NADH and FADH are oxidised to release electrons. The electrons are passed through a series of specialised proteins complexes embedded into the inner membrane. As the electrons are transferred to the next complex, H+ ions are pumped into the intermembrane space. As H+ accumulates in the intermembrane space, the protons are pushed through ATPsynthase following their electrochemical gradient causing phosphorylation of ADP to produce ATP. (ADP + Pi --> ATP. The electrons are accepted by the final electron acceptor, oxygen, that reacts with H+ in the mitochondrial matrix. The oxygen ensures that an H+ electrochemical gradient between mitochondrial matrix and intermembrane space is maintained by using the 2H+ and 2e- to produce water.