TCA cycle yield — The NADH and FADH2 produced by oxidation of one acetyl-CoA in the citric acid cycle generate approximately how many ATP (classical P/O), not counting the substrate-level GTP?

Introduction:ATP yield calculations often distinguish between ATP produced by oxidative phosphorylation from reduced cofactors and ATP (or GTP) produced directly by substrate-level phosphorylation. This question focuses only on ATP generated from the reduced cofactors derived from one acetyl-CoA in the TCA cycle, using classical P/O ratios.

Given Data / Assumptions:

• Per acetyl-CoA in TCA: 3 NADH and 1 FADH2 are produced.
• Classical assumptions: NADH → 3 ATP; FADH2 → 2 ATP.
• GTP (substrate-level) is excluded from the tally here.

Concept / Approach:Multiply the number of each reduced cofactor by its classical ATP yield and sum. Modern bioenergetics often uses ≈ 2.5 ATP per NADH and ≈ 1.5 ATP per FADH2, but many exam contexts retain the classical values for simplicity and historical continuity.

Step-by-Step Solution:

Verification / Alternative check:Electron transport chain stoichiometry in older texts gives these rounded values. With modern P/O estimates, the same calculation yields ≈ 10.5 ATP, but the classical expected answer remains 11 ATP.

Why Other Options Are Wrong:

• 3 or 6 ATP: underestimate by ignoring multiple NADH or FADH2 contributions.
• 15 ATP: exceeds the classical sum from cofactors alone.
• 8 ATP: still undercounts the three NADH contribution.

Common Pitfalls:Accidentally including the 1 GTP/ATP from succinyl-CoA synthetase, which the question explicitly excludes.

Final Answer:11 ATPs.