Oxidative phosphorylation is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. It uses the energy released by the electron transport chain to drive the synthesis of ATP, producing the vast majority of cellular energy.

How Oxidative Phosphorylation Works

The Electron Transport Chain

NADH and FADH2 from glycolysis and the citric acid cycle donate electrons to protein complexes embedded in the inner mitochondrial membrane. These complexes (Complex I through IV) pass electrons through a series of redox reactions, each with a progressively higher reduction potential.

Proton Pumping

As electrons move through the chain, the energy released is used to pump protons (H+) from the mitochondrial matrix into the intermembrane space. Complexes I, III, and IV each contribute to building this proton gradient. The result is a high concentration of protons in the intermembrane space and a low concentration in the matrix.

The Proton Motive Force

The electrochemical gradient created by the proton concentration difference and the membrane potential is called the proton motive force. This force stores potential energy, much like water behind a dam.

ATP Synthesis

Protons flow back into the matrix through ATP synthase (Complex V), a molecular turbine. As protons pass through the enzyme, it rotates, driving the phosphorylation of ADP to ATP. Approximately three to four ATP molecules are produced per ten protons that flow through.

Oxygen as Terminal Electron Acceptor

Oxygen is the final electron acceptor at Complex IV. It accepts electrons and combines with protons to form water. Without oxygen, the electron transport chain backs up, and oxidative phosphorylation stops.

ATP Yield

Complete oxidation of one glucose molecule yields approximately 30–32 ATP molecules through oxidative phosphorylation, far exceeding the 2 ATP produced by glycolysis alone.