When light energy excites electrons in photo system electrons to replace them come from?

In photosystems, when light energy excites electrons, these electrons are primarily replaced by splitting water molecules (H2O) in a process known as photolysis. This reaction occurs in photosystem II and releases oxygen as a byproduct while providing the necessary electrons to maintain the flow of energy. The excited electrons then move through the electron transport chain, ultimately contributing to ATP and NADPH production during photosynthesis.