Q.52 Which one of the following statements is INCORRECT with respect to cyclic
photophosphorylation?
(A) ATP is generated without concomitant formation of NADPH
(B) Electron flows from Photosystem I to cytochrome bf complex
(C) Photosystem II does not participate in cyclic photophosphorylation
(D) Cyclic photophosphorylation occurs when NADP+/NADPH ratio is high

The incorrect statement is (D) “Cyclic photophosphorylation occurs when NADP⁺/NADPH ratio is high.” In reality, cyclic photophosphorylation is favored when NADPH accumulates, i.e., when the NADP⁺/NADPH ratio is low and extra ATP (but not more NADPH) is needed.​

Concept overview: cyclic photophosphorylation

In cyclic photophosphorylation, only Photosystem I (PSI) participates, and electrons excited from P700 cycle back through the electron transport chain to PSI instead of reducing NADP⁺. This cyclic electron flow generates a proton gradient used to synthesize ATP but does not produce NADPH or release oxygen.​

Option (A): ATP is generated without concomitant formation of NADPH

• In cyclic photophosphorylation, electrons from PSI travel through carriers (including the plastoquinone pool and cytochrome b₆f) and return to PSI, powering proton pumping across the thylakoid membrane.​

• Because electrons are cycled back and are not finally transferred to NADP⁺, no NADPH is formed; only ATP is synthesized from the generated proton motive force, so option (A) is correct.​

Option (B): Electron flows from Photosystem I to cytochrome bf complex

• During cyclic electron flow around PSI, electrons excited at P700 are transferred to ferredoxin and then re-injected into the electron transport chain at the plastoquinone/cytochrome b₆f level.​

• In this pathway, PSI donates electrons that ultimately pass through the cytochrome b₆f complex, which uses their energy to pump protons and thus support ATP synthesis, so option (B) is correct.​

Option (C): Photosystem II does not participate in cyclic photophosphorylation

• Canonical cyclic photophosphorylation in higher plants involves PSI and the cytochrome b₆f complex but not PSII; it typically occurs in stromal lamellae regions that lack PSII and NADP⁺ reductase.​

• Since no water splitting, oxygen evolution, or PSII-derived electron input occurs in the cyclic route, option (C) is correct.​

Option (D): Cyclic photophosphorylation occurs when NADP⁺/NADPH ratio is high

• Cyclic electron flow is promoted when the chloroplast has excess NADPH relative to ATP, i.e., NADP⁺ is relatively scarce and the NADP⁺/NADPH ratio is low, so that further linear (non-cyclic) electron flow and NADPH production would be unnecessary or even harmful.​

• Under these conditions, shifting to cyclic photophosphorylation increases ATP output without additional NADPH formation, helping to rebalance the ATP/NADPH demand of the Calvin cycle; therefore, stating that it occurs when the NADP⁺/NADPH ratio is high is wrong, making option (D) the incorrect statement asked for in the MCQ.​

Exam takeaway: cyclic vs non-cyclic in one view

• Non-cyclic photophosphorylation: involves PSII and PSI, splits water, produces ATP, NADPH, and O₂; electrons flow linearly from water to NADP⁺.​

• Cyclic photophosphorylation: involves only PSI and cytochrome b₆f, no water splitting, no NADPH or O₂ production, but extra ATP is generated, especially when ATP demand exceeds NADPH demand.​

For this question, the final answer is: (D) is incorrect.