In the Z-scheme of photosynthesis, which of the following electron flow combinations are NOT correct?
- (i) P680 → PQA → PQB → Cyt b6f → Pheo → PC → P700
- (ii) P700 → A0 → A1 → FeSX → FeSA → FeSB → Fd
- (iii) P680 → Pheo → PQA → PQB → Cyt b6f → PC → P700
- (iv) P700 → A1 → A0 → FeSA → FeSB → FeSX → Fd
- A(iii) and (iv)
- B(i) and (iv)
- C(ii) and (iii)
- D(i) and (ii)
Show answer and explanation
Correct answer: (b) (i) and (iv)
Explanation
In the Z-scheme of the light reaction, electrons flow from water through photosystem II and then photosystem I to NADP⁺. The correct sequences are:
- photosystem II: P680 → pheophytin (the first acceptor) → plastoquinone A → plastoquinone B → cytochrome b6f → plastocyanin → P700. This is sequence (iii);
- photosystem I: P700 → A0 → A1 → FeSX → FeSA → FeSB → ferredoxin. This is sequence (ii).
(i) is wrong because pheophytin must come straight after P680, not after cytochrome b6f. (iv) is wrong because it puts A1 before A0 and changes the order of the iron–sulphur centres. So (i) and (iv) are not correct.