How many ATP are formed in the electron transport chain during complete oxidation of glucose?

• A. 28
• B. 36
• C. 26
• D. 34

Answer: (D)

Energy in the electron transport chain comes from the high-energy electrons carried by NADH and FADH2, and the amount of ATP produced is tied to how many NADH and FADH2 are formed during glucose oxidation. Glucose metabolism yields ten NADH and two FADH2 across glycolysis, pyruvate oxidation, and the Krebs cycle. If we use the common convention that each NADH gives about 3 ATP and each FADH2 about 2 ATP in oxidative phosphorylation, the chain can generate 10 × 3 = 30 ATP from NADH and 2 × 2 = 4 ATP from FADH2, totaling 34 ATP. Note that this value assumes a typical transport of glycolytic NADH into the mitochondria and excludes ATP produced directly by substrate-level phosphorylation (glycolysis and Krebs cycle), which would add extra ATP if counted. Some modern estimates use different shuttle mechanisms and assign around 28–32 ATP to oxidative phosphorylation, but 34 ATP is the standard figure for this question.