The term metabolism which literally means “change” is used to refer to all the enzyme catalyzed chemical reactions taking place within an organism or its individual cells, whether they are involved in growth, reproduction, tissue repair or energy production.
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It is one of the principal vital functions of the organism. The reactions or changes which are concerned with production of heat to maintain the body temperature and supplying of energy for other vital activities constitute “energy metabolism” in contrast to “intermediary metabolism” which is concerned with specific chemical reactions or changes through which the end products of digestion are passed before they could be fully utilized by the body.
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The chemical reactions or changes which undergo during the process of metabolism can be divided under two categories.
Those reactions concerned with the synthesis of cell constituents and cell products from simpler substances are referred to collectively as anabolism.
In general, these do not supply the energy needed by the cell. Indeed, they are usually, endergonic reactions requiring an input of energy.
Energy production is the function of second category of metabolic reactions, collectively called catabolism.
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In catabolism complex molecules are broken down to simpler ones by exergonic processes that ultimately involve energy liberating oxidative reactions.
In metabolism the two categories, e.g., anabolism and catabolism are insolubly connected as there is continuous building up and breaking down of organic compounds.
Moreover in nature anabolism is always in excess when compared with catabolism, it is only because of this, the growth in animals and plants is effected.
Complex substances which become part of the cells and intercellular structures are continuously being formed in the living organisms.
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At the same time complex organic substances are being broken down into simpler ones. The final products of metabolism which cannot be transformed by the organism are eliminated through the specific organs called excretory organs.
Metabolism involves proteins, carbohydrates, fats, water and mineral salts. In addition to these substances the organism requires vitamins.
Metabolism is subject to humoral regulation and regulation by the nervous system. The fact that metabolism in the organs and tissues is controlled by the nervous system was established by Ivan Pavlov and confirmed by Bykov and others.
The influence of nervous system is called trophic influence. The humoral influence on metabolism is exerted through blood by hormones secreted by the endocrine glands.