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

Compared with dark-adapted chloroplasts, illuminated chloroplasts show a thylakoid lumen that has become:

A.more acidic, because water splitting releases H+ into the lumen and the cytochrome b6f complex pumps additional H+ in from the stroma.
B.more basic, because NADPH formation removes protons from the lumen.
C.unchanged in pH, because membranes block all ion movement.
D.more acidic, because Rubisco releases stored CO2 inside the lumen.

Explanation

Core Concept

Illumination starts linear electron flow. Splitting water at Photosystem II deposits protons inside the lumen, and cytochrome b6f moves stromal protons inward as electrons pass to Photosystem I. Both contributions accumulate H+ in the enclosed lumen, dropping its pH well below that of the stroma. ATP synthase then exploits this gradient as protons exit back to the stroma.

Correct Answer

Amore acidic, because water splitting releases H+ into the lumen and the cytochrome b6f complex pumps additional H+ in from the stroma.

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