BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.3. Cellular Respiration

9.3.8. Oxygen Debt and the Immediate Effect of Exercise

The ATP reserves in Cells are quite small. Normally, the body replenishes this supply as it is consumed, but a sudden transition from rest to intense muscular activity requires some time to adapt to this new state. Mechanisms exist that can supply the Muscles with the necessary amount of energy until the intensity of aerobic Respiration increases sufficiently. One such mechanism is Anaerobic respiration. It is important to understand that anaerobic respiration serves as a Complement to aerobic respiration, rather than an alternative to it.

The graph in Fig. 9.10 illustrates how oxygen uptake changes during exercise and immediately after its cessation. To meet the energy demand through aerobic respiration, the body needs to consume 3 liters of oxygen per minute. In this example (see the graph), this level is reached only by the sixth minute of physical exertion. The oxygen debt (The amount of oxygen that was required by the body from external sources via respiration, but was not received) is represented in the graph by zone A. During these first six minutes, various mechanisms described below come into play to ensure The production of the required amount of energy.

Class="center">

Fig. 9.10. Oxygen uptake during Physical Exercise and the recovery period.

9.4. The rate of oxygen uptake increases immediately after the onset of physical activity. How is the delivery of oxygen from the external environment to the cells enhanced during exercise?

Oxygen Reserve

The body maintains certain reserves from which oxygen can be drawn. More oxygen than usual can be extracted from the Lungs, Body Fluids, and Hemoglobin. In muscles, the protein Myoglobin serves to store oxygen. It is very similar in nature to hemoglobin (Fig. 3.36) and, like hemoglobin, is capable of reversible binding with oxygen. However, it releases oxygen only when oxygen levels become very low—that is, after hemoglobin has released the bulk of its oxygen. Therefore, the oxygen stored in myoglobin is used only as a last resort.

Phosphocreatine System

The ATP contained in muscles can sustain maximum exertion for no more than approximately three seconds. Phosphocreatine is another substance that releases energy when its phosphate group is cleaved. This energy is sufficient to synthesize ATP from ADP and P_i. The phosphocreatine reserve in muscles is 2–4 times greater than the ATP reserve, and when necessary, energy from phosphocreatine can be rapidly transferred to ATP. The total energy reserve in ATP and phosphocreatine is sufficient to sustain maximum muscular effort for 8–10 s.

Anaerobic Respiration: The Glycogen-Lactic Acid System

Although anaerobic respiration yields only two molecules of glucose per ATP molecule compared to 38 molecules in aerobic respiration, ATP synthesis proceeds 2.5 times faster in the former (anaerobic respiration produces five molecules of ATP in the same timeframe that aerobic respiration produces two). Anaerobic respiration can therefore supply energy rapidly. The source of glucose for this process is glycogen stored within the muscles. The energy extracted from it is sufficient to sustain maximum muscular activity for 90 s.

All of these systems operate more efficiently with regular exercise.

Thus, we see that the phosphocreatine and anaerobic respiration systems supply energy rapidly, but only for a short duration. The aerobic system can serve as a source of energy indefinitely, provided there is an adequate supply of respiratory substrate. In sports that rely on short, explosive bursts of muscular activity—such as sprinting or weightlifting—energy is supplied primarily by the phosphocreatine system. In a 200-meter run, anaerobic respiration can serve as an additional energy source. In a 400-meter run, it supplies the majority of the energy, and in sports like tennis, squash, or football, practically all the energy during peak exertion comes from this system. Sports that emphasize endurance—such as marathon running, jogging, or cross-country skiing—depend almost entirely on aerobic respiration.

Upon completion of physical activity (Fig. 9.10), oxygen consumption does not immediately return to the resting level (0.25 L/min). During the recovery period, a person continues to breathe heavily for some time. The amount of oxygen consumed during this phase (zone B in Fig. 9.10) constitutes the oxygen debt. This oxygen is utilized:

1. To replenish the body's oxygen reserves, i.e., to restore normal oxygen levels in the lungs, tissue fluids, myoglobin, and hemoglobin.

2. To regenerate phosphocreatine—once exercise ceases, creatine re-attaches a phosphate group, with aerobic respiration providing the necessary energy.

3. The replenishment of body oxygen reserves and the regeneration of phosphocreatine occur rapidly, as evidenced by the steeply descending portion of the curve corresponding to the first minutes of recovery (Fig. 9.10). The slower recovery phase (the shallow part of the curve) represents the period during which lactic acid, accumulated during anaerobic respiration, is cleared from the muscles. Lactic acid enters the bloodstream and is transported from the muscles to the Liver, where it is oxidized to form pyruvic acid and reduced NADH. A portion of this pyruvic acid is directed into the conventional aerobic pathway via the Krebs cycle and undergoes oxidation, resulting in The formation of ATP. This ATP can then be used to convert the remaining pyruvic acid (about 75%) back into glucose through a process that is essentially reversed Glycolysis. In The Heart Muscle during heavy exertion, lactic acid can also be converted to pyruvic acid by Oxidation coupled with NAD, serving as an additional energy source.

9.5. Why does Blood lactic acid concentration continue to rise even after physical activity has ceased and anaerobic respiration has stopped?