Electron transport system and oxidative phosphorylation

The energy held in NADH + H⁺ and FADH₂ is released when they are oxidised through the electron transport system (ETS), a chain of carriers in the inner mitochondrial membrane, with the electrons finally passed to O₂ to form H₂O.

Complex I (NADH dehydrogenase) oxidises NADH made in the matrix and passes the electrons to ubiquinone in the inner membrane.

Ubiquinone also receives reducing equivalents from FADH₂ through complex II, which is generated when succinate is oxidised in the citric acid cycle.

Reduced ubiquinone (ubiquinol) passes its electrons to cytochrome c through the cytochrome bc₁ complex (complex III).

Cytochrome c, a small protein sitting on the outer face of the inner membrane, shuttles electrons as a mobile carrier between complexes III and IV.

Complex IV is cytochrome c oxidase, containing cytochromes a and a₃ and two copper centres.

Electron flow through complexes I to IV is coupled to ATP synthase (complex V). The yield depends on the donor: one NADH gives 3 ATP, one FADH₂ gives 2 ATP.

O₂ acts only at the very end, but it is vital: as the final hydrogen acceptor it pulls hydrogen out of the system and so drives the whole process.

In photophosphorylation, light energy builds the proton gradient; in respiration the energy of oxidation-reduction builds it, so ATP synthesis here is called oxidative phosphorylation.

ATP synthase has two parts: F₁, a peripheral headpiece with the site where ATP is made from ADP and Pi, and F₀, an integral membrane complex forming the proton channel. For each ATP made, 4 H⁺ pass through F₀ from the intermembrane space into the matrix, down the electrochemical proton gradient.