PSII Electron Carriers: Mobile and Tightly Bound Roles

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November 25, 2025
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37. The following are certain statements regarding the PSII electron         carrier during          the          light reaction of photosynthesis:
A. The first electron released from reaction centre P680 is transferred to Q_A to produce a plastosemiquinone.
B. Q_A is the mobile plastoquinone.
C. The first electron transferred from Q_A to Q_B converts Q_B into plastosemiquinone.
D. Q_B is tightly bound to the complex and is not mobile.
Which one of the following options represents the correct statement(s)?
(1) A and B                   (2) B and D
(3) A only                     (4) C only

The correct statement is (4) C only:

A. The first electron released from reaction centre P680 is transferred to QA to produce a plastosemiquinone.
Incorrect. The first electron from P680 goes to pheophytin (not QA). QA accepts electrons after pheophytin receives the primary electron, so the semiquinone is first produced at Pheo, then at QA after electron acceptance from Pheo.
B. QA is the mobile plastoquinone.
Incorrect. QA is a tightly bound plastoquinone in the PSII complex and is not mobile. Its role is to accept a single electron and relay it to QB.
C. The first electron transferred from QA to QB converts QB into plastosemiquinone.
Correct. The first electron transferred to QB from QA reduces QB to the semiquinone (QB- –) state; after a second electron and protonation, QB becomes plastoquinol and is released to the PQ pool.
D. QB is tightly bound to the complex and is not mobile.
Incorrect. QB is weakly bound and is exchangeable. Once fully reduced and protonated, QB diffuses into the thylakoid membrane as plastoquinol.

Introduction

Key phrase: PSII electron carrier light reaction QA QB plastosemiquinone plastoquinone mobility

Understanding the flow of electrons from P680 through QA and QB for plastosemiquinone formation is central to photosynthetic energy conversion. The distinction between mobile and non-mobile plastoquinones is vital for sustaining the electron transport chain during the light reactions.

Explanation of Each Statement

A. Incorrect. Pheophytin receives the first electron; QA does not directly accept from P680.
B. Incorrect. QA is tightly bound, not mobile.
C. Correct. QB is reduced by QA to form a semiquinone, then further reduced and released.
D. Incorrect. QB is mobile and leaves the PSII complex as plastoquinol after reduction.

Key Facts: QA and QB Plastosemiquinones

QA: Non-mobile, tightly bound one-electron acceptor.
QB: Mobile, becomes plastosemiquinone after first electron, leaves as plastoquinol.