Light absorbed by chlorophylls and accessory pigments drives electrons from water through PS II (P680) and PS I (P700) to NADP⁺, releasing O₂; the proton gradient built on the way makes ATP by chemiosmosis.
How it's asked: Early experiments (Priestley, Ingenhousz, Sachs, Engelmann, van Niel), pigment colours on chromatography, reaction-centre wavelengths, the Z-scheme sequence, where water splitting occurs, cyclic vs non-cyclic photophosphorylation, and the location of CF0 and CF1.
Why this is true
Accessory pigments (chlorophyll b, xanthophylls, carotenoids) absorb wavelengths that chlorophyll a misses and pass the energy to a reaction-centre chlorophyll a, giving PS II (P680) and PS I (P700); they also protect chlorophyll a from photo-oxidation. Excited P680 hands an electron to a primary acceptor, and the electron runs downhill through cytochromes to PS I, where light boosts it again before it reduces NADP⁺ to NADPH + H⁺ — an uphill–downhill–uphill path shaped like a Z. The hole left in PS II is filled by splitting water, so the O₂ of photosynthesis comes from water, as van Niel inferred. Protons pile up in the thylakoid lumen for three reasons: water is split on the lumen side, the PS II primary acceptor passes electrons to a hydrogen carrier that moves protons from the stroma into the lumen, and NADP reductase on the stroma side consumes stromal protons. Protons can return to the stroma only through the CF0 channel of ATP synthase, and this flow drives CF1 to make ATP on the stroma side, where the Calvin cycle needs it. If electrons from PS I cycle back to the chain instead of reducing NADP⁺, protons are still pumped, so ATP is made without NADPH or O₂.
Key points
Non-cyclic electron flow (Z-scheme)
- 1Water splitting2H₂O → 4H⁺ + O₂ + 4e⁻ on the lumen side, linked to PS II
Cyclic vs non-cyclic photophosphorylation
| Feature | Non-cyclic | Cyclic |
|---|---|---|
| Photosystems | PS II and PS I | PS I only |
| Electron source | Water | Electrons return to P700 |
| Products | ATP + NADPH; O₂ evolved | ATP only; no O₂ |
| Location | Grana lamellae | Stroma lamellae |
| Light | Wavelengths that excite both photosystems | Can run when only light beyond 680 nm is available |
Common traps
- PS I has P700 and PS II has P680 — the numbering reflects order of discovery, not the order of action (PS II acts first).
- Protons accumulate in the lumen, not the stroma; ATP is made on the stroma side as they flow out through CF0.
Worked example
Isolated thylakoids are illuminated only with far-red light of 700 nm. ATP formation continues, but no O₂ is released and NADP⁺ is not reduced.
High-yield
PS I = P700, PS II = P680; water split with PS II on lumen side. Stroma lamellae lack PS II and NADP reductase → cyclic → ATP only. CF0 channel, CF1 makes ATP facing stroma.
Quick check
Q1.The splitting of water during photosynthesis is associated with:
Q2.Cyclic photophosphorylation occurs in stroma lamellae because they lack: