ETS or the Electron transport chain is the pathway through which the electron pass from one carrier to another. The ETS is found in the inner mitochondrial membrane.

Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by NADH dehydrogenase (complex I), and electrons are then 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 citric acid cycle.

The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc 1 complex (complex III). The cytochrome c acts as a mobile carrier between complex III and cytochrome c oxidase complex, containing cytochrome a and a₃, along with copper centres (complex IV).

During the transfer of electrons from each complex, ATP is produced from ADP and inorganic phosphate by the action ATP synthase (complex V).

The amount of ATP produced depends on the molecule, which has been oxidized. 2 ATP molecules are produced by the oxidation of one molecule of NADH. One molecule of FADH2, on oxidation, gives 3 ATP molecules.