Metabolism is the process by which living things grow and repair their bodies and produce energy needed for life. Metabolism includes all the chemical changes that take place in the protoplasm of the cell. The changes which build up new protoplasm from simpler materials are called anabolic metabolism. The changes which break down parts of the protoplasm are catabolic metabolism.
All metabolic processes use food as the raw material for supplying energy and building new tissues. Foods include water, minerals, vitamins, carbohydrates (starches and sugars), proteins, and fats. The latter three are changed by digestion to simple sugar, amino acids, fatty acids, and glycogen, which can be assimilated. Glucose, fats and amino acids may be used to form a basic part of protoplasm, oxidized to give the body the heat and energy it needs, or stored for future use.
In plants there are four anabolic processes and two catabolic processes. plant cells produce sugars and starches from carbon dioxide and water in photosynthesis. They recombine the atoms of some of these carbohydrates to produce fats and proteins. Plant cells can change any of these food substances into compounds which can be stored. Plants are able to grow and repair worn out or injured parts by converting amino acids, carbohydrates, and fat in living protoplasm. This conversion is called synthesis.
The catabolic processes of plants are respiration and digestion. Respiration is the chemical combination of foods with oxygen releasing the stored energy of foods. This energy is necessary for further chemical changes and movement. Digestion in the cell changes complex foods into simpler food which can be oxidized in respiration, used for growth, or changed and stored.
Animal digestion is a catabolic process. Food is taken in and broken down into simpler chemicals by the process of digestión. In protozoans, sponges, and partly in coelenterates, digestion is a process occurring within the cells. In higher animals food is digested outside the cell and the cell absorbs the predigested food. Forty percent of digested carbohydrates and fats, and eighty to eighty-five per cent of digested proteins are oxidized in the cell to produce energy. The rest are stored as body fat or glycogen. Proteins not used in repair are changed to carbohydrates and oxidized or stored. Amino acids from protein are recombined into proteins characteristic of the animal. When an organism is performing its normal functions, metabolism is carried on at a uniform rate. When one has not eaten for several hours and lies quietly, metabolism is carried on at a slow rate, called the basal metabolic rate. This rate in humans is twenty to twenty-five per cent greater in children than adults, and eight per cent higher in males. It declines slowly after age twenty. Basal metabolic rate is largely controlled by the thyroid gland.
