How do cytosolic glycolysis and mitochondrial ATP production differ in substrates, products, and overall ATP yield per glucose?

• A. Cytosolic glycolysis yields 4 ATP per glucose and no NADH; mitochondria yield all NADH.
• B. Cytosolic glycolysis yields 2 ATP and 2 NADH; mitochondrial processes yield about 26–30 ATP per glucose.
• C. Cytosolic glycolysis yields 30 ATP; mitochondria yield 2 ATP.
• D. Cytosolic glycolysis yields no ATP; mitochondrial processes yield all ATP.

Answer: (B)

In aerobic glucose oxidation, the energy payoff is split between cytosolic glycolysis and mitochondrial respiration. Glycolysis in the cytosol converts one glucose to two pyruvate, producing a net of 2 ATP by substrate-level phosphorylation and generating 2 NADH. Those NADH must be shuttled into the mitochondria to fuel oxidative phosphorylation, and the exact ATP gained from them depends on the shuttle system (about 2.5 ATP per cytosolic NADH with the malate-aspartate shuttle, or about 1.5 ATP with the glycerol-3-phosphate shuttle).

Inside the mitochondria, the two acetyl-CoA units derived from glucose enter the TCA cycle, yielding 2 GTP (equivalent to 2 ATP), 6 NADH, and 2 FADH2 per glucose. The NADH and FADH2 feed the electron transport chain to drive most of the ATP production, roughly 20–25 ATP from NADH and FADH2 combined depending on the exact coupling efficiency and shuttle type, plus the 2 ATP from glycolysis and the 2 ATP equivalent from the TCA step, giving a total on the order of 26–30 ATP per glucose.

So, cytosolic glycolysis provides a small, direct ATP payoff and the NADH to be used by mitochondria, while mitochondrial respiration supplies the majority of ATP, yielding about 26–30 ATP per glucose.