How electrons power the pump that makes ATP
The electron transport chain (ETC) is a series of protein complexes (I, II, III, IV) embedded in the inner mitochondrial membrane. NADH and FADH2, the electron carriers from the citric acid cycle and fatty acid oxidation, donate electrons to the chain. These electrons jump from complex to complex, losing energy at each step. The energy released pumps protons from the mitochondrial matrix into the intermembrane space, creating a gradient.
Complex IV, the final step, passes electrons to oxygen, the ultimate electron acceptor. Oxygen combines with electrons and protons to form water. This coupling of electron transfer to proton pumping is the critical innovation: oxygen cannot take electrons directly from food; the chain steps it down and harnesses the energy.
The proton gradient and ATP synthesis
The proton gradient (high concentration in the intermembrane space, low in the matrix) stores energy like a battery. Protons flow back through ATP synthase, a remarkable molecular machine, and the energy released drives the phosphorylation of ADP to ATP. This is chemiosmotic coupling: the electrochemical gradient, not chemical bonds, directly powers ATP synthesis.
In total, glucose oxidation yields about 2.5 ATP per NADH and 1.5 per FADH2. The citric acid cycle produces 3 NADH and 1 FADH2 per acetyl-CoA, plus 1 ATP directly, so roughly 10 ATP per acetyl-CoA. Since one glucose yields 2 acetyl-CoA (after glycolysis), the total is approximately 30-32 ATP per glucose, far more than the ATP yielded from glucose fermentation alone.