Step by Step
Old
The old number: 36-38 ATP
Older textbooks stated 36-38 ATP per glucose. This used P/O ratios of 3 per NADH and 2 per FADH₂ and assumed 100% coupling efficiency. This is the theoretical maximum under ideal conditions — rarely achieved in living cells.
New
The modern number: ~30-32 ATP
Current P/O ratios based on actual H⁺ stoichiometry: NADH ≈ 2.5 ATP, FADH₂ ≈ 1.5 ATP. From one glucose: Glycolysis: 2 ATP + 2 NADH (cytoplasmic). Pyruvate oxidation: 2 NADH (mitochondrial). Krebs cycle: 2 GTP + 6 NADH + 2 FADH₂. ETC: 10 NADH × 2.5 + 2 FADH₂ × 1.5 = 25 + 3 = 28 ATP. Total ≈ 30-32 ATP.
Memory trick: Old textbook = 38. Real life = 30-32. The difference = leaky mitochondria, transport costs.
Why
Why the actual yield is lower
Several factors reduce efficiency: (1) Cytoplasmic NADH (from glycolysis) must use a shuttle to enter the mitochondria — costing ATP. (2) The H⁺ gradient is used not just for ATP synthesis but also for mitochondrial transport and heat. (3) Proton leak across the IMM (thermogenin in brown fat is an extreme example).
Sub
Substrate-level vs oxidative phosphorylation
Substrate-level phosphorylation: direct ATP synthesis (glycolysis steps 7,10 and Krebs GTP). No membrane gradient needed. Oxidative phosphorylation: uses the H⁺ gradient across the IMM via ATP synthase. Produces ~90% of total ATP. Without O₂, only substrate-level phosphorylation works (net 2-4 ATP per glucose).
Applied Walkthrough
1
Glycolysis: 2 ATP (substrate-level) + 2 NADH (cytoplasmic — worth ~1.5 each via malate-aspartate shuttle = 3 ATP).
2
Pyruvate oxidation (2 pyruvates): 2 NADH (mitochondrial — worth 2.5 each = 5 ATP).
3
Krebs cycle (2 turns): 2 GTP + 6 NADH (worth 2.5 each = 15 ATP) + 2 FADH₂ (worth 1.5 each = 3 ATP).
4
Total: 2 + 3 + 5 + 2 + 15 + 3 = 30 ATP. Add 2 GTP = ~32 ATP total. Compare to anaerobic: only 2 ATP.
Exam Application
Exams test both the old (36-38) and new (~30-32) ATP numbers — know which your course uses. Key breakdown: glycolysis=2 ATP, Krebs=2 GTP, ETC=~26-28 ATP. Know that ~90% of ATP comes from oxidative phosphorylation (ETC), not substrate-level phosphorylation. Without oxygen, cells make only 2 net ATP from glycolysis.
⚠ Common Trap
Students use 38 ATP when their exam expects 30-32 (or vice versa — check your course). Also: the cytoplasmic NADH from glycolysis is worth LESS than mitochondrial NADH because it costs energy to shuttle electrons into the mitochondria. The shuttle mechanism used (malate-aspartate vs glycerol-3-phosphate) affects the final count.
✓ Quick Self-Check
1. What is the modern estimate for ATP yield per glucose?
~30-32 ATP (using P/O ratios of 2.5 per NADH and 1.5 per FADH₂).
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2. Why is the actual ATP yield lower than the theoretical maximum?
Proton leak, shuttle costs for cytoplasmic NADH, and use of the H⁺ gradient for transport beyond just ATP synthesis.
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3. What percentage of ATP comes from oxidative phosphorylation?
~90% — the ETC/ATP synthase generates the vast majority of ATP from complete glucose oxidation.
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4. How much ATP does anaerobic glycolysis produce?
Only 2 net ATP per glucose — no ETC, so only substrate-level phosphorylation.
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5. What is the difference between substrate-level and oxidative phosphorylation?
Substrate-level: direct ATP synthesis from a phosphorylated substrate (no membrane gradient needed). Oxidative phosphorylation: uses the H⁺ gradient across the IMM via ATP synthase.
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