Q.39 Arrange the following in the increasing order of amount of ATP generated by metabolism
of one molecule of the following compounds.
(i) Anaerobic catabolism of starch with 300 glucose units
(ii) Aerobic catabolism of glucose
(iii) Aerobic catabolism of acetate
(iv) Aerobic catabolism of palmitate
(A) (ii), (iv), (iii), (i) (B) (iii), (ii), (i), (iv)
(C) (iv), (ii), (i), (iii) (D) (iii), (ii), (iv), (i)
Anaerobic catabolism of acetate yields the least ATP, while starch with 300 glucose units under anaerobic conditions produces the most among the options.
Correct Answer
(D) (iii), (ii), (iv), (i)
This arranges the compounds from lowest to highest ATP yield: acetate < glucose < palmitate < starch (300 units).
ATP Calculation Breakdown
Each option’s net ATP is calculated based on standard biochemical pathways, assuming complete metabolism where applicable.
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(iii) Aerobic catabolism of acetate: Acetate (2C, CH₃COO⁻) enters as acetyl-CoA. One turn of TCA cycle yields ~10 ATP (3 NADH × 2.5 + 1 FADH₂ × 1.5 + 1 GTP), minus 2 ATP for activation. Net: ~8 ATP per acetate.
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(ii) Aerobic catabolism of glucose: One glucose (6C) via glycolysis, pyruvate dehydrogenase, TCA (2 turns), and ETC yields net 30-32 ATP (2 from glycolysis, 2 from TCA, 10 from NADH/FADH₂ in glycolysis/PDH, 20 from TCA/ETC). Standard value used: 30 ATP.
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(iv) Aerobic catabolism of palmitate: Palmitate (C₁₆H₃₂O₂, 8 acetyl-CoA units) requires 7 β-oxidation cycles (7 FADH₂ + 7 NADH = 14 + 17.5 = 31.5 ATP). 8 acetyl-CoA × 10 ATP = 80 ATP. Minus 2 ATP activation. Net: ~109.5 ATP (rounded to 108-110).
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(i) Anaerobic catabolism of starch (300 glucose units): Starch hydrolyzes to 300 glucose. Anaerobic glycolysis per glucose nets 2 ATP (4 produced – 2 invested). Total: 300 × 2 = 600 ATP.
Comparison Table
| Compound | Pathway Summary | Net ATP Yield | Order Rank |
|---|---|---|---|
| (iii) Acetate | Aerobic (acetyl-CoA + TCA) | ~8 ATP | 1st |
| (ii) Glucose | Aerobic (glycolysis + TCA + ETC) | ~30 ATP | 2nd |
| (iv) Palmitate | Aerobic (β-oxidation + TCA) | ~108 ATP | 3rd |
| (i) Starch (300 glucose) | Anaerobic (glycolysis only) | 600 ATP | 4th |
Why Other Options Wrong
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(A) (ii), (iv), (iii), (i): Wrong—acetate (iii) yields less than glucose (ii); palmitate (iv) far exceeds both.
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(B) (iii), (ii), (i), (iv): Wrong—starch (i, 600 ATP) exceeds palmitate (iv, ~108 ATP).
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(C) (iv), (ii), (i), (iii): Wrong—palmitate highest among first three; ignores acetate’s low yield.
