Final answer:
In photosystem II, water molecules are split to replace lost electrons. Splitting one water molecule releases two electrons, two hydrogen atoms, and one oxygen atom. This process is called photolysis.
Step-by-step explanation:
In photosystem II (PSII), the molecules of water are split to replace electrons that have been lost. The splitting of water releases electrons, hydrogen ions, and oxygen atoms. Splitting one water molecule releases two electrons, two hydrogen atoms, and one oxygen atom. This process is called photolysis or photodissociation. The extracted low-energy electrons from water then replace the lost electrons in the chlorophyll of PSII.