Question

When light energy excites electrons in photosystem II, where do the electrons to replace them come from?

Answer 1

Oh we have just done this in biology recently... photolysis occurs where it splits apart water molecules to make electrons and hydrogen ions (the H+ is used to reduce NaDP later on) and also O₂ is made as a by product

basically this is the equation 2H₂O⇒4e⁻ + 4H⁺  where the electrons replace the lost ones in chlorophyll

hope this helped :)