Oxidation/reduction reactions in cells involve transfer of electrons from the molecule being oxidized to the molecule being reduced. The molecule oxidized is the electron donor and the molecule reduced is the electron acceptor.

Common acceptors of electrons (also called electron carriers) include the flavin coenzymes, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), and the nicotinamide coenzymes nicotinamide adenine dinucleotide (NAD⁺) and nicotinamide adenine dinucleotide phophate (NADP+) (Figure 14.7). FAD, FMN, NAD⁺, and NADP+ can accept electrons in pairs, but FAD and FMN can also accept single electrons via formation of a semiquinone intermediate (Figure 14.8).

The electron carriers FADH2 and NADH readily donate electrons to the electron transport system. NADPH, on the other hand, usually donates its electrons directly to molecules undergoing biosynthesis (such as in fatty acid synthesis).