A biochemical reaction involving the transfer of a negatively charged electron from one organic compound to another organic compound or to oxygen. When a compound loses an electron, or is oxidized, another compound gains the electron, or is reduced. Oxidation-reduction (redox) reactions represent the main source of biological energy. Redox reactions occur simultaneously, and one does not occur without the other. The principal sources of reductants for animals are the numerous breakdown products of the major foodstuffs: carbohydrates, fats, and proteins. Energy release from these substances occurs in a stepwise series of hydrogen and electron transfers to molecular oxygen.