How do photosystems I and II cooperate for photosynthesis?

The pairs of electrons pass from photosystem II to the photosystem I by an electron transport chain. This passage generates a proton gradient or proton motive forces on both sides of the thylakoid membrane which activate the ATP synthesis. Photosystem I is excited by a light photon. The excited electron is then transmitted in a short chain of electron transport to NADP+. This can reduce the NADP+ into NADPH. The electrons removed from the P700 molecule are replaced by those coming from photosystem II.