Electron transport and ATP synthase

The NADH and FADH₂ made so far carry high-energy electrons. The electron transport system (ETS), in the inner mitochondrial membrane, takes these electrons and passes them along a chain of carriers to oxygen, which then forms water.

The chain, step by step:

• Complex I (NADH dehydrogenase) takes electrons from NADH and passes them to ubiquinone. FADH₂ feeds its electrons to ubiquinone through complex II.
• Ubiquinone (now ubiquinol) passes them to complex III (cytochrome bc₁).
• From there, the small mobile protein cytochrome c carries them to complex IV, cytochrome c oxidase (which has cytochromes a and a₃ and two copper centres).
• Complex IV hands the electrons to oxygen, the final hydrogen (electron) acceptor, forming water.

As electrons move, the energy released is used to make ATP. One NADH gives 3 ATP; one FADH₂ gives 2 ATP. Oxygen works only at the very end, but it drives the whole process: without it to take the electrons away, the chain would jam.

Here the energy for pumping protons comes from oxidation-reduction reactions, not from light, so this way of making ATP is called oxidative phosphorylation. It works by the same chemiosmosis idea as in chloroplasts: a proton gradient across a membrane drives ATP synthase.

ATP synthase (also called complex V) has two parts: F₀, buried in the membrane, which forms the proton channel, and F₁, the headpiece, which makes ATP. For every ATP made, 4 H⁺ flow through F₀ from the intermembrane space into the matrix.