Alternative Title: catabolic reaction

Catabolism, the sequences of enzyme-catalyzed reactions by which relatively large molecules in living cells are broken down, or degraded. Part of the chemical energy released during catabolic processes is conserved in the form of energy-rich compounds (e.g., adenosine triphosphate [ATP]).

Energy is released in three phases. In the first, such large molecules as those of proteins, polysaccharides, and lipids are broken down; small amounts of energy are released in the form of heat in these processes. In the second phase, the small molecules are oxidized, liberating chemical energy to form ATP as well as heat energy, to form one of the three compounds: acetate, oxaloacetate, or α-oxoglutarate. These are oxidized to carbon dioxide during the third phase, a cyclic reaction sequence called the tricarboxylic acid (or Krebs) cycle. Hydrogen atoms or electrons from the intermediate compounds formed during the cycle are transferred (through a succession of carrier molecules) ultimately to oxygen, forming water. These events, the most important means for generating ATP in cells, are known as terminal respiration and oxidative phosphorylation.

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in metabolism

Figure 1: Biological energy carriers.
Weeping willow (Salix babylonica).
Metabolic pathways and cycles are either catabolic (energy-releasing) or anabolic (energy-consuming). Catabolic reactions break down complex metabolites into simpler ones, whereas anabolic reactions build up (biosynthesize) new molecules. When chemical bonds are broken, energy is released, which drives anabolic reactions to form new bonds. The energy released generally has been stored in...
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