Final answer:
H2O is required for photosynthesis to provide electrons to replace those lost from chlorophyll. When H2O is split by light, it releases electrons, hydrogen ions, and oxygen atoms.
Step-by-step explanation:
H2O is required for photosynthesis because it serves as a source of electrons to replace those lost from chlorophyll during the light reactions. When H2O molecules are split by light in a process called photolysis, they are broken into hydrogen ions, electrons, and oxygen atoms. The electrons from H2O replace the lost electrons in chlorophyll, while the hydrogen ions and high-energy electrons from chlorophyll continue the energy transformation after the light reactions. The oxygen atoms produced are released as a byproduct.