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

A herbicide binds the plastoquinone pool of thylakoid membranes and prevents electrons from moving between photosystem II and photosystem I. Which immediate consequence best explains why treated plants stop growing?

A.Electrons can no longer reach photosystem I and NADP+, so NADPH output collapses and the Calvin cycle loses its reducing power
B.Water splitting stops instantly because plastoquinone supplies the electrons used to oxidize water
C.The herbicide dissolves thylakoid membranes so protons leak freely into the stroma
D.Rubisco is destroyed because reactive radicals accumulate in the stroma

Explanation

Core Concept

Plastoquinone shuttles excited electrons from photosystem II toward the cytochrome b6f complex and onward to photosystem I. Blocking that shuttle strands electrons at PSII, so very few arrive at photosystem I and ferredoxin NADP+ reductase. NADPH production therefore plunges, and without NADPH the reduction of 3-PGA in the Calvin cycle cannot proceed. Carbon fixation stops even though rubisco itself remains untouched, so new sugars are no longer built for growth.

Correct Answer

AElectrons can no longer reach photosystem I and NADP+, so NADPH output collapses and the Calvin cycle loses its reducing power

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