Final answer:
During the light-dependent reactions of photosynthesis, water molecules are split in the process called photolysis, releasing H+ ions, electrons, and oxygen. This occurs in the thylakoid space of the chloroplasts, facilitated by the enzyme photosystem II, contributing to the generation of ATP.
Step-by-step explanation:
During the light-dependent reactions, H+ ions build up in the thylakoid space when water molecules are split. This process, also known as photolysis, takes place inside the chloroplasts of plant cells. The splitting of water molecules is catalyzed by the enzyme photosystem II and it releases electrons, oxygen, and hydrogen ions. The electrons are used to replace those lost by chlorophyll in photosystem II, while the oxygen is released as a byproduct. The build-up of hydrogen ions inside the thylakoid space creates a concentration gradient that is used by ATP synthase to generate ATP from ADP and an inorganic phosphate group during the next steps of photosynthesis.