The series of redox reactions taking place between compounds involving electron transfer from donors to acceptors is known as the electron transport chain. In complex I (NADH dehydrogenase) which is the main site of production of superoxide where the NADH loses two electrons which are transferred to ubiquinone. In complex IV (cytochrome oxidase) two molecules of water are produced due to the transfer of electrons from four molecules of cytochrome C to the molecule oxygen.