Fig. 3. The oxidative phosphorylation complexes in the electron transport chain (ETC) of mitochondrion.

Complex I (Co-I) receives electrons from NADH. Complex II (Co-II) represents a point of intersection between tricarboxylic acid cycle and electron transport. Mobile electron carriers, like coenzyme Q (CoQ) and cytochrome c, move electrons between protein complexes. Complex III (Co-III) is the CoQ-cytochrome c oxidoreductase. Complex IV (Co-IV) is also known as cytochrome c oxidase (COX). This complex terminates the flow of electrons through the ETC, reducing oxygen to water. Finally, complex V (Co-V) generates ATP from the energy stored by the proton gradient established through the previous step by step work.