Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologyhardmcq1 pt

The herbicide DCMU blocks electron transfer from Photosystem II to plastoquinone in treated, illuminated leaves. Which pattern of photosynthetic output follows?

A.Water splitting continues normally because photolysis sits downstream of the blocked step.
B.NADPH keeps accumulating because Photosystem I draws fresh electrons straight from water.
C.Carbon fixation speeds up because unused ATP floods the Calvin cycle.
D.O2 evolution and NADPH production both stop, though cyclic electron flow around Photosystem I can still generate some ATP.

Explanation

Core Concept

Cutting the PSII-to-plastoquinone link halts linear electron flow at its source: water can no longer pass electrons onward, so O2 evolution and NADPH synthesis cease. Photosystem I and its acceptors remain functional, however, letting electrons circle PSI through ferredoxin and cytochrome b6f. That cyclic loop still moves protons and permits some ATP synthesis despite the block.

Correct Answer

DO2 evolution and NADPH production both stop, though cyclic electron flow around Photosystem I can still generate some ATP.

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