Figure 416

Chemical structure of ATP. Its breakdown to ADP and Pi is accompanied by the release of 7 kcal of energy per mol.

processes that require energy. ATP is often referred to as the energy currency of the cell. By analogy, if the amount of usable energy released by the catabolism of one molecule of glucose were equivalent to a $10 bill, then the energy released by the hydrolysis of one molecule of ATP would be worth about a quarter. The energy-requiring machinery of a cell uses only quarters— it will not accept $10 bills. Transferring energy to ATP is the cell's way of making change. However, the amount of energy released in a reaction is the same whether it is released all at once (as in combustion) or in small steps, as occurs physiologically.

Energy is continuously cycled through ATP in a cell. A typical ATP molecule may exist for only a few seconds before it is broken down to ADP and Pj, with the released energy used to perform a cell function. Equally rapidly, the products of ATP hydrolysis, ADP and Pj, are converted back into ATP through coupling to reactions that release energy during the catabolism of carbohydrates, fats, or proteins (Figure 4-17).

(From catabolism of fuel molecules)

The total amount of ATP in the body is sufficient to maintain the resting functions of the tissues for only about 90 s. Thus, energy must be continuously transferred from fuel molecules to ATP.

Only about 40 percent of the energy released by the catabolism of fuel molecules is transferred to ATP, the remaining 60 percent appearing as heat, which is used to maintain the high body temperature found in birds and mammals. Increased metabolic activity, as occurs during exercise, releases increased amounts of heat, producing an elevation in body temperature.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Fuel

Catabolism

molecules

Temperature Across The Human Body
Heat energy 60%

Energy-requiring cell functions

Force and movement

Active transport across membranes

Molecular synthesis

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