Chemiosmotic hypothesis explains

The chemiosmotic hypothesis explains the mechanism of ATP synthesis. In photosynthesis , ATP synthesis is linked to the development of a proton gradient across a thylakoid membrane. The protons are accumulated in the lumen of thylakoids.

Within the chloroplast, splitting of water, movement of electrons across the photosystem leads to accumulation of protons in the lumen of thylakoids while consumption of electrons by the NADP reductase enzyme ( located on the stroma side of the membrane) along with electrons for the reduction of NADP+ to NADPH+ H+ decreases the proton concentration in  the stroma .

 This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen. Breakdown of this gradient due to the movement of protons across the membrane to the stroma through the transmembrane channel F0 of ATP synthase leads to release of energy.This energy is captured by the F1 component of ATP synthase and utilized to make ATP from ADP +Pi..