Cyclic Photophosphorylation: Mechanism and When It Occurs
Biology · Photosynthesis in Higher Plants · NEET
Cyclic photophosphorylation is the making of ATP when only Photosystem I (PS I, P700) is working. The electron leaves P700, passes through the electron carriers, and comes back to P700 again, so it moves in a circle. This makes only ATP, no NADPH and no oxygen. Memory hook: "One system, one circle, one product" = PS I only, electron goes round, only ATP.
In cyclic photophosphorylation the electron from P700 (PS I) passes through the electron carriers and returns to P700, making only ATP. PS II, water splitting, oxygen and NADPH are all absent.
Your doubts, answered
Does cyclic photophosphorylation produce NADPH?
No. Cyclic photophosphorylation makes only ATP. The electron from P700 is not passed to NADP+, it is sent back to P700 in a circle. So no NADPH + H+ is formed. Only non-cyclic photophosphorylation (PS II plus PS I working in series) makes both ATP and NADPH.
Why does cyclic photophosphorylation use only PS I?
In cyclic flow the electron must return to the same reaction centre it came from. P700 (PS I) sends out an electron, the carriers pass it along, and it comes back to P700. PS II is not needed because water is not split and no electron gap has to be filled from water. So only PS I is functional here.
Where in the chloroplast does cyclic photophosphorylation occur?
It happens in the stroma lamellae, as per NCERT. The stroma lamellae membranes have PS I but lack PS II and also lack the NADP reductase enzyme. Because NADP reductase is missing, NADPH cannot be made there, so only ATP is produced. The grana membranes have both PS I and PS II.
Does cyclic photophosphorylation release oxygen?
No. Oxygen comes only from the splitting of water (photolysis), which needs PS II. Since PS II is not involved in cyclic flow, water is not split and no oxygen is released. This is a common NEET point of confusion.
When does cyclic photophosphorylation happen?
It happens when only light beyond 680 nm is available (only PS I active), or when the cell needs extra ATP but not NADPH. For example, the Calvin cycle uses more ATP than NADPH, so cyclic flow makes the extra ATP to balance the demand.
⚠️ The NEET trap ✗ Cyclic photophosphorylation involves both PS I and PS II and makes ATP and NADPH. ✓ Cyclic photophosphorylation involves only PS I (P700) in the stroma lamellae and makes only ATP, no NADPH and no oxygen. Both photosystems and both products belong to non-cyclic flow. 🧠 Read the location and the products together, not separately.
Real NEET questions
NEET 2019 / 2021 / ReNEET 2026
Which of the following statements is incorrect?
A · Grana lamellae have both PS I and PS II
B · Cyclic photophosphorylation involves both PS I and PS II ✓
C · Both ATP and NADPH + H+ are synthesized during non-cyclic photophosphorylation
D · Stroma lamellae have PS I only and lack NADP reductase
Solution: Cyclic photophosphorylation occurs when only PS I is functional; the electron is circulated within PS I and only ATP is made. It does not involve PS II. So the statement that it involves both photosystems is incorrect. All the other statements are correct as per NCERT.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
