28. In photosynthetic electron transport, electrons travel through carriers organized in the “Z-scheme”. The following are indicated as directions of electron flow:
A. P680–àPQ_A–àPQ_B–àCytb₆f–àPheo–àPC–àP700

B. P700–àA₀–àA₁–àFeS_X–à FeS_A–à FeS_B–àFd

C. P680–àPheo–àPQ_A–àPQ_B–àCytb₆f–àPC–àP700

D. P700–àA₁–àA₀–àFeS_B–à FeS_A–à FeS_x–àFd

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

The correct answer is (2) B and C:

• B: P700 → A0 → A1 → FeSX → FeSA → FeSB → Fd

• C: P680 → Pheo → PQA → PQB → Cytb6f → PC → P700

Statement B correctly illustrates Photosystem I electron flow, and C correctly illustrates the Z-scheme flow linking PSII and PSI. Here is a detailed explanation of each option:

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Z-Scheme Electron Transport: Sequence and Carriers

Introduction

Key phrase: Z-scheme electron transport pathway order PSII PSI carriers photochemistry

The Z-scheme describes the linear electron flow in photosynthesis, beginning at Photosystem II (PSII), traveling through intermediates, and ending at Photosystem I (PSI) before reducing NADP+. The correct order of electron flow is essential to understand energy conversion mechanisms in green plants.

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Explanation of Each Statement

• A. P680 → PQA → PQB → Cytb6f → Pheo → PC → P700

  • Incorrect. Pheo (pheophytin) receives electrons first after P680 excitation, before PQA or PQB. The correct order from P680 is Pheo → PQA → PQB → Cytb6f → PC → P700; therefore, the order in A is not accurate.

• B. P700 → A0 → A1 → FeSX → FeSA → FeSB → Fd

  • Correct. This reflects the electron transfer sequence within PSI: P700 (chlorophyll a) becomes excited, transfers electrons to A0 (chlorophyll), then A1 (phylloquinone), passes through iron-sulfur clusters (FeSX, FeSA, FeSB), and finally to ferredoxin (Fd).

• C. P680 → Pheo → PQA → PQB → Cytb6f → PC → P700

  • Correct. This sequence is the textbook Z-scheme for linear electron flow merging PSII and PSI: electrons excited at P680 go to pheophytin, then quinones (QA and QB), cytochrome b6f, plastocyanin, and ultimately PSI (P700).

• D. P700 → A1 → A0 → FeSB → FeSA → FeSX → Fd

  • Incorrect. The actual flow in PSI is P700 → A0 → A1 → FeSX → FeSA → FeSB → Fd; order of A0 and A1 is reversed in D, which is not correct for PSI.

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Key Facts: Z-Scheme Photosynthetic Electron Flow

• PSII (P680) to Pheo → QA → QB → Cytb6f → PC → PSI (P700).

• PSI (P700) to A0 → A1 → FeSX → FeSA → FeSB → ferredoxin (Fd).

• The electron carriers are precisely ordered for efficient energy conversion and NADP+ reduction.