A series of molecules that transfers electrons via a sequence of biochemical reduction-oxidation (redox) reactions to effect a stepwise change in free energy. An electron transport chain, also known as a respiratory chain, forms the final stage of cellular respiration. The components of this chain are located in the inner membrane of the mitochondria and consist of multiprotein complexes (labelled I to IV) with tightly bound prosthetic groups (cofactors and coenzymes) essential to catalysing the reactions of the chain. Each component is present in multiple copies in a single mitochondrion. NADH or FADH₂, generated by the Krebs cycle, transfer their electrons through a chain of electron carrier molecules, including iron-sulphur proteins, ubiquinone (Q), and a series of cytochromes (cyt), that undergo reversible redox reactions, accepting electrons and then donating them to the next carrier in the chain—a process known as electron flow. Cytochrome oxidase combines electrons and hydrogen ions with oxygen—the final electron acceptor in the chain—to form water (see illustration). This process is coupled to the formation of ATP by the chemiosmotic mechanism (see also oxidative phosphorylation). An electron transport chain also occurs in the thylakoid membranes of chloroplasts in photosynthesis; the carrier molecules include plastoquinone, plastocyanin, and ferredoxin. See photophosphorylation.

The mitochondrial electron transport chain and chemiosmotic synthesis of ATP