Question

Describe the light reaction of photosynthesis with special reference to non-cyclic and cyclic photophosphorylation.

Answer 1

The photochemical phase of photosynthesis is also known as a light reaction. It is responsible for the formation of high-energy chemical intermediates, ATP and NADPH, and it includes light absorption, water splitting and release of oxygen. It takes place in the granal thylakoids of chloroplasts.

The production of assimilatory power in photosynthesis occurs through two processes- non-cyclic photophosphorylation and cyclic photophosphorylation. These are the main processes of light reaction which generates ATP and NADPH.

1. Non-cyclic photophosphorylation: Non-cyclic photophosphorylation involves both Photosystem I and Photosystem II. These two photosystems work in series, first PS II and the PS I. The two photosystems are connected through an electron transport chain. Both ATP and $NADPH+H^{+}$ are synthesised by this kind of electron flow.
2. Cyclic photophosphorylation: Cyclic photophosphorylation is a process of photophosphorylation in which an electron released by the excited photocentre is returned to it after passing through a series of electron carriers. It takes place in the stroma lamellae membrane of chloroplasts. The process of cyclic photophosphorylation involves only photosystem I. The excited electron does not pass on to $NAD{P}^{+}$ and is cycled back to the PS I complex through the electron transport chain. It synthesises ATP only.