During photosynthesis, the energy from sunlight is used to split water molecules. what happens to the hydrogen ions that are split from the water molecules?
a. they flow out of the thylakoid, providing energy to add a phosphate group to adp.
b. they build up in the thylakoid, providing energy to break apart atp molecules.
c. they build up in the thylakoid, where they bond to each other to create atp.
d. they flow out of the thylakoid, where they stimulate enzymes to break apart atp.

The correct answer is A. They flow out of  the thylakoid, providing energy to add a phosphate group to ADP.

Photolysis (photo means light and lysis means break) is the term used for the splitting of water during photosynthesis. Photolysis of water serves dual purpose for the plants:

1. electron released by photolysis is used in electron transport chain which is used to reduce NAD+ to NADPH.

2. protons () released are used to generate chemiosmotic potential which is used to synthesize ATP by ATP synthase. H^{+} ions accumulated in the thylakoid lumen travel down the concentration gradient through ATP synthase. This provides energy to the ATP synthase to add phosphate group to the ADP.

3. is released as a byproduct.