the electrons that move from photosystem II must be replaced for the cycle to continue. This replacement is achieved by electrons available due to the splitting of water (photolysis). The text explicitly states: "The electrons needed to replace those removed from photosystem I are provided by photosystem II... The electrons that were moved from photosystem II must be replaced. This is achieved by electrons available due to splitting of water" . Oxygen is a byproduct of this reaction, not the source of electrons.