Electron transport chain
Also known as: ETC, respiratory chain
The electron transport chain is a series of protein complexes in the inner mitochondrial membrane that transfers electrons from NADH and FADH2 to oxygen, pumping protons to create the gradient that powers ATP synthesis.
The electron transport chain (ETC) is the final stage of aerobic cellular respiration, carried out by a series of protein complexes (I-IV) embedded in the inner mitochondrial membrane. Electron carriers generated by glycolysis and the citric acid cycle — NADH and FADH2 — donate their high-energy electrons to the chain: NADH at complex I and FADH2 at complex II. Electrons then pass through coenzyme Q and cytochrome c to complex IV, where oxygen serves as the final electron acceptor, combining with electrons and protons to form water.
As electrons move down the chain, complexes I, III, and IV pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient. Protons flow back into the matrix through ATP synthase, which uses that flow to phosphorylate ADP into ATP — a mechanism called chemiosmosis, and the heart of oxidative phosphorylation. The process is efficient: complete oxidation of one glucose molecule yields roughly 30-32 ATP, the great majority produced here rather than in glycolysis.
Because everything depends on oxygen accepting the final electrons, the chain halts without it, forcing cells into anaerobic metabolism and lactate production. Classic poisons target specific steps: cyanide and carbon monoxide inhibit complex IV, while uncouplers dissipate the proton gradient, releasing energy as heat instead of ATP.
The ETC is tested across surprisingly different exams: USMLE Step 1 emphasizes the complexes, poisons, and uncouplers; the MCAT ties it into bioenergetics and thermodynamics; and the CSCS covers it as the oxidative energy system that fuels endurance exercise.
Key takeaways
- The ETC is a set of inner-mitochondrial-membrane complexes that pass electrons from NADH and FADH2 to oxygen, the final electron acceptor.
- Complexes I, III, and IV pump protons into the intermembrane space, building an electrochemical gradient.
- ATP synthase uses the proton gradient to make ATP — chemiosmosis, the core of oxidative phosphorylation.
- One glucose yields roughly 30-32 ATP with the ETC, versus 2 from glycolysis alone.
- Inhibitors like cyanide (complex IV) and uncouplers are high-yield on USMLE Step 1; the MCAT and CSCS test the ETC in bioenergetics.