29. Which one of the following is the correct order of electron transport during light reaction in the thylakoid membrane of chloroplast?
(1) P680–àCytochrome b₆f —à PC–àPQ
(2) P680–àPC–àCytochrome b₆f–àPQ
(3) P680–àPQ–àPC–àCytochrome b₆f
(4)P680–àPQ–àCytochrome b₆f–à 4 PC

The correct answer is (4): P680 → PQ → Cytochrome b6f → PC. This order traces electron flow during the light reaction in the thylakoid membrane: electrons excited at P680 (PSII) move to plastoquinone (PQ), then to the cytochrome b6f complex, followed by plastocyanin (PC), and finally arrive at PSI (P700).​

Electron Transport Chain in Thylakoid Membrane

Introduction

Key phrase: electron transport thylakoid membrane order PSII PQ cytochrome b6f PC photosynthesis

Photosynthetic electron flow in chloroplasts follows a tightly regulated pathway across the thylakoid membrane. Proper sequence ensures optimal capture, transfer, and utilization of solar energy for production of ATP and NADPH.​

Explanation of Each Option

• (1) P680 → Cytochrome b6f → PC → PQ

  • Incorrect. The correct order has PQ immediately after P680; cytochrome b6f acts after PQ and before PC.​

• (2) P680 → PC → Cytochrome b6f → PQ

  • Incorrect. PC receives electrons after cytochrome b6f, not directly from P680; PQ should precede cytochrome b6f.​

• (3) P680 → PQ → PC → Cytochrome b6f

  • Incorrect. PC comes after cytochrome b6f, not before; the proper flow is PQ to cytochrome b6f, then to PC.​

• (4) P680 → PQ → Cytochrome b6f → PC

  • Correct. In the Z-scheme: electrons excited by PSII (P680) first pass to plastoquinone (PQ), then to cytochrome b6f, followed by the mobile carrier plastocyanin (PC), before being donated to PSI (P700).​

Key Facts: Photosynthetic Electron Pathway

• Sequence: P680 (PSII) → PQ → cytochrome b6f → PC → P700 (PSI).

• Cytochrome b6f links PSII and PSI through electron transfer and proton pumping.

• This order is essential for efficient photosynthesis and subsequent ATP, NADPH formation.​