When water molecules are split to replace the electrons lost by Photosystem II during photosynthesis, the following are produced:

* Electrons: These electrons are used to replace those lost by Photosystem II, allowing the process of photosynthesis to continue.

* Oxygen (O2): The oxygen atoms from the water molecules combine to form molecular oxygen (O2), which is released as a byproduct of photosynthesis.

* Hydrogen ions (H+): These ions accumulate in the thylakoid lumen, contributing to the proton gradient that drives ATP synthesis.

Therefore, the splitting of water molecules in photosynthesis is a critical step that provides the electrons necessary to power the light-dependent reactions and results in the release of oxygen as a byproduct.