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metabolism, sum of all biochemical processes involved in life. Two subcategories of metabolism are anabolism, the building up of complex organic molecules from simpler precursors, and catabolism, the breakdown of complex substances into simpler molecules, often accompanied by the release of energy. Organic molecules involved in these processes are called metabolites, and their interconversions are catalyzed by enzymes. The transformation of one molecule into another, and then into another and another in sequence, is termed a metabolic pathway; the intermediates in these pathways are often identified with the aid of a chemical tracer. Exercise, food, and environmental temperature influence metabolism. Basal metabolism is the caloric expenditure of an organism at rest; it represents the minimum amount of energy required to maintain life at normal body temperature. The basal metabolism rate is usually measured indirectly by calculation from measurements of the amounts of oxygen and carbon dioxide exchanged during breathing under certain standard conditions, i.e., complete rest in a room temperature of 68°F (20°C), 12 to 14 hours after ingestion of food. A less cumbersome method of estimating basal metabolic rate involves the quantitative assay of the hormone thyroxine, known to regulate the body's rate of metabolism. Often the word metabolism is associated with a particular organic compound or class of compounds, as in phenylalanine metabolism or amino acid metabolism. In this usage the word refers to the sum of all interconversions, both anabolic and catabolic, in which the particular compound or class of compounds is involved.
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