The two photosystems in the light reactions of photosynthesis are connected by a series of proteins and electron carriers known as the electron transport chain.
During the light reactions, which take place in the thylakoid membranes of chloroplasts, photons of light are absorbed by pigments in both Photosystem I (PSI) and Photosystem II (PSII). When light energy is absorbed, it excites electrons, which are then transferred to an electron acceptor. In the case of PSII, the primary electron acceptor is plastoquinone, which then passes the electrons through the electron transport chain.
The connection between the two photosystems occurs as the electrons move from PSII through the electron transport chain and are eventually passed to PSI. This movement not only allows for the flow of electrons but also contributes to the creation of a proton gradient that is essential for ATP synthesis. It’s this series of reactions that ultimately captures and converts light energy into chemical energy, in the form of ATP and NADPH, which are used in the Calvin cycle.