Question
How many ATP molecules are produced per turn of the Krebs cycle? Trace the complete cycle from Acetyl CoA entry to oxaloacetate regeneration, listing all products at each step.
This is a direct NEET 2023 question type. The examiners love asking “per turn” vs “per glucose” — and that distinction alone decides whether you score or lose 4 marks.
Solution — Step by Step
Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). The enzyme is citrate synthase. This step is irreversible — the cell commits Acetyl CoA to oxidation here.
Citrate is rearranged to isocitrate, then oxidative decarboxylation gives α-ketoglutarate (5C). One CO₂ is released, one NADH is produced. Think of this as the cycle “exhaling” its first carbon.
Another oxidative decarboxylation. α-ketoglutarate (5C) → Succinyl CoA (4C), releasing the second CO₂ and producing the second NADH. At this point, both carbons that entered as Acetyl CoA have left as CO₂.
Succinyl CoA → Succinate yields 1 GTP (equivalent to 1 ATP — this is substrate-level phosphorylation, not oxidative). Succinate → Fumarate produces 1 FADH₂. Fumarate → Malate adds water (hydration). Each step is enzyme-specific; NEET asks FAD-linked dehydrogenase for the succinate step.
Malate is oxidised to oxaloacetate, producing the third NADH. The cycle is now complete — oxaloacetate is free to accept another Acetyl CoA.
Products per turn of the Krebs cycle:
| Product | Count |
|---|---|
| CO₂ | 2 |
| NADH | 3 |
| FADH₂ | 1 |
| GTP (≈ ATP) | 1 |
Direct ATP per turn: 1 GTP + 2.5×3 (from NADH) + 1.5×1 (from FADH₂) = 10 ATP (using modern P/O ratios from NCERT 11th, Class 11 Bio chapter 14).
Why This Works
The Krebs cycle is fundamentally a machine for extracting electrons, not ATP. The three NADHs and one FADH₂ carry those electrons to the electron transport chain, where the real ATP bonanza happens.
The cycle must regenerate oxaloacetate to keep running — that’s why it’s a cycle and not a pathway. Every intermediate is technically catalytic; only Acetyl CoA’s two carbons are consumed per turn.
The GTP produced directly is substrate-level phosphorylation — same mechanism as glycolysis’s ATP production. No membrane potential needed; the enzyme succinyl CoA synthetase just transfers the phosphate group directly.
Alternative Method — Counting for Per Glucose (NEET Trap Question)
When the question asks per glucose molecule, remember: one glucose produces 2 Acetyl CoA after pyruvate decarboxylation. So the cycle turns twice.
And the pyruvate decarboxylation step itself adds another 2 NADH (often missed). Total from Krebs + pyruvate oxidation: 8 NADH + 2 FADH₂ + 2 GTP per glucose.
NEET frequently frames this as “how many NADH are produced in mitochondria per glucose?” Answer: 8 (2 from pyruvate decarboxylation + 6 from Krebs). The 2 NADH from glycolysis are produced in the cytoplasm — a separate story involving the malate-aspartate or glycerol-3-phosphate shuttle.
Common Mistake
Students write “3 ATP per turn of Krebs cycle” — confusing the GTP with total ATP yield. The 1 GTP is only the direct yield. The NADH and FADH₂ are reducing equivalents, not ATP. Their ATP equivalent (~10 ATP total per turn) comes from oxidative phosphorylation in the inner mitochondrial membrane, not from the Krebs cycle itself. NEET 2023 specifically tested this distinction — “substrate-level phosphorylation in Krebs cycle yields ___ ATP” — and the answer is 1, not 3.
Also watch out: students often confuse which step produces FADH₂. It’s specifically the succinate dehydrogenase step (succinate → fumarate), not malate dehydrogenase. The mnemonic — succinate dehydrogenase is the only Krebs enzyme embedded in the inner mitochondrial membrane, which is why it uses FAD rather than NAD⁺.