Biochemistry
Electron Transport Chain
Series of complexes that transfer electrons to O₂ and pump protons — drives ATP synthesis
The electron transport chain (ETC) is a series of protein complexes in the inner mitochondrial membrane (or bacterial plasma membrane) that transfer electrons from NADH/FADH₂ to oxygen. Energy released pumps H⁺ across membrane → electrochemical gradient. ATP synthase uses this gradient to make ATP (chemiosmotic coupling — Mitchell 1961, Nobel 1978). Four main complexes (I-IV); plus Q (ubiquinone) and cytochrome c shuttle electrons. Final electron acceptor: O₂ → H₂O. Couples electron flow to ATP synthesis. Most ATP from cellular respiration generated here (~26-28 per glucose).
- LocationInner mitochondrial membrane (eukaryotes); plasma (bacteria)
- SourceNADH, FADH₂ from Krebs cycle
- Final acceptorO₂ → H₂O
- OutputH⁺ gradient (drives ATP synthesis)
- ComplexesI, II, III, IV; plus Q and cyt c
- ATP per NADH~2.5 (theoretical max 3)
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Why ETC matters
- Energy. Most cellular ATP from here.
- Aerobic respiration. Why we need oxygen.
- Mitochondrial diseases. ETC defects cause severe disease.
- Cancer. Altered respiration in cancer cells.
- Aging. Mitochondrial decline with age.
- Toxicology. Many poisons target ETC.
- Drug discovery. Targeting in cancer, parasitic infections.
Common misconceptions
- ETC makes ATP directly. Makes H⁺ gradient; ATP synthase makes ATP.
- O₂ accepts electrons immediately. Multi-step transfer through complexes.
- ETC simple sequence. Multi-component, regulated process.
- Same yield from NADH and FADH₂. NADH 2.5 ATP; FADH₂ 1.5.
- ETC only in mitochondria. Bacterial plasma membrane too.
- Energy directly used. Stored in H⁺ gradient first.
Frequently asked questions
How does ETC work?
NADH and FADH₂ donate electrons to chain. Electrons pass through complexes I-IV (or II-IV for FADH₂). At each step, energy released — used to pump H⁺ from matrix to intermembrane space. Final acceptor: O₂ (Complex IV) → H₂O. Result: H⁺ gradient. ATP synthase uses gradient to make ATP. NADH yields ~2.5 ATP; FADH₂ ~1.5.
What are the four complexes?
(1) Complex I (NADH dehydrogenase): NADH → Q; pumps 4 H⁺. (2) Complex II (succinate dehydrogenase): FADH₂ → Q; doesn't pump H⁺. (3) Complex III (cytochrome bc1): Q → cyt c; pumps 4 H⁺. (4) Complex IV (cytochrome oxidase): cyt c → O₂ → H₂O; pumps 2 H⁺. Plus Q (mobile in membrane) and cyt c (mobile in intermembrane space) shuttle electrons.
What's chemiosmotic theory?
Peter Mitchell (1961). Electron flow through ETC pumps H⁺ across membrane → electrochemical gradient. Gradient drives ATP synthesis when H⁺ flows back through ATP synthase. Initially controversial (people thought direct chemical coupling); now well-established. Mitchell Nobel 1978. Foundation of bioenergetics.
What's ATP synthase?
Molecular machine that synthesizes ATP from ADP + Pi using H⁺ gradient. F0 (membrane portion): rotates as H⁺ flows through. F1 (cytoplasmic): synthesizes ATP via rotational motor mechanism. Boyer (Nobel 1997) explained mechanism: F1 has 3 catalytic sites cycling through binding, ATP synthesis, release. ~3 H⁺ per ATP made. Walker (Nobel 1997) determined structure.
What happens without O₂?
ETC stops. Without final electron acceptor, electrons accumulate; NADH can't be reoxidized; Krebs cycle stops. Cells switch to fermentation: glycolysis only, regenerating NAD⁺ via lactate or ethanol. Far less ATP. Important: cells with high ETC dependence (cardiac muscle) suffer most from O₂ deprivation. Hypoxia/ischemia cause cell death.
What about poisons of ETC?
Many toxins inhibit ETC. (1) Cyanide: blocks Complex IV (cytochrome oxidase). Death rapid. (2) Carbon monoxide: also blocks Complex IV. (3) Rotenone (pesticide): Complex I. (4) Antimycin: Complex III. (5) Azide: Complex IV. Result: cells can't make ATP; rapid death. Why CO and CN are deadly.
What's the proton gradient strength?
~3-4 pH units across membrane (matrix more basic, intermembrane more acidic). Plus electrical potential ~140 mV (matrix negative). Combined proton motive force (PMF): ~200 mV. Drives ATP synthesis when H⁺ flows back through ATP synthase. Also drives some other transport (Ca²⁺ uptake into mitochondria).