Q.102 Which ONE of the following combinations of products will result, when 3 molecules of acetyl CoA
is fed into TCA cycle?
(A) 1 ATP, 2 CO2, 3 NADH, and 1 FADH2
(B) 3 ATP, 6 CO2, 9 NADH, and 3 FADH2
(C) 3 ATP, 3 CO2, 3 NADH, and 3 FADH2
(D) 38 ATP, 6 CO2, 3 NADH, and 12 FADH2
TCA Cycle Products from 3 Acetyl CoA Molecules
Discover the correct products when 3 acetyl CoA enters the TCA cycle, with detailed analysis of all options for exam prep.
Correct Answer
The correct option is (B) 3 ATP, 6 CO2, 9 NADH, and 3 FADH2.
Per acetyl CoA molecule in the TCA (Krebs) cycle, one turn produces 2 CO2, 3 NADH, 1 FADH2, and 1 ATP (or GTP equivalent via substrate-level phosphorylation). For 3 molecules, multiply yields: 6 CO2, 9 NADH, 3 FADH2, and 3 ATP.
TCA Cycle Overview
Acetyl CoA (2 carbons) condenses with oxaloacetate (4 carbons) to form citrate (6 carbons), followed by oxidations, decarboxylations, and reductions[web:1]. Key steps release 2 CO2 (isocitrate to α-ketoglutarate; α-ketoglutarate to succinyl CoA), generate 3 NADH (isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, malate dehydrogenase), 1 FADH2 (succinate dehydrogenase), and 1 ATP (succinyl CoA synthetase).
This matches standard biochemistry for complete oxidation per turn.
Option Analysis
- (A) 1 ATP, 2 CO2, 3 NADH, 1 FADH2: Matches single acetyl CoA only, undercounts for 3 molecules.
- (B) 3 ATP, 6 CO2, 9 NADH, 3 FADH2: Exact match—3x per-turn products.
- (C) 3 ATP, 3 CO2, 3 NADH, 3 FADH2: Wrong CO2 (should be 6) and NADH (should be 9) counts.
- (D) 38 ATP, 6 CO2, 3 NADH, 12 FADH2: Confuses direct TCA yields with full ETC ATP (~10 ATP per NADH, ~6 per FADH2, totaling ~30 ATP from 3 turns); lists exaggerated ATP and incorrect reduced cofactor numbers.
| Option | ATP | CO2 | NADH | FADH2 | Why Incorrect? |
|---|---|---|---|---|---|
| A | 1 | 2 | 3 | 1 | For 1 acetyl CoA only |
| B | 3 | 6 | 9 | 3 | Correct |
| C | 3 | 3 | 3 | 3 | Low CO2/NADH |
| D | 38 | 6 | 3 | 12 | Includes ETC ATP; wrong cofactors |
