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

Oligomycin plugs the proton channel of ATP synthase in respiring mitochondria supplied abundantly with ADP, Pi, and oxygen. What happens to proton pumping by the electron transport chain, and why?

A.Pumping speeds up because blocked protons reroute through Complex I instead
B.Nearly all pumping stops because the gradient climbs to its ceiling and the resulting backpressure stalls electron flow through the complexes
C.Pumping continues at full speed because the complexes ignore the gradient they create
D.Pumping reverses direction, hauling protons inward to discharge the mounting gradient

Explanation

Core Concept

With the rotor channel blocked, expelled protons lack a return path, so the electrochemical gradient steepens dramatically. Pushing further protons outward eventually becomes energetically impossible: the free energy released by descending electrons can no longer force additional charges against the maximal gradient. Electron transport consequently grinds down, NADH stays reduced, and oxygen consumption collapses. Uncouplers revive flow by leaking protons inward, which is why they raise oxygen consumption in these poisoned mitochondria.

Correct Answer

BNearly all pumping stops because the gradient climbs to its ceiling and the resulting backpressure stalls electron flow through the complexes

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