electron transport chain

electron transport chain (electron transport system) A sequence of biochemical reduction–oxidation reactions that effects the transfer of electrons through a series of carriers. An electron transport chain, also known as the respiratory chain, forms the final stage of aerobic respiration. In the mitochondria, NADH or FADH₂, generated by the Krebs cycle, transfer their electrons through a chain of carrier molecules, including ubiquinone and a series of cytochromes, that undergo reversible reduction–oxidation 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 to form water (see illustration). This process is coupled to the formation of ATP (see chemiosmotic theory; 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.