Electron Transport System and Oxidative Phosphorylation

The electron transport system (ETS), located in the inner mitochondrial membrane, is a chain of protein complexes and mobile carriers that passes electrons from NADH and FADH2 to oxygen, using the released energy to pump protons and build a gradient. Oxidative phosphorylation is the partner process in which ATP synthase uses that gradient to make ATP. Together they produce the great majority of ATP in aerobic respiration.

This matters because roughly 26 to 28 ATP of the total about 30 to 32 per glucose come from oxidative phosphorylation. It is the reason we breathe oxygen — oxygen is the final electron acceptor that keeps the chain flowing. Poisons like cyanide kill precisely by blocking this chain.

Key Concepts

Complex I (NADH dehydrogenase) accepts electrons from NADH and pumps 4 H+. Complex II (succinate dehydrogenase) accepts FADH2 electrons from the Krebs cycle but pumps no protons, which is why FADH2 yields less ATP. Ubiquinone (CoQ) and cytochrome c are mobile carriers. Complex III pumps 4 H+ and Complex IV pumps 2 H+ while passing electrons to oxygen, forming water: ½O2 + 2e- + 2H+ → H2O. ATP synthase (Complex V) lets protons flow back, rotating to join ADP + Pi → ATP. Total pumped per NADH ≈ 10 H+, per FADH2 ≈ 6 H+.

Formulas

Final step: ½O2 + 2H+ + 2e- → H2O; oxygen is the terminal acceptor, without which electrons back up and the chain halts. ATP yield: ATP per NADH ≈ 10 H+ pumped ÷ about 4 H+ per ATP (including phosphate transport) = 2.5 ATP. ATP per FADH2 ≈ 6 ÷ 4 = 1.5 ATP. Oxidative phosphorylation overall: NADH + H+ + ½O2 + 2.5ADP + 2.5Pi → NAD+ + H2O + 2.5ATP (average stoichiometry). Assumptions: intact membrane, normal coupling, no uncoupling; yields are statistical averages, not fixed integers, which is why total per glucose is quoted as about 30-32 ATP.

Worked Example

Calculate total ATP from one glucose using modern values. Glycolysis: net 2 ATP + 2 cytosolic NADH via malate-aspartate shuttle = 2 × 2.5 = 5 ATP, subtotal 7. Link reaction: 2 NADH = 2 × 2.5 = 5 ATP, subtotal 12. Krebs (2 turns): 6 NADH = 15, 2 FADH2 = 3, 2 GTP = 2, subtotal 20. Grand total = 7 + 5 + 20 = 32 ATP. Using the glycerol-phosphate shuttle instead (cytosolic NADH = 1.5 each), total = 30 ATP. Older textbooks used 3 and 2 ATP per NADH/FADH2 and quoted 38; modern measured values give 30-32.

Common Misconceptions

• Oxygen makes ATP directly. Correction: Oxygen only accepts electrons to form water; ATP is made by ATP synthase using the proton gradient.
• Each NADH always gives exactly 3 ATP. Correction: Measured yields average 2.5 due to transport costs and proton leak; older integer values are outdated.
• Complex II pumps protons like the others. Correction: Complex II pumps none, so FADH2 entering there yields less ATP than NADH.

Real-World Applications

Medicine explains cyanide and carbon monoxide poisoning as Complex IV blockade, stopping ATP production within minutes. Brown fat uncoupling protein deliberately short-circuits the gradient for heat in infants. Metformin, a diabetes drug, mildly affects Complex I. Exercise physiologists measure oxygen consumption to estimate oxidative phosphorylation rates in athletes. Researchers designing artificial mitochondria and bio-batteries copy this chemiosmotic machinery. Endurance training increases mitochondrial number, raising oxidative capacity and stamina.

The ETS passes electrons from NADH and FADH2 to oxygen, pumping about 10 and 6 protons respectively, and ATP synthase converts that gradient into 2.5 and 1.5 ATP. This chemiosmotic payoff is aerobic life's main energy engine.