The Krebs cycle left us with 10 NADH and 2 FADH₂ per glucose. Oxidative phosphorylation turns their electrons into ATP. It works like the membrane potential of a neuron: a pump builds a gradient of charged particles across a membrane, and the gradient does work as it runs down.
- Electron transport: NADH gives electrons to complex I; FADH₂ gives them to complex II. They pass through ubiquinone (Q), complex III, cytochrome c and complex IV, losing energy at each step.
- Pumping: complexes I, III and IV use that energy to pump H⁺ from the matrix into the intermembrane space.
- Oxygen: at complex IV, oxygen takes the spent electrons and H⁺, forming water. It is the final electron acceptor.
- Chemiosmosis: H⁺ flows back into the matrix through ATP synthase, which spins and joins ADP and phosphate into ATP.
Key idea
Electrons go along the chain and end on O₂. Protons (H⁺), not electrons, flow through ATP synthase. Each NADH yields about 2.5 ATP and each FADH₂ about 1.5; with the 4 ATP from earlier stages, a glucose gives about 30 to 32 ATP.
Why it works
Why does FADH₂ give less ATP? It hands its electrons to complex II, which does not pump H⁺, so they skip the first pumping step. Why is oxygen essential? Without it, complex IV cannot unload electrons; the chain backs up, pumping stops, NADH cannot be recycled to NAD⁺, and the Krebs cycle halts too. Only glycolysis, with fermentation to recycle NAD⁺, can continue.
Worked examples
Worked example: Estimating the ATP
Use 2.5 ATP per NADH and 1.5 per FADH₂ to estimate the ATP from one glucose.
- 10 NADH × 2.5 = 25 ATP.
- 2 FADH₂ × 1.5 = 3 ATP.
- Oxidative phosphorylation: about 28 ATP; plus 2 from glycolysis and 2 from the Krebs cycle: 32.
- Moving the NADH made in glycolysis into the mitochondrion can cost some, so books give 30 to 32.
Answer: About 30 to 32 ATP per glucose.
Worked example: A leaky membrane
A chemical makes the inner mitochondrial membrane leaky to H⁺. Electron transport keeps running. What happens to ATP production and to the cell's temperature?
- H⁺ leaks back into the matrix without passing through ATP synthase.
- The gradient is lost, so little ATP is made.
- The energy of the gradient is released as heat instead.
Answer: ATP production falls and more heat is released. (Brown fat uses a protein that does this on purpose to keep some mammals warm.)
Common mistake
Saying electrons pass through ATP synthase. Electrons travel along the chain and end on oxygen; it is the flow of protons back through ATP synthase that makes ATP. A check: ATP synthase is driven by the H⁺ gradient, and anything that destroys the gradient stops ATP synthesis even while electrons keep flowing.