Ultimately, the electrons taken when water is split in photosystem II during the process of photolysis.
Photosystems loose electrons during light reactions.Their lost electrons are replaced by photolysis of water
H20 - NADPH - Calvin Cycle
The supply of electrons in photosynthesis comes from water.
why are electrons important to photosynthesis.
Water 
Photolysis.
The flow of electrons in photosynthesis has four sequential steps. Here is the sequence: from water to photosystem II then photosystem I and lastly NADP.
The supply of electrons in photosynthesis comes from water.
why are electrons important to photosynthesis.
No it does not. But photosynthesis provides electrons
no
sunlight to excite the electrons
Water 
Photolysis.
they are the reason the whole thing happens
Yes. Photosynthesis involves the transfer of electrons. Photosynthesis causes a chemical change. All chemical changes involve either the transfer of electrons or a change in the configuration of electrons. No other way exists to make a chemical change.
Light is needed to excite electrons in the electron transport chain to commence photosynthesis.
Oxygen is not use.Oxygen is produced in photosynthesis when breaking of water molecules to obtain electrons.
The flow of electrons in photosynthesis has four sequential steps. Here is the sequence: from water to photosystem II then photosystem I and lastly NADP.