The Electron Transport Chain
High-Yield Summary
- The ETC sits in the inner mitochondrial membrane's cristae. Electrons flow through Complexes I-IV toward increasing reduction potential, releasing energy used to pump protons — building the proton-motive force (not making ATP directly).
- Complex I: NADH entry point, pumps protons, passes electrons to coenzyme Q. Complex II: FADH₂ entry point (= succinate dehydrogenase), does NOT pump protons — why FADH₂ yields less ATP than NADH. Complex III: passes electrons to cytochrome c, pumps protons. Complex IV: transfers electrons to O2 (final acceptor, forms water), pumps protons.
- Proton pumping occurs at Complexes I, III, and IV only.
- Cytosolic NADH from glycolysis needs a shuttle to enter the ETC: glycerol-3-phosphate shuttle enters at Complex II (faster, less ATP); malate-aspartate shuttle enters at Complex I (slower, full ATP yield) — this explains the 30-32 ATP range for glucose.
Key Terms
- Proton-motive force
- Electrochemical gradient (concentration + charge) across the inner mitochondrial membrane that stores energy for ATP synthase.
- Coenzyme Q (ubiquinone)
- Mobile electron carrier shuttling electrons from Complex I or II to Complex III.
- Cytochrome c
- Small soluble protein shuttling electrons from Complex III to Complex IV.
- Malate-aspartate shuttle
- Moves cytosolic NADH electrons into the matrix via oxaloacetate/malate, entering at Complex I — full ATP yield.
- Glycerol-3-phosphate shuttle
- Moves cytosolic NADH electrons via DHAP/glycerol-3-phosphate, entering at Complex II as FADH₂ — reduced ATP yield, faster.
The Four Complexes
| Complex | Electron Source / Pumps Protons? |
|---|---|
| Complex I (NADH-CoQ oxidoreductase) | NADH / Yes |
| Complex II (succinate-CoQ oxidoreductase) | FADH₂ (succinate) / No |
| Complex III (CoQ-cytochrome c oxidoreductase) | Reduced CoQ / Yes |
| Complex IV (cytochrome c oxidase) | Reduced cytochrome c / Yes |
NADH Shuttle Systems
| Shuttle | Entry Point / ATP Yield / Speed |
|---|---|
| Glycerol-3-phosphate shuttle | Complex II (as FADH₂) / Lower / Faster |
| Malate-aspartate shuttle | Complex I (as NADH) / Higher (full yield) / Slower |
Common MCAT Trap
- Complex II does not pump protons — it's the same enzyme as succinate dehydrogenase from the citric acid cycle, and this is exactly why FADH₂ yields less ATP than NADH (1.5 vs. 2.5 ATP).
- Cytosolic NADH from glycolysis can't cross the inner mitochondrial membrane directly — it must be shuttled, and which shuttle is used changes the final ATP count.
- Oxygen is the FINAL electron acceptor (at Complex IV), not a direct participant in proton pumping at earlier complexes.
Quick Recall
Which complex does NOT pump protons, and why does that matter?
What is the final electron acceptor of the ETC?
Which shuttle preserves the full ATP yield from cytosolic NADH?