any biochemical process that involves the transfer of an electron or hydrogen atom, sometimes with accompanying atoms or groups, from an oxidizable molecule to a reducible one. Oxidation-reduction reactions in organisms are catalyzed by enzymes of the oxidoreductase class. The energy that is released in oxidation-reduction reactions is stored in the chemical bonds of adenosine triphosphate (ATP) molecules and other high-energy compounds. Oxidation-reduction reactions that take place in any living cell include the reactions in the tricarboxylic acid cycle and the electron-transfer reactions in respiration, photosynthesis, fermentation, glycolysis, and fatty-acid oxidation and synthesis.