Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologyhardmcq1 pt

Electrons arriving on FADH2 support synthesis of less ATP than the same number of electrons arriving on NADH. Which structural fact explains the shortfall?

A.FADH2 molecules are larger and physically obstruct proton channels.
B.FADH2 enters the chain at Complex II, downstream of Complex I, so its electrons skip one proton-pumping station.
C.FADH2 carries only half as many electrons as NADH does.
D.Oxygen refuses electrons whose history includes Complex II.

Explanation

Core Concept

NADH deposits electrons at Complex I, which uses the released energy to pump protons before passing them along the chain. FADH2, reduced during the Krebs cycle by succinate dehydrogenase, feeds electrons directly into Complex II, which pumps nothing. Those electrons still traverse Complexes III and IV, but one full pumping step is missing, so a smaller proton-motive force forms per FADH2 oxidized.

Correct Answer

BFADH2 enters the chain at Complex II, downstream of Complex I, so its electrons skip one proton-pumping station.

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