Q.Compare cyclic and non-cyclic photophosphorylation in detail, covering the photosystem(s) involved, the path taken by electrons, the products formed, whether O2 is evolved, and the physiological circumstances under which each is favoured.
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Concept understanding — Cyclic and Non-Cyclic Photophosphorylation
The Big Picture: Why Plants Need Two Kinds of Light Reactions
Photosynthesis has two jobs: make ATP (the energy currency) and make NADPH (the reducing power). Calvin cycle needs both — 3 ATP and 2 NADPH for every molecule of G3P it builds. But here's the problem: the light reactions don't always produce ATP and NADPH in that exact ratio. Sometimes the cell needs extra ATP without extra NADPH. That's where the two pathways come in.
Think of it like a factory. Non-cyclic photophosphorylation is the main assembly line — it produces both ATP and NADPH, plus oxygen as a byproduct. Cyclic photophosphorylation is a backup loop — it only makes ATP, no NADPH, no oxygen. The plant switches between them depending on what the Calvin cycle needs at that moment.
Non-Cyclic Photophosphorylation: The Main Line
This is the standard Z-scheme you see in every textbook. It involves both photosystems — PS II and PS I — working in series.
Step by step:
Light hits PS II (P680). An electron gets excited and is passed to a primary acceptor.
That electron is replaced by splitting water: 2H2O→4H++4e−+O2. This is the only source of oxygen in photosynthesis.
The excited electron travels down an electron transport chain (ETC) from PS II to PS I. As it moves through the cytochrome b6f complex, it pumps protons (H+) into the thylakoid lumen, building a gradient.
That proton gradient drives ATP synthase — photophosphorylation happens here. ATP is made.
The electron, now lower in energy, reaches PS I (P700). Light hits PS I, re-exciting the electron to a much higher energy level.
This high-energy electron is passed to ferredoxin (Fd) and then to NADP+ reductase, which reduces NADP+ to NADPH.
2H2O+2NADP++3ADP+3PilightO2+2NADPH+3ATP
The key point: electrons flow from water to NADP+ in a one-way path. They never return to where they started. That's why it's called non-cyclic.
Important
Non-cyclic photophosphorylation produces ATP, NADPH, and O2. It requires both PS II and PS I.
Cyclic Photophosphorylation: The Shortcut
Sometimes the Calvin cycle has enough NADPH but needs more ATP. The plant can't just make ATP without making NADPH — unless it uses a different route.
In cyclic photophosphorylation, only PS I is involved. PS II and water splitting are completely bypassed.
How it works:
Light excites PS I (P700). An electron is ejected and picked up by ferredoxin.
Instead of going to NADP+, the electron is shunted back to the cytochrome b6f complex (via plastoquinone).
It travels down the ETC again, pumping protons — generating ATP.
The electron eventually returns to PS I, filling the hole it left.
The electron goes in a circle: PS I → ferredoxin → plastoquinone → cytochrome b6f → plastocyanin → PS I. No water is split, no oxygen is released, no NADPH is made.
ADP+PilightATP(only ATP, no NADPH, no O2)
Watch out
A common mistake: thinking cyclic photophosphorylation produces NADPH. It does not. The electron never reaches NADP+ reductase — it loops back to PS I.
Why Two Pathways?
The Calvin cycle uses ATP and NADPH in a 3:2 ratio. Non-cyclic photophosphorylation gives roughly a 3:2 ratio (3 ATP, 2 NADPH). But under certain conditions — like high light intensity or when the cell is doing other metabolic work — the demand for ATP can exceed that ratio.
Cyclic photophosphorylation provides extra ATP without upsetting the NADPH balance. It's a fine-tuning mechanism.
Tip
| Feature | Non-Cyclic | Cyclic |
|---|---|---|
| Photosystems involved | PS II and PS I | Only PS I |
| Electron source | Water | PS I itself (recycled) |
| Oxygen produced | Yes | No |
| NADPH produced | Yes | No |
| ATP produced | Yes | Yes |
| Electron path | Linear (water → NADP+) | Circular (returns to PS I) |
The Bottom Line
Non-cyclic photophosphorylation is the main event — it captures light energy to make both ATP and NADPH, while releasing oxygen from water. Cyclic photophosphorylation is a supplement — it runs when the cell needs extra ATP, using only PS I and recycling electrons without making NADPH or oxygen.
Both pathways are essential for a plant to balance its energy budget under changing conditions.
Cyclic and Non-Cyclic Photophosphorylation is taught as part of Photosynthesis in Higher Plants in the Class 11 NCERT Biology textbook, which is why students often look up "Cyclic and Non-Cyclic Photophosphorylation important questions class 11" or "Cyclic and Non-Cyclic Photophosphorylation short notes" while preparing for CBSE boards and NEET Botany.
[!TLDR]
Cyclic: PS I alone, closed loop, ATP only, no O2/NADPH; favoured for extra ATP demand or far-red light. Non-cyclic: both photosystems, linear flow, ATP+NADPH+O2; the default route.
[!ANSWER]
Cyclic photophosphorylation: PS I alone, electron returns to PS I via cytochrome b6f, produces ATP only. Non-cyclic: both PS I and PS II, linear electron flow, produces ATP + NADPH + O2. Cyclic dominates when extra ATP is needed (e.g. C4/CAM) or under far-red light.
Non-cyclic photophosphorylation is the default, linear route of electron flow already described in the light reaction: electrons ejected from PS II are replaced by electrons from water splitting, travel via plastoquinone, cytochrome b6f and plastocyanin to PS I, and are ultimately used, via ferredoxin and NADP+ reductase, to reduce NADP+ to NADPH -- producing ATP (via the proton gradient), NADPH and O2 together, and requiring both photosystems operating in sequence. Cyclic photophosphorylation, by contrast, involves only Photosystem I operating independently: the electron ejected from photoactivated P700 is redirected back through the cytochrome b6f complex and plastocyanin to PS I itself, forming a closed loop rather than a linear path. Because the electron never reaches NADP+ reductase, no NADPH is formed; and because PS II and the water-splitting complex are not involved, no water is split and no O2 is evolved. The only product of cyclic flow is ATP, generated from the proton gradient the cyclic electron's passage through cytochrome b6f still contributes to. Physiologically, cyclic photophosphorylation is favoured whenever a chloroplast's demand for ATP exceeds its demand for NADPH -- for example, in C4 or CAM leaves, both of which require additional ATP beyond what the Calvin cycle alone consumes -- and it is also favoured under far-red light beyond roughly 680 nm, wavelengths that PS I (P700) can still use effectively but that PS II (P680) can barely absorb, a condition under which the linear, both-photosystems-required non-cyclic route would otherwise stall.
[!ANSWER]
Cyclic (PS I alone, closed loop): ATP only, no NADPH, no O2; favoured when extra ATP is needed (C4/CAM) or under far-red light. Non-cyclic (both photosystems, linear flow): ATP + NADPH + O2 together; the default route under normal light.
Build the comparison feature by feature -- photosystem(s) involved, electron path shape (loop vs line), products, and physiological trigger -- rather than describing each mode as an isolated narrative.
Assuming cyclic photophosphorylation is simply a "lesser" or incomplete version of non-cyclic flow -- it is a distinct, physiologically purposeful route used specifically when extra ATP (without extra NADPH) is required.
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2021Set ANNUAL1 mark
Q.What is photo-phosphorylation?
›Reveal solutionSolution
ATP is generated in the thylakoid membrane during the light reaction, using energy from the electron transport chain set up by light absorption.
During the light reaction of photosynthesis, light energy absorbed by the photosystems drives electron flow through an electron transport chain embedded in the thylakoid membrane. This electron flow pumps protons (H+) into the thylakoid lumen, building up a proton gradient across the membrane. As these protons flow back out through the enzyme ATP synthase, the energy released is used to synthesize ATP from ADP and inorganic phosphate — this process is called photophosphorylation. It occurs in two forms: cyclic photophosphorylation (only Photosystem I involved, produces only ATP) and non-cyclic photophosphorylation (both PS I and PS II involved, produces ATP, NADPH and O2).
✓Final answer
Light-driven synthesis of ATP from ADP + Pi during the light reaction of photosynthesis.