Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologyhardmcq1 pt

During the light-dependent reactions, the interior of the thylakoid becomes far more acidic than the surrounding stroma. Which process adds protons to the thylakoid lumen most directly?

A.Water molecules are split at photosystem II, releasing hydrogen ions inside the lumen
B.Carbon fixation by rubisco builds sugars in the stroma
C.NADP+ accepts electrons and protons along the stromal face
D.Hydrogen ions reenter the lumen through ATP synthase channels

Explanation

Core Concept

Photolysis at the oxygen-evolving complex of photosystem II deposits protons directly into the lumen as water donates electrons to the reaction center. The cytochrome complex adds still more protons by pumping them across the thylakoid membrane during electron flow. The resulting gradient powers ATP synthase as protons exit back into the stroma.

Correct Answer

AWater molecules are split at photosystem II, releasing hydrogen ions inside the lumen

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