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Photosynthesis

AP Biology· difficulty 3/5

The illustration shows a thylakoid disk with lumen interior and stroma exterior.

Thylakoid PSII b6f PSI ATP synthase Lumen (high H+) Stroma (low H+) H+

Where is the highest concentration of H+ during active photosynthesis?

  • A

    Stroma

  • B

    Cytoplasm

  • C

    Thylakoid lumen

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  • D

    Outer chloroplast envelope

Explanation

Photolysis (in the lumen) and proton pumping by cyt b6f both load H+ into the lumen. ATP synthase lets H+ flow back to the stroma, building ATP.

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