Estimate ATP yield per glucose under ideal conditions.

• A. 28 ATP
• B. 30–32 ATP
• C. 24 ATP
• D. 36 ATP

Answer: (B)

ATP yield from glucose in aerobic respiration comes from glycolysis, pyruvate oxidation, the TCA cycle, and oxidative phosphorylation, and it depends on how efficiently electrons from NADH are shuttled into the mitochondria. In glycolysis you net 2 ATP and produce 2 NADH; pyruvate oxidation gives 2 more NADH; the TCA cycle yields 2 ATP (as GTP), 6 NADH, and 2 FADH2. That adds up to 10 NADH and 2 FADH2, plus 4 ATP from substrate-level phosphorylation. If each NADH yields about 2.5 ATP and each FADH2 about 1.5 ATP in oxidative phosphorylation, the oxidative-phosphorylation contribution is 10×2.5 = 25 ATP from NADH plus 2×1.5 = 3 ATP from FADH2, totaling 28 ATP. Adding the 4 ATP from substrate-level phosphorylation gives 32 ATP. In practice, the exact amount depends on how the cytosolic NADH from glycolysis is shuttled into the mitochondria; some shuttles give about 1.5 ATP per NADH, lowering the total to around 30 ATP. That’s why the commonly cited range is about 30–32 ATP per glucose under ideal aerobic conditions, with 32 being the maximum.