Redox chemistry involves the transfer of electrons from one reactant to another. Reducing agents donate electrons while oxidizing agents accept electrons. Biological redox systems, like cytochromes, are important for cellular functions such as forming water from hydrogen. Cytochromes undergo one electron redox reactions and participate in electron transport chains, transferring electrons from reduced flavoproteins to oxygen to form water. Many biological redox systems are pH dependent, and electrode potentials are typically listed at pH 7.0 for comparisons. Other important redox systems include NADH/NAD, flavins, and various substrate systems.