Q.71 The theoretical maximum number of ATP molecules produced from aerobic oxidation of glucose
by eukaryotic cells is
(A) 38 (B) 24 (C)12 (D) 8
The theoretical maximum ATP yield from complete aerobic oxidation of one glucose molecule in eukaryotic cells is 38, assuming optimal shuttle systems (malate-aspartate) transfer glycolytic NADH electrons efficiently into mitochondria. This classic calculation accounts for all substrate-level and oxidative phosphorylation yields. The correct answer is (A) 38.
Correct Answer
(A) 38
ATP Yield Breakdown
Glycolysis: 2 ATP (net substrate-level) + 2 NADH (5-6 ATP via shuttles).
Pyruvate decarboxylation: 2 NADH (5 ATP).
TCA cycle: 6 NADH (18 ATP) + 2 FADH2 (4 ATP) + 2 GTP (2 ATP).
Total: 4 substrate-level + 34 oxidative = 38 ATP (theoretical maximum using 3 ATP/NADH, 2 ATP/FADH2 ratios).
Option Analysis
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(A) 38: Correct theoretical maximum for eukaryotes with malate-aspartate shuttle (glycolytic NADH yields 3 ATP each). Prokaryotes hit exactly 38; eukaryotes typically 36-38.
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(B) 24: Wrong. Underestimates oxidative phosphorylation; possibly confuses with partial pathways or realistic yields (~30 ATP accounting for proton leak).
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(C) 12: Wrong. Roughly glycolysis + TCA substrate-level only, ignoring ETC (34 ATP from reduced carriers).
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(D) 8: Wrong. Anaerobic glycolysis (2 ATP net) + minimal extras; ignores aerobic ETC entirely.
Introduction to Maximum ATP Yield
Maximum ATP aerobic glucose oxidation eukaryotes calculations define GATE Life Sciences biochemistry, where Q.71 tests the classic 38 ATP theoretical yield vs realistic ~30-32. Assuming perfect P/O ratios (3 ATP/NADH, 2 ATP/FADH2), eukaryotes reach 38 via optimal shuttles.
Complete ATP Accounting Table
| Stage | Substrate ATP | NADH (×3 ATP) | FADH2 (×2 ATP) | Total |
|---|---|---|---|---|
| Glycolysis | 2 net | 2 (6) | – | 8 |
| Pyruvate → Acetyl-CoA | – | 2 (6) | – | 6 |
| TCA (×2) | 2 GTP | 6 (18) | 2 (4) | 24 |
| Grand Total | 4 | 10 (30) | 2 (4) | 38 |
Shuttle note: Malate-aspartate (3 ATP/NADH) vs glycerol-phosphate (2 ATP/NADH) = 38 vs 36.
Q.71 Why 38 Beats Others
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38: Theoretical max (exam standard).
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24: ~Realistic yield (proton leak, transport costs).
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12: Cytosolic/TCA substrate only.
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8: Embden-Meyerhof anaerobic.
Answer: (A)—GATE expects textbook theoretical.
Realistic vs Theoretical Debate
Modern estimates: ~2.5 ATP/NADH, ~1.5 ATP/FADH2 → ~30 ATP. But exams test 38 as “theoretical maximum” per classic bioenergetics.
GATE Biochemistry Tips
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Mnemonic: “Glyco8, Pyruvate6, TCA24 = 38.”
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High-yield: Malate shuttle = +2 ATP vs glycerol.
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PYQ trap: Prokaryotes always 38 (no shuttle cost).
Master maximum ATP aerobic glucose oxidation eukaryotes for guaranteed biochemistry success.
