The electron transport chain, otherwise referred to as the respiratory
chain, has a series of four large protein complexes through which the electron pass from lower to higher redox potentials.
Only two complexes, however, are common to the oxidation of both NADH and FADH2. The flow of electrons causes
pumping of protons from the matrix side to the intermembrane space. This maintains a proton motive force composed of a
pH gradient and a membrane potential. The protons flow back into the matrix along various pathways but particularly
through the ATP synthesizing complex embedded in the membrane during ATP synthesis. The central feature of this
process of oxidative phosphorylation is the generation of proton gradient and its accompanying membrane potential.