Cyclic Photophosphorylation
சுழற்சி ஒளி பாஸ்போரிலேற்றம்
Stop confusing Cyclic with Non-Cyclic Photophosphorylation. In the stroma lamellae, there is no NADP reductase enzyme. If you think NADPH is produced here, you have already lost four marks.
The idea
When only PS I is functional, the electron is circulated within the photosystem. The excited electron returns to P700 via the electron transport chain. This process synthesizes ATP but produces neither NADPH nor oxygen. It occurs exclusively in the stroma lamellae membranes, which lack both PS II and NADP reductase.
Why it matters
To meet the cell's demand for ATP when NADPH levels are sufficient or when light wavelengths are beyond 680 nm.
NEET lens
NEET focuses on the 'Where' and 'What'. Memorize: Stroma lamellae = Cyclic = ATP only. Traps involve options claiming oxygen evolution or NADPH synthesis in this cycle. Always check if the question specifies the location; if it's stroma lamellae, PS II is absent.
Clinical significance
This mechanism acts as a metabolic safety valve during high light intensity. If plants cannot balance the ATP/NADPH ratio, photo-oxidative damage occurs to the thylakoid membranes. Ecosystem stability depends on this precise ATP production to sustain the Calvin cycle under varying light conditions.
Introduction
Cyclic photophosphorylation is a type of light reaction involving only Photosystem I (PSI), where electrons cycle back from ferredoxin to the plastoquinone pool, producing ATP but no NADPH and no O₂.
Core concept
Pathway: Light → PSI (P700) → excited electrons → ferredoxin → (instead of going to NADP⁺) → back to plastoquinone → cytochrome b6f → plastocyanin → PSI. Protons pumped through cytochrome b6f complex → drives ATP synthesis. Products: ONLY ATP. No NADPH produced. No water split. No O₂ released. Occurs when: (1) NADPH/NADP⁺ ratio is high (NADP⁺ is limiting), (2) Extra ATP needed.
Go deeper
Physiological significance: Calvin cycle needs ATP and NADPH in 3:2 ratio. Non-cyclic photophosphorylation produces them in roughly 3:2. But certain conditions (e.g., stressed conditions) may shift the ratio needed. Cyclic photophosphorylation provides flexibility — can generate extra ATP without more NADPH.
From Class 10
The normal light reactions use both photosystems. But sometimes only PSI is used, and electrons cycle back — this is cyclic photophosphorylation.