Answer:
The electrons needed to start Photosystem II come from water molecules.
During the light-dependent reactions of photosynthesis, light energy is used to split water molecules into oxygen, protons (H+), and electrons (e-). The electrons released from water are then used to replace the electrons lost by Photosystem II when it absorbs light energy. This process is called photolysis, and it occurs in the thylakoid membranes of the chloroplasts.
The electrons released from water are initially transferred to the chlorophyll molecules in Photosystem II, which then use the energy from the absorbed light to transfer the electrons to an electron transport chain. The electron transport chain then uses the energy from the electrons to pump protons across the thylakoid membrane, which generates a proton gradient that is used to produce ATP through chemiosmosis. The electrons eventually reach Photosystem I, where they are re-energized by light energy and transferred to another electron transport chain that generates NADPH, another energy-rich molecule that is used to drive the synthesis of carbohydrates during the light-independent reactions.