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MBBS QUESTION #10340
Question 1
In the electron transport chain, NADH donates electrons at Complex I, whereas FADH2 (generated by succinate dehydrogenase, Complex II) enters at a point that bypasses Complex I entirely. This explains why oxidation of NADH yields more ATP than oxidation of an equivalent amount of FADH2, because:
  • FADH2 bypasses the proton-pumping step of Complex I, translocating fewer protons overall and thus generating less ATP per molecule oxidized✔️
  • Both NADH and FADH2 enter the chain at the identical point with identical ATP yield
  • NADH enters the chain directly at Complex IV, bypassing Complexes I-III
  • FADH2 enters at Complex I, the same site used by NADH
Correct Answer Explanation
Because FADH2 delivers electrons directly to Complex II (bypassing the proton-pumping activity of Complex I), fewer protons are ultimately translocated across the inner mitochondrial membrane per FADH2 oxidized compared with per NADH oxidized — the classical basis for NADH being assigned a higher ATP yield (traditionally ~2.5 ATP) than FADH2 (~1.5 ATP) in ATP-yield calculations.