The next few chapters deal with metabolism in the body, which means the chemical processes that make it possible for the cells to continue living. It is not the puipose of this textbook to present the chemical details of all the various cellular reactions, because this lies in the discipline of biochemistry. Instead, these chapters are devoted to (1) a review of the principal chemical processes of the cell and (2) an analysis of their physiologic implications, especially the manner in which they fit into the overall concept of homeostasis.
'Active ion transport - Musciù «mlraclion •Synthesis oI molecules
Release of Energy from Foods, and the Concept of "Free Energy"
A great proportion of the chemical reactions in the cells is concerned with making the energy in foods available to the various physiologic systems of the cell. For instance, energy is required for muscle activity, secretion by the glands, maintenance of membrane potentials by the nerve and muscle fibers, synthesis of substances in the cells, absorption of foods from the gastrointestinal tract, and many other functions.
Coupled Reactions. All the energy foods—carbohydrates, fats, and proteins—can be oxidized in the cells, and during this process, large amounts of energy are released. These same foods can also be burned with pure oxygen outside the body in an actual fire, also releasing large amounts of energy; in this case, however, the energy is released suddenly, all in the form of heat. The energy needed by the physiologic processes of the cells is not heat but energy to cause mechanical movement in the case of muscle function, to concentrate solutes in the case of glandular secretion, and to effect other functions. To provide this energy, the chemical reactions must be "coupled" with the systems responsible for these physiologic functions. This coupling is accomplished by special cellular enzyme and energy transfer systems, some of which are explained in this and subsequent chapters.
"Free Energy." The amount of energy liberated by complete oxidation of a food is called the free energy of oxidation of the food, and this is generally represented by the symbol AG. Free energy is usually expressed in terms of calories per mole of substance. For instance, the amount of free energy liberated by complete oxidation of 1 mole (180 grams) of glucose is 686,000 calories.
Adenosine triphosphate (ATP) is an essential link between energy-utilizing and energy-producing functions of the body (Figure 67-1). For this reason, ATP has been called the energy currency of the body, and it can be gained and spent repeatedly.
Energy derived from the oxidation of carbohydrates, proteins, and fats is used to convert adenosine diphosphate (ADP) to ATP, which is then consumed by the various reactions of the body that are necessary for (1) active transport of molecules across cell membranes; (2) contraction of muscles and performance of mechanical work; (3) various synthetic reactions that create hormones, cell membranes, and many other essential molecules of the body; (4) conduction of nerve impulses; (5) cell division and growth; and (6) many other physiologic functions that are necessary to maintain and propagate life.
ATP is a labile chemical compound that is present in all cells. It has the chemical structure shown in Figure 67-2. From this formula, it can be seen that ATP is a combination of adenine, ribose, and three phosphate radicals. The last two phosphate radicals are connected with the remainder of the molecule by high-energy bonds, which are indicated by the symbol
The amount of free energy in each of these high-energy bonds per mole of ATP is about 7300 calories under standard conditions and about 12,000 calories under the usual conditions of temperature and concentrations of the reactants in the body. Therefore, in the body, removal of each of the last two phosphate radicals liberates about 12,000 calories of energy. After loss of one phosphate radical from ATP, the compound becomes ADP, and after loss of the second phosphate radical, it becomes adenosine monophosphate (AMP).
The interconversions among ATP, ADP, and AMP are the following:
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