Cyclic vs Non-Cyclic Photophosphorylation

OCR specification module 5.2.1(f)(i) requires you to distinguish between cyclic and non-cyclic photophosphorylation. Both produce ATP, but only non-cyclic also produces reduced NADP and O₂. Many students find it hard to remember which is which — this lesson explains the electron paths clearly and highlights why plants need both.

Key Definitions:

• Non-cyclic photophosphorylation — the production of ATP using light energy, in a linear electron pathway starting from water and ending at NADP.
• Cyclic photophosphorylation — the production of ATP using light energy, in which electrons cycle back to PSI instead of reducing NADP.
• PSI (Photosystem I) — reaction centre P700; can operate cyclically on its own.
• PSII (Photosystem II) — reaction centre P680; required for non-cyclic flow; splits water.

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The Two Pathways at a Glance

flowchart LR
    subgraph NonCyclic[Non-Cyclic Photophosphorylation]
        W1[H2O] -->|Photolysis| P2a[PSII]
        P2a --> ETC_a[ETC - H+ pumping] --> P1a[PSI]
        P1a --> FD1[Ferredoxin] --> NADP1[Reduced NADP]
    end
    subgraph Cyclic[Cyclic Photophosphorylation]
        P1b[PSI] --> FD2[Ferredoxin]
        FD2 --> ETC_b[ETC - H+ pumping]
        ETC_b --> P1b
    end

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Non-Cyclic Photophosphorylation

• Uses both PSI and PSII.
• Electrons follow a linear path: H₂O → PSII → ETC → PSI → Ferredoxin → NADP.
• Water is split (photolysis) to replace the electrons lost from PSII.
• Products: ATP, reduced NADP and O₂.
• Provides both the reducing power (reduced NADP) and the energy (ATP) needed for the Calvin cycle.
• This is the main pathway of photosynthesis under normal conditions.

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Cyclic Photophosphorylation

• Uses only PSI.
• Electrons excited at PSI are passed to ferredoxin, but instead of going to NADP they are passed back through an electron transport chain to PSI. The electrons "cycle" around.
• As the electrons pass through the ETC, the energy released pumps H⁺ into the thylakoid space — the proton gradient drives ATP synthase, making extra ATP.
• No photolysis (no water is split).
• No reduced NADP is produced.
• No O₂ is produced.
• Only product: ATP.

Why Do Plants Need Cyclic Photophosphorylation?

The Calvin cycle uses more ATP than reduced NADP — for every three CO₂ molecules fixed, the cycle uses 9 ATP and 6 reduced NADP (a ratio of 3:2). Non-cyclic photophosphorylation alone would produce ATP and reduced NADP in roughly equal amounts, leaving a shortfall of ATP. Cyclic photophosphorylation allows the plant to top up ATP supplies without accumulating unneeded reduced NADP. It therefore acts as a balancing mechanism to match ATP:reduced NADP supply to Calvin cycle demand.

Cyclic photophosphorylation is also important in guard cells, where ATP is needed to pump K⁺ ions into the cell to open the stomata — but no Calvin cycle operates in guard cells.

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