Oxidation/reduction reactions involve the transfer of electrons from a molecule being oxidized (the electron donor) to a molecule being reduced (the electron acceptor). Because one or more electrons are transferred, neither oxidation nor reduction can occur without the other occurring simultaneously. An example of an oxidation/reduction reaction is the following reaction, catalyzed by succinate dehydrogenase, from the citric acid cycle:
Succinate is the electron donor (so it is oxidized) and FAD is the electron acceptor (so it is reduced). The products of the reaction are fumarate (oxidized) and FADH2 (reduced). Cells typically use a common set of electron acceptors, such as FAD, NAD+ and NADP+. Many of these molecules deposit their electrons into the electron transport system.