Q.73 Choose the correct sequence of electron transport clutång electron transpon chain of light
reaction:
(A) Pheophytin
(B) Plastoquinone
(C) Cytb6-f complex
(D) Plastocyanin
The correct sequence of electron transport in the light reaction’s electron transport chain is (A) Pheophytin → (B) Plastoquinone → (C) Cytb6-f complex → (D) Plastocyanin.
Option Breakdown
Pheophytin acts as the primary electron acceptor in Photosystem II (PSII), capturing excited electrons from chlorophyll P680 after light absorption. Plastoquinone (PQ) then receives these electrons from pheophytin, functioning as a mobile carrier that shuttles them while facilitating proton translocation into the thylakoid lumen. The cytochrome b6f complex (Cytb6-f) accepts electrons from reduced plastoquinone, pumps protons to build the gradient for ATP synthesis, and passes electrons onward. Plastocyanin, a soluble copper protein, finally transports these electrons from Cytb6-f to Photosystem I (PSI).
The electron transport chain light reaction sequence is a cornerstone of photosynthesis, powering ATP and NADPH production in thylakoid membranes. This sequence starts in Photosystem II (PSII) and drives non-cyclic electron flow, essential for competitive exams like GATE Life Sciences.
Core Components Explained
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Pheophytin (A): First stable acceptor in PSII; receives high-energy electrons from excited P680 chlorophyll, preventing recombination.
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Plastoquinone (B): Lipid-soluble carrier; oxidized PQ (Q_A, Q_B) reduces to PQH₂, releasing 2H⁺ to the lumen.
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Cytb6-f Complex (C): Proton-pumping complex akin to Complex III in mitochondria; oxidizes PQH₂ via Q-cycle, boosting pmf.
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Plastocyanin (D): Cu-containing mobile carrier; diffuses electrons to PSI’s P700, linking PSII to PSI.
This electron transport chain light reaction sequence ensures efficient energy conversion, with water splitting replenishing PSII electrons.
Visual Flow in Table
| Step | Component | Role in Sequence |
|---|---|---|
| 1 | Pheophytin (A) | Accepts e⁻ from PSII P680* |
| 2 | Plastoquinone (B) | Transfers e⁻, forms PQH₂ (2H⁺ released) |
| 3 | Cytb6-f (C) | Pumps H⁺ via Q-cycle |
| 4 | Plastocyanin (D) | Delivers e⁻ to PSI P700 |
For GATE prep, memorize Pheophytin → Plastoquinone → Cytb6-f → Plastocyanin as the PSII-to-PSI bridge.
