The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS).
This pathway is present in the inner mitochondrial membrane.
• Electrons from NADH (produced in the mitochondria matrix) are oxidized by an NADH dehydrogenase (Complex I). After that, electrons are transferred to ubiquinone which is located within the inner membrane.
• Ubiquinone also receives reducing equivalents via FADH2 (Complex II). FADH2 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 bc1 complex (complex III).
• Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV.
• Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3, and two copper centres.
• When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V). This coupling is necessary for the production of ATP from ADP and inorganic phosphate. The nature of the electron donor decides the number of ATP molecules synthesized.