Metabolism as you know is a process by which living organisms acquire and use energy. A large number of biochemical reactions take place in the living organisms during various physiological activities. The sum total of all the biochemical activities of the organism is called metabolism. Broadly two types of biochemical reactions take place in the body. In one, 'simple substances combine to form complex substances. This is called anabolism and involves acquisition and storage of energy. The second, catabolism, produces simpler substances as a result of the break-down of complex ones, releasing energy. Metabolic reactions occur continuously and simultaneously in living organisms and when they cease the organisms die.
Figure: Metabolic (anabolic and catabolic) reactions occur continuously in every living organism.
This figure shows the relationships of some metabolic activities. Some of the nutrients ingested by the organism are used to synthesize needed materials and cell parts; other nutrients are used as fuel for cellular respiration, a process that captures energy stored in food. This energy is needed for synthesis and for other forms of cellular work. Cellular respiration also requires oxygen, which is provided, by the process of gas exchange. Wastes from the cells such as carbon dioxide and water must he excreted from the body.