Oxidative phosphorylation

Oxidative phosphorylation is the final stage of aerobic respiration, in which energy from electrons stripped off food molecules is used to attach a phosphate group to ADP, making ATP. It has two linked parts. The electron transport chain moves electrons to oxygen and pumps protons across the inner mitochondrial membrane, and chemiosmosis lets those protons flow back into the matrix through an enzyme called ATP synthase.

The idea that a proton gradient couples the two parts is the chemiosmotic hypothesis, proposed by Peter Mitchell in 1961 and now firmly supported. ATP synthase works like a tiny turbine: protons passing through the part embedded in the membrane make it rotate, and the rotation changes the shape of the enzyme's head so that ADP and phosphate are joined. When oxygen is available, this process supplies roughly 90 percent of the ATP a cell makes from glucose, far more than glycolysis and the citric acid cycle produce directly.

If the membrane becomes leaky to protons, the energy escapes as heat instead of being stored in ATP. Brown fat uses a special uncoupling protein in exactly this way to help keep newborn babies warm.