FOOD AND ENERGYAll organisms depend on energy from food for life. Carbohydrates, fats, and proteins are synthesized in plants during periods of available sunlight and stored in tubers (potatoes) or roots (sugar maples), to be drawn on during periods when new growth calls for large energy expenditure. Food energy is expressed in calories. (In energy metabolism this unit usually refers to the large calorie, or kilocalorie: the amount of heat energy required to raise the temperature of 1 kg of water by 1° C.) Carbohydrates have an average value of 4.1 calories per gram, proteins have 5.7 calories per gram, and fats have an average of 9.3 calories per gram. Organisms rely more heavily on one or another of these foods to suit particular needs. An arctic fox, for example, depends almost entirely on lightweight, high-energy-yielding fats. Seeds, which must be light in weight yet contain large amounts of energy, are likely to contain a high percentage of oils. A sugar maple, however, which leads a fixed existence and has ample storage space in its roots, relies almost entirely on carbohydrates in the form of sucrose. When foods—especially in the form of carbohydrates and fats—are burned in the animal system, they yield the same calories per gram as when undergoing rapid combustion in a laboratory calorimeter. Mechanical engines, in fact, yield the same number of calories per weight of fuel as animal systems. Mechanical and animal systems also yield large amounts of heat energy and relatively small amounts of work energy. Animal muscle yields only about one calorie of work for every four given up as heat. In animal systems, however, heat does not go entirely wasted. It is needed (especially by warm-blooded animals) to maintain body temperature and to induce metabolic reactions, which at lower temperatures would take place too slowly to be able to maintain bodily functions. Although living cells conform to the same laws of energy transformation as do machines, their modes of functioning are infinitely more versatile. One unique characteristic of living systems is their ability to consume their own tissues after they have exhausted all other food-energy stores. Another is that instead of radically releasing energy through rapidly combusting compounds, as an automobile engine does, living cells release energy in step-by-step chemical reactions. The energy yielded by one chemical reaction drives other reactions, enabling a gradual release of work energy with minimum fatigue to the cells.
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