Non-cyclic photophosphorylation involves both PSI and PSII. The chlorophyll molecules at the reaction centres absorb light. Electrons are passed to electron carriers. A proton gradient is set up between the stroma and the inside of the thylakoid by actively transporting hydrogen ions into the thylakoid space using the energy from the electrons. The proton gradient is used to synthesise ATP by chemiosmosis.The electrons from PSII replace the lost electrons from PSI. A water-splitting enzyme splits water into protons, electrons and molecular oxygen as a waste product. This is called photolysis. The electrons replace those lost by PSII whilst the protons combine with NADP and the electrons from PSI to form reduced NADP.