Electron transport system (ETS) is present in the inner membrane of mitochondrial. Electrons from NADH produced in the mitochondrial matrix during TCA cycle are oxidised by an NADH dehydrogenase (complex I), and are transferred to ubiquinone located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH2 (complex II) that is generated during oxidation of succinate in the TCA cycle. The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc1 complex (complex III) and finally the electrons are passed on to O2 resulting in the formation of H2O.