There can be a number of chemical processes by which a compounds can be broken down into simpler substances. The most common and wide-spread of such processes is Catabolism. Along with anabolism, catabolism make up the metabolism process for living organisms. In catabolism, complex chemical molecules (such as proteins, polysaccharides, etc.) are broken down into simpler molecules (such as amino acids, monosaccharides, etc.). This is often accompanied by release of energy in the form of ATP (adenosine triphosphate) molecules and intermediate metabolites (which can be used by the organism in the anabolic processes). The energy thus generated is used for operation and maintenance of cells (and consequently, the body).
Other processes that break down chemical substances into simpler substances, include depolymerization, decomposition, etc.
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