With the help of a diagram, explain briefly the process of cyclic photophosphorylation.

Explanation

(a) In cyclic photophosphorylation only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to the cyclic flow of electrons.
(b) This takes place in stroma lamellae.
(c) Electron in the reaction centre of PS I gets excited when they receive red light of wavelength 700 nm.
(d) In this process, the electrons from the reaction centre of P700 are conveyed to e-acceptor.
(e) The excited electron is cycled back to PS I complex through electron transport system.
(f) The cyclic flow results in the synthesis of ATP.
(g) Cyclic photophoshorylation also occurs when only light of wavelength beyond 680 nm is available.
(h) In green plants, cyclic photophosphorylation is an additional source of ATP required for chloroplast activities over and above that is required in the Calvin cycle.