Question
Compare Photosystem I (PS I) and Photosystem II (PS II) in terms of their reaction centre, location in thylakoid, and role. Differentiate between cyclic and non-cyclic photophosphorylation. Which process produces both ATP and NADPH, and which produces only ATP?
(NEET 2023, similar pattern)
Solution — Step by Step
| Feature | Photosystem I (PS I) | Photosystem II (PS II) |
|---|---|---|
| Reaction centre | P700 (absorbs 700 nm) | P680 (absorbs 680 nm) |
| Location | Stroma lamellae + grana | Grana thylakoids only |
| Electron acceptor | Ferredoxin → NADP⁺ | Pheophytin → PQ |
| Source of electrons | PS II (via ETC) or cyclic | Water (photolysis: ) |
PS II is called “II” because it was discovered second, even though it acts first in the Z-scheme. Don’t let the numbering confuse you.
This is the main pathway. Both PS II and PS I work in series:
- PS II absorbs light → P680 gets excited → electrons pass through the electron transport chain (PQ → Cyt b6f → PC)
- ATP is synthesised as electrons flow downhill through the ETC (chemiosmosis)
- PS I absorbs light → P700 gets excited → electrons go to ferredoxin → NADP⁺ reductase reduces NADP⁺ to NADPH
- PS II replaces its lost electrons by splitting water (photolysis of water)
Products: ATP + NADPH + O₂. This is non-cyclic because electrons flow in one direction — from water to NADP⁺.
Only PS I participates. Excited electrons from P700 go to ferredoxin, but instead of reducing NADP⁺, they cycle back through Cyt b6f and plastocyanin to P700.
This cyclic flow generates a proton gradient → ATP only. No NADPH is produced, no water is split, no is evolved.
This happens when the cell needs more ATP than NADPH (e.g., for the Calvin cycle, which uses 3 ATP per 2 NADPH).
Why This Works
The two photosystems work like a relay. PS II generates the initial high-energy electrons (using water as the electron source) and PS I boosts them to an even higher energy level to reduce NADP⁺. The energy released as electrons flow downhill between the two systems drives proton pumping across the thylakoid membrane, creating the gradient for ATP synthesis.
Cyclic photophosphorylation exists as a “top-up” mechanism for ATP. The Calvin cycle consumes ATP and NADPH in a 3:2 ratio, but non-cyclic photophosphorylation doesn’t produce them in exactly that ratio. Cyclic flow makes up the ATP deficit.
Alternative Method — Remember Using “PLAN”
P — Photolysis of water occurs at PS II (not PS I) L — Light absorbed at P680 (PS II) and P700 (PS I) A — ATP from both cyclic and non-cyclic; NADPH only from non-cyclic N — Non-cyclic involves both PS I and PS II; cyclic involves only PS I
NEET frequently asks: “Which photophosphorylation produces O₂?” Answer: only non-cyclic, because only non-cyclic involves PS II and the photolysis of water. Cyclic photophosphorylation has no water splitting, so no oxygen evolution.
Common Mistake
The most common NEET error: students say “PS I comes first because it’s numbered I.” Wrong. In the Z-scheme, PS II acts first — it absorbs light, splits water, and sends electrons to the ETC. PS I acts second, receiving electrons from PS II via plastocyanin. The numbering is historical, not sequential. Getting this backwards can cost you 4 marks on a single MCQ.