The word chemiosmotic means simply that there is osmotic gradient for a chem.... in photosynthesis. There is accumulation of h+ ions in the lumen of thylakoids. This gradient is because of many reasons. Firstly, the hydrolysis of water occurring on the inner side of membrane sends its h+ ion to the lumen. Secondly, the h+ ion required for formation of NADPH is taken from stroma. This all cause the gradient......... this accumulating h+ ion are then send back to the stroma through ATP pump…. through its f0 - f1 channel.., having ATPase enzyme and so formation of ATP occurs. This all occurs during light rxn... and so ATP synthesis is a chemiosmotic phenomenon.