Oxidative Phosphorylation
High-Yield Summary
- The ETC builds a proton gradient (proton-motive force) but does NOT make ATP directly — ATP synthase does, via chemiosmotic coupling.
- ATP synthase has two parts: F₀ (membrane-spanning proton channel, drives mechanical rotation as protons flow down their gradient) and F₁ (catalytic head, uses that rotation to phosphorylate ADP → ATP).
- Losing the proton gradient (e.g., damaged inner mitochondrial membrane) stops ATP synthase entirely and halts oxidative phosphorylation.
- Total yield per glucose: ~4 ATP direct (2 from glycolysis + 2 GTP from TCA), ~25 ATP from 10 NADH (×2.5), ~3 ATP from 2 FADH₂ (×1.5) = approximately 30-32 ATP.
- The range (30 vs. 32) depends on which shuttle moves glycolysis's cytosolic NADH into the mitochondria: malate-aspartate (Complex I, full yield) vs. glycerol-3-phosphate (Complex II, reduced yield).
Key Terms
- Chemiosmotic coupling
- Coupling proton flow down its gradient directly to ATP synthesis, carried out by ATP synthase.
- F₀
- Membrane-spanning proton channel portion of ATP synthase; proton flow drives its physical rotation.
- F₁
- Catalytic head of ATP synthase; uses rotational energy to phosphorylate ADP into ATP.
Total ATP Yield From One Glucose
4 ATP (direct) + 25 ATP (10 NADH × 2.5) + 3 ATP (2 FADH₂ × 1.5) ≈ 30–32 ATP
- 4 ATP = 2 from glycolysis substrate-level phosphorylation + 2 GTP from 2 TCA turns
- 10 NADH = 2 glycolysis + 2 PDC + 6 TCA (2 turns × 3)
- 2 FADH₂ = From 2 TCA turns (1 each)
- The range exists because glycolysis's 2 cytosolic NADH must be shuttled into the mitochondria — malate-aspartate (Complex I) preserves full yield, glycerol-3-phosphate (Complex II) yields less.
Energy Flow: Electrons to ATP
- 1Electrons from NADH/FADH₂ move through ETC Complexes I-IV
- 2Energy released pumps protons into the intermembrane space, building the proton-motive force
- 3Protons flow back into the matrix through ATP synthase's F₀ channel, driving rotation
- 4F₁ uses that rotational energy to phosphorylate ADP + Pi → ATP
Common MCAT Trap
- The ETC itself makes zero ATP — all ATP synthesis is at ATP synthase, downstream of the proton gradient the ETC builds.
- 30-32 ATP is a range, not a fixed number, specifically because of shuttle choice for glycolytic NADH — don't treat it as a single hardcoded value on the exam.
- A compromised inner mitochondrial membrane (proton leak) stops ATP synthase even if the ETC itself is intact — the gradient, not the complexes, is what ATP synthase needs.
Quick Recall
What are ATP synthase's two functional regions?
What is the total approximate ATP yield from one glucose molecule?
Why does the ATP yield vary between 30 and 32?