Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologymediummcq1 pt

During oxidative phosphorylation, what happens to the electrons at the very end of the electron transport chain, and why is this handoff necessary?

A.They combine with oxygen and protons to form water; without this sink the chain backs up and ATP production stalls
B.They are warehoused in the intermembrane space for later reuse
C.They convert NAD+ directly back into NADH, extending the energy supply
D.They escape the cell as heat because the chain cannot retain them safely

Explanation

Core Concept

At the end of the mitochondrial electron-transport chain, complex IV transfers electrons to oxygen, which is reduced to water. Removing electrons at the end of the chain allows continued electron flow and supports proton pumping by complexes I, III, and IV. Complex II transfers electrons into the chain but does not pump protons, so 'every earlier complex' is incorrect.

Correct Answer

AThey combine with oxygen and protons to form water; without this sink the chain backs up and ATP production stalls

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