Why does the cell bother with cyclic photophosphorylation when non-cyclic already gives ATP and NADPH?
Aarav Kulkarni11th Sample
11 Oct 2026 · 1 min read
Checklist time for the light reaction. I've got the non-cyclic route down (I think):
- PS II (P680) absorbs light, an electron is excited and goes to a primary acceptor, then through the electron transport chain (with plastoquinone, cytochrome b6f, plastocyanin) to PS I.
- PS II gets its electron replaced by splitting of water: .
- PS I (P700) absorbs light and passes the electron on, finally to NADP⁺, forming NADPH.
- The proton gradient across the thylakoid membrane drives ATP synthesis.
So non-cyclic makes ATP, NADPH and O₂.
Now cyclic: only PS I is involved, the electron returns to PS I through the cytochrome complex, making ATP only. No NADPH, no oxygen. And NCERT says cyclic happens when only wavelengths beyond 680 nm are available.
My problem is the why. If non-cyclic already gives both ATP and NADPH, why does a cell need a route that gives only ATP? Also I have a doubt on location: NCERT says cyclic occurs in stroma lamellae. But what's special about stroma lamellae? And water splitting: on which side of the thylakoid does it happen, and which ions are needed?
Sorry for the long one, it's a checklist, it's what I do 😅
7 answers & replies
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[+][–]Meera Patel11thSample· 8d ago
The Calvin cycle needs more ATP than NADPH. Per CO₂ fixed in the C3 cycle, you need 3 ATP and 2 NADPH, i.e. a 3:2 ratio. Non-cyclic flow on its own is generally thought to fall a bit short on ATP for that ratio, so the cell tops up with extra ATP from cyclic flow, which gives ATP without using up the electrons needed for NADPH. In C4 plants, the extra ATP demand is even higher.
[+][–]Siddharth RaoDropperSample· 8d ago
Location: grana thylakoids have both PS I and PS II and NADP reductase, so they are where non-cyclic happens. Stroma lamellae (the unstacked membranes) have PS I and ATP synthase but lack PS II and NADP reductase. With no PS II, there's no water splitting, no O₂, and no NADPH formed there, so cyclic flow is what's possible there. That's the logic behind NCERT's statement.
[+][–]Aarav Kulkarni11thSample· 8d agoAuthor
The stroma lamellae thing is the missing piece in my checklist, adding "PS I + ATP synthase only" next to it.
[+][–]Ananya Reddy11thSample· 8d ago
Water splitting happens on the inner (lumen) side of the thylakoid membrane, associated with PS II. Mn is the essential ion (in the oxygen-evolving complex), and Ca²⁺ and Cl⁻ are also required. NCERT says these ions (Mn, Ca, Cl) are involved. The 4 H⁺ released go into the lumen, which adds to the proton gradient and helps ATP synthesis.
[+][–]Kavya NaikDropperSample· 8d ago
Doesn't the cyclic pathway also release some oxygen since light is involved? I thought O₂ always comes with light reactions.
[+][–]Nisha Verma12thSample· 8d ago
Kavya, no. O₂ comes from splitting water by PS II. In cyclic, PS II isn't used, no water is split, so no O₂ and no NADPH. An easy summary: cyclic = PS I only → ATP only. Non-cyclic = PS II + PS I → ATP + NADPH + O₂. An MCQ will often try that "light reaction therefore O₂" trap.
[+][–]Ishita GoyalDropperSample· 8d ago
Extra "when": cyclic can run when the NADPH is already high and NADP⁺ isn't available to accept electrons (so the chain is clogged), or when the light is only far-red (beyond 680 nm), where PS II can't be excited. This is a good point to mention in a descriptive answer. For one-mark MCQs just know the difference in products.
