Question

What is photophosphorylation ? Distinguish between cyclic and non-cyclic photophosphorylation. Give schematic sketches of both.

Answer 1

The production of ATP by the light-dependent reactions is called photophosphorylation, as it uses light as an energy source.

Photophosphorylation may be either a cyclic process or a non-cyclic process.

Cyclic photophosphorylation	Non-cyclic photophosphorylation
1. Predominant in bacteria.	Predominant in green plants.
2. Only photosystem I (PS1) is involved.	Both photosystem 1 and 2 (PS1 and 2) are involved.
3. Since PS I is involved, the active reaction centre is P₇₀₀.	The active reaction centre is P₆₈₀.
4. The movement of electrons is cyclic.	The movement of electrons is non-cyclic.
5. This process produces no NADPH and no O₂, but it does make ATP.	This process produces both NADPH and ATP, and O₂ is evolved as a by-product.
6. Photolysis (water-splitting) is absent.	Photolysis (water-splitting) is present.
7. The electrons travel back to PS1 and complete the cycle.	The electrons from PS1 accepted by NADPH, making the cycle incomplete.