Oxidative phosphorylationElectron transport chain

Oxidative phosphorylation is the final stage of aerobic energy production. Electrons carried by NADH and FADH2 pass along a chain of protein complexes in the inner mitochondrial membrane, pumping protons across it; the resulting gradient drives ATP synthase, and oxygen accepts the electrons at the end, forming water.

It matters because this is where the overwhelming majority of ATP comes from — far more per unit of fuel than glycolysis manages. It is also the reason oxygen delivery limits endurance performance: without the final electron acceptor, the whole chain backs up.

In practice this pathway is why VO2 max is a meaningful number at all. Maximum oxygen consumption is essentially a measure of how fast you can run this process at full tilt.