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 beoxidized 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 cou-pling is accomplished by special cellular enzyme and energy transfer systems.
“Free Energy.” The amount of energy liberated by complete oxidation of a food iscalled the free energy of oxidation of the food, and this is generally represented by the symbol DG. 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.
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