catabolism (kətăbˈəlĭzˌəm) [key], subdivision of metabolism involving all degradative chemical reactions in the living cell. Large polymeric molecules such as polysaccharides, nucleic acids, and proteins are first split into their constituent monomeric units, such as amino acids, after which the monomers themselves can be broken down into such simple cellular metabolites as lactic acid, acetic acid, carbon dioxide, ammonia, and urea. The first set of reactions provides the cell with monomers with which it can construct new polymeric molecules. The second set of reactions usually involves the process of oxidation and is accompanied by a release of chemical free energy, not all of which is lost as heat, but is partially conserved through the coupled synthesis of adenosine triphosphate. The hydrolysis of this compound is subsequently used to drive almost every energy-requiring reaction in the cell. Thus catabolism also provides the source of chemical energy necessary for the maintenance of the living cell.
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