Q.Why is photolysis of water accompained with 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:

1. Light hits PS II (P680). An electron gets excited and is passed to a primary acceptor.
2. That electron is replaced by splitting water: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$. This is the only source of oxygen in photosynthesis.
3. The excited electron travels down an electron transport chain (ETC) from PS II to PS I. As it moves through the cytochrome $b_6f$ complex, it pumps protons ($H^+$) into the thylakoid lumen, building a gradient.
4. That proton gradient drives ATP synthase — photophosphorylation happens here. ATP is made.
5. The electron, now lower in energy, reaches PS I (P700). Light hits PS I, re-exciting the electron to a much higher energy level.
6. This high-energy electron is passed to ferredoxin (Fd) and then to NADP$^+$ reductase, which reduces NADP$^+$ to NADPH.

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.

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:

1. Light excites PS I (P700). An electron is ejected and picked up by ferredoxin.
2. Instead of going to NADP$^+$, the electron is shunted back to the cytochrome $b_6f$ complex (via plastoquinone).
3. It travels down the ETC again, pumping protons — generating ATP.
4. The electron eventually returns to PS I, filling the hole it left.

The electron goes in a circle: PS I → ferredoxin → plastoquinone → cytochrome $b_6f$ → plastocyanin → PS I. No water is split, no oxygen is released, no NADPH is made.

Why Two Pathways? …