18. Oxidation of one molecule of glucose via the glycerol-phosphate shuttle produces
(A) 32 molecules of ATP
(B) 32 molecules of NADPH
(C) 30 molecules of ATP
(D) 30 molecules of NADPH

Oxidation of one glucose molecule through the glycerol-phosphate shuttle yields 30 ATP molecules, making option (C) correct. This shuttle, active in tissues like skeletal muscle and brain, transfers electrons from cytosolic NADH to FADH2, bypassing Complex I for lower energy efficiency. Understanding this process clarifies ATP calculations in aerobic respiration.

Glycerol-Phosphate Shuttle Mechanism

Cytosolic NADH from glycolysis reduces dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate via cytoplasmic glycerol-3-phosphate dehydrogenase. Glycerol-3-phosphate then diffuses to the mitochondrial inner membrane, where mitochondrial glycerol-3-phosphate dehydrogenase oxidizes it back to DHAP, reducing FAD to FADH2. FADH2 donates electrons to ubiquinone (Complex II), producing about 1.5 ATP per FADH2 instead of 2.5 ATP per NADH via the malate-aspartate shuttle.

Complete ATP Yield Calculation

Glycolysis nets 2 ATP and 2 NADH (each yielding 1.5 ATP via shuttle, totaling 3 ATP). Pyruvate decarboxylation yields 2 NADH (5 ATP total, mitochondrial). TCA cycle produces 6 NADH (15 ATP), 2 FADH2 (3 ATP), and 2 GTP (2 ATP). Overall: 2 + 3 + 5 + 15 + 3 + 2 = 30 ATP per glucose.

Why Not 32 ATP?

The 32 ATP figure applies to the malate-aspartate shuttle in liver, heart, and kidney, where cytosolic NADH electrons enter as NADH at Complex I (2.5 ATP each). Glycolytic NADH then contributes 5 ATP (not 3 ATP), raising the total to 32 ATP. Glycerol-phosphate shuttle’s FADH2 entry reduces yield by 2 ATP.

Ruling Out NADPH Options

NADPH is not produced in glucose oxidation via glycolysis, pyruvate dehydrogenase, or TCA cycle; these steps generate NADH and FADH2 for ATP synthesis. NADPH arises in pentose phosphate pathway for biosynthesis, not oxidative phosphorylation. Options (B) and (D) are incorrect as they confuse reducing equivalents and energy currency.