Electron Transport System (ETS) and Oxidative Phosphorylation

The following steps in the respiratory process are to release and utilise the energy stored in NADH + H+ and FADH2. This is accomplished when they are oxidised through the electron transport system and the electrons are passed on to O2, resulting in the formation of H2O. The metabolic pathway through which the electron passes from one carrier to another is called the electron transport system (ETS), and it is present in the inner mitochondrial membrane. The sequence of events is:

• Electrons from NADH produced in the mitochondrial matrix during the citric acid cycle are oxidised by an 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), which is generated during the oxidation of succinate in the citric acid cycle.
• The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via the 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 complex IV.
• Complex IV refers to the 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) for the production of ATP from ADP and inorganic phosphate. The number of ATP molecules synthesised depends on the nature of the electron donor:

• Oxidation of one molecule of NADH gives rise to 3 molecules of ATP.
• Oxidation of one molecule of FADH2 produces 2 molecules of ATP.

Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor. Unlike photophosphorylation, where light energy is utilised for the production of the proton gradient, in respiration it is the energy of oxidation-reduction that is utilised for the same process. It is for this reason that the process is called oxidative phosphorylation. …