LIGHT REACTION ( Cyclic Phosphorylation )

Cyclic photophosphorylation:

 The rise in NADPH and deficit of ATP may stimulate a temporary shift from a non-cyclic to cyclic electron flow until ATP supply catches up the demand. 

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In this mechanism only PS-1 is utilized. It absorbs energy in the form of photons. When energy reaches the reaction centre of PS- the electrons are boosted up to higher energy level. Such excited electrons are first captured by primary electron acceptor of PS-1, then they move through an electron transport chain containing ferridoxin, cytochrome b-f complex and plastocyanin. When electrons are passed from cytochrome be-f complex an ATP is generated by chemiosmosis. Finally, the electrons after losing the energy return back to P700 chlorophyll in PS-I reaction centre. There is no production of NADPH, no occurrence of photolysis of water and therefore, no release of oxygen.