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

How does cyclic electron flow in chloroplasts differ from the linear electron flow that supplies the Calvin cycle?

A.Electrons travel from water through both photosystems to NADP+ exactly as in linear flow, only twice as fast
B.Photosystem II splits carbon dioxide to release oxygen while photosystem I assembles glucose
C.The pathway oxidizes surplus NADPH back to NADP+ to shield the cell from excess reducing power
D.Electrons excited in photosystem I return through plastoquinone and cytochrome b6f, driving extra ATP synthesis but yielding no NADPH and no oxygen

Explanation

Core Concept

In the cyclic route photosystem I passes its excited electrons to ferredoxin, but instead of reducing NADP+ they flow back through plastoquinone and the cytochrome b6f complex toward plastocyanin. Proton pumping along this loop enlarges the thylakoid gradient and drives additional ATP synthesis. Because photosystem II never participates, water is not split, so neither O2 nor NADPH is produced.

Correct Answer

DElectrons excited in photosystem I return through plastoquinone and cytochrome b6f, driving extra ATP synthesis but yielding no NADPH and no oxygen

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