Explain how during light reaction of photosynthesis, ATP synthesis is a chemiosmotic phenomenon.

Explanation

In light reaction plants trap solar readiation by photosynthetic pigments which convert light energy into chemical energy. Main event of light reaction is photosphorylation, i.e., formation of ATP from ADP+Pi by using energy of excited electron movement through electron transport chain, present in thylakoid membrane.
Chemiosomosis is the movemet of ions across a selectively permeable membrane, down the electrochemical/proton gradient.
Chemiosomosis hypothesis of ATP formation was first proposed by Mitchell (1961) according to which the enzyme ATP synthase generates ATP via a membrane, proton pump and proton gradient. ATP synthase allows ions `O_(2)` protons to pass through membrane and proton pump.
This creales a high concentration of proton `(H^(+))` in the lumen and hence diffuses across the membrane to active ATPase, releasing ATP molecules. One molecule of ATP is released for every two `(H^(+))` ions passing Through ATPase.