Illustration of the electron transport chain, a series of protein complexes located in the inner membrane of mitochondria. Here a series of reactions occurs in which electrons are transported to meet up with oxygen from respiration. The overall electron chain transport reaction is: 2H+ + 2e+ + 1/2O2 ---> H2O + energy. This reaction actually involves eight or more steps. A series of redux reactions moves electrons from a donor (NADH or QH2) to an acceptor (O2). A proton gradient is created across the inner membrane by three proton pumps (I -- NADH dehydrogenase, II -- succinate dehydrogenase, and IV -- cytochrome bc1 complex). The resulting proton gradient allows production of ATP via ATP synthase. This is a reversible process. Every use of the electron transport chain results in the production of three ATP molecules.