Step by Step
Loc
Location — inner mitochondrial membrane
The ETC is embedded in the inner mitochondrial membrane (IMM). It consists of four protein complexes (I, II, III, IV) and two mobile carriers (ubiquinone/CoQ and cytochrome c). The intermembrane space (IMS) accumulates H⁺ pumped by Complexes I, III, and IV.
Flow
Electron flow through the complexes
NADH → Complex I → CoQ → Complex III → Cyt c → Complex IV → O₂ (water). FADH₂ → Complex II → CoQ (bypasses Complex I — fewer H⁺ pumped). Complex II does NOT pump protons — key exam point. O₂ is the final electron acceptor, reduced to H₂O.
Memory trick: NADH takes the express lane (Complex I). FADH₂ takes the slower lane (Complex II, skips I). Both end at oxygen.
Chemi
Chemiosmosis — ATP synthesis
H⁺ pumped into the IMS creates a proton gradient (proton-motive force). H⁺ flows back into the matrix through ATP synthase (Complex V) — the rotary motor that phosphorylates ADP → ATP. Each NADH ≈ 2.5 ATP. Each FADH₂ ≈ 1.5 ATP. This is oxidative phosphorylation.
Memory trick: ATP synthase = a water wheel. H⁺ flowing through it spins the wheel and makes ATP.
Inh
ETC inhibitors — clinical relevance
Complex I: rotenone (pesticide). Complex III: antimycin A. Complex IV: cyanide (CN⁻) and carbon monoxide (CO) — bind cytochrome a₃, block O₂ binding → cells can't make ATP → rapid death. Uncouplers (DNP, thermogenin in brown fat): allow H⁺ to leak back without making ATP → energy released as heat.
Applied Walkthrough
1
NADH from glycolysis, pyruvate oxidation, and the Krebs cycle delivers electrons to Complex I.
2
Electrons flow through CoQ to Complex III, then cytochrome c, then Complex IV. At each step, H⁺ is pumped into the IMS.
3
The resulting H⁺ gradient drives H⁺ back through ATP synthase → rotational energy → ATP synthesis (oxidative phosphorylation).
4
O₂ accepts the final electrons at Complex IV → reduced to H₂O. Without O₂, electrons back up and the entire ETC stops.
Exam Application
Exams test the order of complexes (I→CoQ→III→Cyt c→IV), that Complex II doesn't pump protons, that O₂ is the final electron acceptor, the yield per NADH (~2.5 ATP) vs FADH₂ (~1.5 ATP), and ETC inhibitors (cyanide/CO block Complex IV). Chemiosmosis and the proton-motive force are essential concepts.
⚠ Common Trap
Students think Complex II pumps protons — it does NOT. This is why FADH₂ produces less ATP than NADH (FADH₂ enters at CoQ, bypassing Complex I's proton pumping). Also: the ETC does not directly make ATP — it builds the H⁺ gradient. ATP synthase (Complex V) makes the ATP using that gradient.
✓ Quick Self-Check
1. Where is the ETC located?
In the inner mitochondrial membrane.
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2. What is the final electron acceptor in the ETC?
Oxygen (O₂) — reduced to water (H₂O) at Complex IV.
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3. Why does FADH₂ produce less ATP than NADH?
FADH₂ enters the ETC at CoQ (bypassing Complex I) — fewer protons are pumped, so less ATP is generated (~1.5 vs ~2.5 per NADH).
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4. How does cyanide poisoning kill cells?
Cyanide binds cytochrome a₃ in Complex IV, blocking O₂ binding — the ETC stops, the H⁺ gradient collapses, and ATP synthesis ceases.
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5. What does an uncoupler do?
It allows H⁺ to leak back into the matrix without passing through ATP synthase — the gradient is dissipated as heat instead of ATP.
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