolysis stage of carbohydrate degradation, because the chemical reactions for the breakdown of glucose to pyruvic acid do not require oxygen.
This process is extremely wasteful of glucose, because only 24,000 calories of energy are used to form ATP for each molecule of glucose metabolized, which represents only a little over 3 per cent of the total energy in the glucose molecule. Nevertheless, this release of glycolytic energy to the cells, which is called anaerobic energy, can be a lifesaving measure for up to a few minutes when oxygen becomes unavailable.
Formation of Lactic Acid During Anaerobic Glycolysis Allows Release of Extra Anaerobic Energy. Thelaw of mass actionstates that as the end products of a chemical reaction build up in a reacting medium, the rate of the reaction decreases, approaching zero. The two end products of the glycolytic reactions (see Figure 67–5) are (1) pyruvic acid and (2) hydrogen atoms combined with NAD+ to form NADH and H+. The buildup of either or both of these would stop the glycolytic process and prevent further formation of ATP. When their quantities begin to be excessive, these two end products react with each other to form lactic acid, in accordance with the fol-lowing equation:
Thus, under anaerobic conditions, the major portion of the pyruvic acid is converted into lactic acid, which diffuses readily out of the cells into the extracellular fluids and even into the intracellular fluids of other less active cells. Therefore, lactic acid represents a type of “sinkhole” into which the glycolytic end products can disappear, thus allowing glycolysis to proceed far longer than would otherwise be possible. Indeed, gly-colysis could proceed for only a few seconds without this conversion. Instead, it can proceed for several minutes, supplying the body with considerable extra quantities of ATP, even in the absence of respiratory oxygen.
Reconversion of Lactic Acid to Pyruvic Acid When Oxygen Becomes Available Again. When a person begins to breatheoxygen again after a period of anaerobic metabolism, the lactic acid is rapidly reconverted to pyruvic acid and NADH plus H+. Large portions of these are immedi-ately oxidized to form large quantities of ATP. This excess ATP then causes as much as three fourths of the remaining excess pyruvic acid to be converted back into glucose.
Thus, the large amount of lactic acid that forms during anaerobic glycolysis is not lost from the body because, when oxygen is available again, the lactic acid can be either reconverted to glucose or used directly for energy. By far the greatest portion of this reconversion occurs in the liver, but a small amount can also occur in other tissues.
Use of Lactic Acid by the Heart for Energy. Heart muscleis especially capable of converting lactic acid to pyruvic acid and then using the pyruvic acid for energy. This occurs to a great extent during heavy exercise, when large amounts of lactic acid are released into the blood from the skeletal muscles and consumed as an extra energy source by the heart.
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