30. A ‘Z’ scheme describes electron transport in 0₂^– evolving photosynthetic organisms. The direction of electron flow is presented in the following sequences:

A.P680^*–àPheophytin–àQ_A–àQ_B–àPC–àCytochrome b₆f–àP700

B. P700^*–àPhylloquinone–àFeS_A–àFeS_B–àFeS_X–àFd

C. P680^*–àPheophytin–àQ_A–àQ_B–àCytochrome b₆f–àPC–àP700

D. P700^*–àPhylloquinone–àFeS_X–àFeS_A–àFeS_B–àFd

Which one of the following combinations is correct?
(1) A and B      (2) B and C
(3) C and D      (4) D and A

The correct answer is (3) C and D:

• C. P680 → Pheophytin → QA → QB → Cytochrome b6f → PC → P700*

  • This is the correct order of electron flow in the Z-scheme starting at PSII (P680*), then via pheophytin, quinones QA and QB, cytochrome b6f, plastocyanin (PC), and finally to PSI (P700). This correctly follows the canonical sequence of carriers in oxygenic photosynthesis.​​

• D. P700 → Phylloquinone → FeSX → FeSA → FeSB → Fd*

  • This is the correct order of electron flow in PSI after P700 absorption: electrons pass to phylloquinone, then sequentially through the iron-sulfur centers (FeSX, FeSA, FeSB), and finally to ferredoxin (Fd), which ultimately transfers electrons to NADP+.​

Z-Scheme Electron Flow: Thylakoid Pathways

Introduction

Key phrase: Z-scheme electron flow photosynthesis sequence PSII PSI carriers

The Z-scheme describes the energy and electron movement in the light reactions of photosynthesis, highlighting the direction and order through specialized protein complexes and cofactors from water to NADP+.​

Explanation of Each Statement

• A. P680 → Pheophytin → QA → QB → PC → Cytochrome b6f → P700*

  • Incorrect. The electron sequence should go P680* → Pheophytin → QA → QB → Cytochrome b6f → PC → P700. PC follows cytochrome b6f, not precedes it.​

• B. P700 → Phylloquinone → FeSA → FeSB → FeSX → Fd*

  • Incorrect. The electron flow post-P700 should be Phylloquinone → FeSX → FeSA → FeSB → Fd. The order of iron-sulfur centers is not correct in this option.​

• C. P680 → Pheophytin → QA → QB → Cytochrome b6f → PC → P700*

  • Correct. This matches the established Z-scheme pathway for PSII to PSI electron transfer, with carriers precisely ordered.​

• D. P700 → Phylloquinone → FeSX → FeSA → FeSB → Fd*

  • Correct. This is the correct downhill electron flow for PSI after light absorption and primary charge separation.​

Key Facts: Z-Scheme Electron Transport

• Sequence PSII: P680* → Pheo → QA → QB → Cytb6f → PC → PSI (P700).​

• Sequence PSI: P700* → Phylloquinone → FeSX → FeSA → FeSB → Fd.​

• This organization supports efficient energy conversion and oxygen evolution in green plants.