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Chapter: Medical Physiology: Energetics and Metabolic Rate

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Anaerobic Versus Aerobic Energy

Anaerobic energy means energy that can be derivedfrom foods without the simultaneous utilization of oxygen; aerobic energy means energy that can be derived from foods only by oxidative metabolism.

Anaerobic Versus Aerobic Energy

Anaerobic energy means energy that can be derivedfrom foods without the simultaneous utilization of oxygen; aerobic energy means energy that can be derived from foods only by oxidative metabolism. it is noted that carbohydrates, fats, and proteins can all be oxidized to cause synthesis of ATP. However, carbohydrates are theonly significant foods that can be used to provide energy without the utilization of oxygen; this energy releaseoccurs during glycolytic breakdown of glucose or glyco-gen to pyruvic acid. For each mole of glucose that is split into pyruvic acid, 2 moles of ATP are formed. However, when stored glycogen in a cell is split to pyruvic acid, each mole of glucose in the glycogen gives rise to 3 moles of ATP. The reason for this difference is that free glucose entering the cell must be phosphorylated by using 1 mole of ATP before it can begin to be split; this is not true of glucose derived from glycogen because it comes from the glycogen already in the phosphorylated state, without the additional expenditure of ATP. Thus,the best source of energy under anaerobic conditions is the stored glycogen of the cells.

Anaerobic Energy Utilization During Hypoxia. One of the primeexamples of anaerobic energy utilization occurs in acute hypoxia.When a person stops breathing, there is already a small amount of oxygen stored in the lungs and an additional amount stored in the hemoglobin of the blood. This oxygen is sufficient to keep the metabolic processes functioning for only about 2 minutes. Contin-ued life beyond this time requires an additional source of energy. This can be derived for another minute or so from glycolysis—that is, the glycogen of the cells split-ting into pyruvic acid, and the pyruvic acid becoming lactic acid, which diffuses out of the cells.

Anaerobic Energy Utilization During Strenuous Bursts of Activity Is Derived Mainly from Glycolysis. Skeletal muscles canperform extreme feats of strength for a few seconds but are much less capable during prolonged activity. Most of the extra energy required during these bursts of activ-ity cannot come from the oxidative processes because they are too slow to respond. Instead, the extra energy comes from anaerobic sources: (1) ATP already present in the muscle cells, (2) phosphocreatine in the cells, and (3) anaerobic energy released by glycolytic breakdown of glycogen to lactic acid.

The maximum amount of ATP in muscle is only about 5 mmol/L of intracellular fluid, and this amount can maintain maximum muscle contraction for no more than a second or so. The amount of phosphocreatine in the cells is three to eight times this amount, but even by using all the phosphocreatine, maximum contraction can be maintained for only 5 to 10 seconds.

Release of energy by glycolysis can occur much more rapidly than can oxidative release of energy. Conse-quently, most of the extra energy required during stren-uous activity that lasts for more than 5 to 10 seconds but less than 1 to 2 minutes is derived from anaerobic gly-colysis. As a result, the glycogen content of muscles during strenuous bouts of exercise is reduced, whereas the lactic acid concentration of the blood rises. After the exercise is over, oxidative metabolism is used to recon-vert about four fifths of the lactic acid into glucose; the remainder becomes pyruvic acid and is degraded and oxidized in the citric acid cycle. The reconversion to glucose occurs principally in the liver cells, and the glucose is then transported in the blood back to the muscles, where it is stored once more in the form of glycogen.

Oxygen Debt Is the Extra Consumption of Oxygen After Completion of Strenuous Exercise. After a period of strenuous exer-cise, a person continues to breathe hard and to consume large amounts of oxygen for at least a few minutes and sometimes for as long as 1 hour thereafter. This addi-tional oxygen is used (1) to reconvert the lactic acid that has accumulated during exercise back into glucose, (2) to reconvert adenosine monophosphate and ADP to ATP, (3) to reconvert creatine and phosphate to phos-phocreatine, (4) to re-establish normal concentrations of oxygen bound with hemoglobin and myoglobin, and (5) to raise the concentration of oxygen in the lungs to its normal level. This extra consumption of oxygen after exercise is over is called repaying the oxygen debt.


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