22. Which one of the following is the function of cytochrome b₆f complex?
(1) Oxidation of water to O₂
(2) Oxidation of plasto-hydroquinone (PQH₂)
(3) Reduction of NADP⁺ to NADPH.
(4) Production of ATP.

The primary function of the cytochrome b6f complex is (2) oxidation of plasto-hydroquinone (PQH2). Cytochrome b6f links Photosystem II to Photosystem I by oxidizing PQH2 (plastoquinol), facilitating electron transfer, and contributing to the proton gradient necessary for ATP synthesis.

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Cytochrome b6f Complex in Photosynthesis

Introduction

Key phrase: cytochrome b6f oxidizes plasto-hydroquinone PQH2 photosynthesis

The cytochrome b6f complex sits at the heart of the thylakoid electron transport chain, accepting electrons from plasto-hydroquinone, oxidizing it, and transferring electrons to plastocyanin for Photosystem I, while actively pumping protons into the lumen to generate a chemiosmotic gradient for ATP production.

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Explanation of Each Option

• (1) Oxidation of water to O2

  • Incorrect. Water splitting and oxygen evolution occur at Photosystem II, not at the cytochrome b6f complex.

• (2) Oxidation of plasto-hydroquinone (PQH2)

  • Correct. Cytochrome b6f oxidizes PQH2, transferring its electrons into the transport chain and contributing to the proton gradient for ATP synthesis.

• (3) Reduction of NADP+ to NADPH

  • Incorrect. Reduction of NADP+ to NADPH is the function of Photosystem I and ferredoxin-NADP+ reductase, not cytochrome b6f.

• (4) Production of ATP

  • Indirectly correct. The cytochrome b6f complex does not directly synthesize ATP but facilitates ATP formation by generating the necessary proton gradient that powers ATP synthase.

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Key Functions of Cytochrome b6f

• Oxidizes PQH2 and passes electrons to plastocyanin.

• Pumps protons into thylakoid lumen, fueling ATP synthesis.

• Regulates electron flow between photosystems and upholds photosynthetic efficiency.