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ATP + Creatine

Note particularly that the higher energy level of the high-energy phosphate bond in phosphocreatine (1000 to 1500 calories per mole greater than that in ATP) causes the reaction between phosphocreatine and ADP to proceed rapidly toward the formation of new ATP every time even the slightest amount of ATP expends its energy elsewhere. Therefore, the slightest usage of ATP by the cells calls forth the energy from the phos-phocreatine to synthesize new ATP. This effect keeps the concentration of ATP at an almost constant high level as long as any phosphocreatine remains. For this reason, we can call the ATP-phosphocreatine system an ATP "buffer" system. One can readily understand the importance of keeping the concentration of ATP nearly constant, because the rates of almost all the metabolic reactions in the body depend on this constancy.

Anaerobic Versus Aerobic Energy

Anaerobic energy means energy that can be derived from foods without the simultaneous utilization of oxygen; aerobic energy means energy that can be derived from foods only by oxidative metabolism. In the discussions in Chapters 67 through 69, it is noted that carbohydrates, fats, and proteins can all be oxidized to cause synthesis of ATP. However, carbohydrates are the only significant foods that can be used to provide energy without the utilization of oxygen; this energy release occurs 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 prime examples 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. Continued 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 splitting into pyruvic acid, and the pyruvic acid becoming lactic acid, which diffuses out of the cells, as described in Chapter 67.

Anaerobic Energy Utilization During Strenuous Bursts of Activity Is Derived Mainly from Glycolysis. Skeletal muscles can perform 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 activity 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. Consequently, most of the extra energy required during strenuous 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 reconvert 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 exercise, 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 additional 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 phosphocreatine, (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.

The principle of oxygen debt is discussed further in Chapter 84 in relation to sports physiology; the ability of a person to build up an oxygen debt is especially important in many types of athletics.

Summary of Energy Utilization by the Cells

With the background of the past few chapters and of the preceding discussion, we can now synthesize a composite picture of overall energy utilization by the cells, as shown in Figure 72-1. This figure demonstrates the anaerobic utilization of glycogen and glucose to form ATP and the aerobic utilization of compounds derived from carbohydrates, fats, proteins, and other substances to form additional ATP. In turn, ATP is in reversible equilibrium with phosphocreatine in the cells, and because larger quantities of phosphocreatine are present in the cells than ATP, much of the cells' stored energy is in this energy storehouse.

Energy from ATP can be used by the different functioning systems of the cells to provide for synthesis and growth, muscle contraction, glandular secretion, nerve impulse conduction, active absorption, and other cellular activities. If greater amounts of energy are demanded for cellular activities than can be provided by oxidative metabolism, the phosphocreatine storehouse is used first, and then anaerobic breakdown of glycogen follows rapidly. Thus, oxidative metabolism

Figure 72-1

Overall schema of energy transfer from foods to the adenylic acid system and then to the functional elements of the cells. (Modified from Soskin S, Levine R: Carbohydrate Metabolism. Chicago: University of Chicago Press, 1946, 1952.)

Glycogen

Glycogen

Pyruvic acid

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