Question
Describe the electron transport chain (ETC) with its four complexes and ATP synthase. How does oxidative phosphorylation generate ATP?
(NEET, CBSE Class 11 — Respiration in Organisms)
Solution — Step by Step
NADH donates its electrons to Complex I on the inner mitochondrial membrane. The electrons are transferred to ubiquinone (Coenzyme Q). Complex I pumps 4 H from the matrix to the intermembrane space.
FADH donates electrons directly to Complex II (which is also the enzyme succinate dehydrogenase from the Krebs cycle). Electrons are passed to ubiquinone. Complex II does not pump protons — this is why FADH produces fewer ATP than NADH.
Ubiquinone carries electrons to Complex III, which transfers them to the mobile carrier cytochrome c. Complex III pumps 4 H to the intermembrane space.
Cytochrome c delivers electrons to Complex IV, which transfers them to O — the final electron acceptor. Oxygen combines with H and electrons to form water: . Complex IV pumps 2 H.
The proton gradient (high H in intermembrane space, low in matrix) drives protons back through ATP synthase (F-F complex). This flow of H down the gradient provides energy to synthesise ATP from ADP + Pi. This mechanism is called chemiosmosis (proposed by Peter Mitchell).
graph LR
A["NADH"] -->|"2e⁻"| B["Complex I"]
C["FADH₂"] -->|"2e⁻"| D["Complex II"]
B -->|"Ubiquinone"| E["Complex III"]
D -->|"Ubiquinone"| E
E -->|"Cytochrome c"| F["Complex IV"]
F -->|"e⁻ + H⁺ + ½O₂"| G["H₂O"]
H["H⁺ gradient"] -->|"Chemiosmosis"| I["ATP Synthase → ATP"]Why This Works
The ETC is a series of redox reactions where electrons flow from high-energy carriers (NADH, FADH) to low-energy acceptor (O). At each step, the energy released is used to pump protons across the membrane, creating an electrochemical gradient. This gradient is the “stored energy” that ATP synthase harnesses.
Per NADH: approximately 2.5 ATP (passes through Complexes I, III, IV — pumps 10 H). Per FADH: approximately 1.5 ATP (enters at Complex II, skips Complex I — pumps only 6 H).
Approximately 4 H are needed per ATP molecule synthesised by ATP synthase.
Alternative Method — The Proton Motive Force
Think of the ETC as a dam. Complexes I, III, IV are the pumps that fill the reservoir (proton gradient). ATP synthase is the turbine that uses the water flow (proton flow) to generate electricity (ATP). Without O to accept electrons at the end, the whole chain stops — just like blocking the outflow of a dam.
For NEET, the key point about cyanide poisoning: cyanide blocks Complex IV. Without Complex IV, electrons cannot reach O, the entire ETC stops, proton gradient collapses, and no ATP is made. This is why cyanide is lethal — it shuts down aerobic ATP production.
Common Mistake
Students mix up Complex II and Complex IV. Complex II is where FADH enters (succinate dehydrogenase — same enzyme as in Krebs cycle). Complex IV is where O is consumed. Also, Complex II does NOT pump protons, which is why FADH yields less ATP than NADH. This difference is a NEET favourite.