16. The donor and acceptor of electron for cytb/f₆ are
(1) PQ and UQ        (2) UQ and Cyt C
(3) UQ and PC        (4) PQ and PC

The correct answer is (4): PQ (plastoquinone) and PC (plastocyanin) serve as the donor and acceptor of electrons for the cytochrome b6f complex in the photosynthetic electron transport chain. PQ donates electrons to cytochrome b6f, which then passes them to PC, facilitating the connection between PSII and PSI.​

Donor and Acceptor of Electrons for Cytochrome b6f

Introduction

Key phrase: cytochrome b6f electron donor acceptor PQ plastocyanin photosynthesis

The cytochrome b6f complex acts as a key electron transporter in the thylakoid membrane, linking Photosystem II (PSII) and Photosystem I (PSI). Understanding its electron donors and acceptors, especially plastoquinone (PQ) and plastocyanin (PC), is critical for grasping the fundamentals of the photosynthetic electron transport chain.​

Explanation of Each Option

• (1) PQ and UQ

  • Incorrect. PQ (plastoquinone) is the correct electron donor, but ubiquinone (UQ) is involved in mitochondrial respiration, not in chloroplasts; it is not the electron acceptor for cytochrome b6f in photosynthesis.​

• (2) UQ and Cyt C

  • Incorrect. UQ and cytochrome c are integral to the mitochondrial electron transport chain, not the chloroplast’s thylakoid electron flow.​

• (3) UQ and PC

  • Incorrect. While PC (plastocyanin) is the correct acceptor, UQ is not involved in chloroplast electron transfer.​

• (4) PQ and PC

  • Correct. PQ collects electrons from PSII and donates them to the cytochrome b6f complex, which then transfers those electrons to PC. PC carries them to PSI, completing the key link in oxygenic electron transport.​

Key Facts: Cytochrome b6f Electron Carriers

• PQ (plastoquinone): Electron donor from PSII to cytochrome b6f.

• PC (plastocyanin): Electron acceptor from cytochrome b6f to PSI.

• This pathway is exclusive to chloroplast electron transport and critical for energy conversion.​