Final answer:
Hydrogen ions build up in the thylakoid space during light-dependent reactions when water is split in the PSII's P680 reaction center, contributing to an electrochemical gradient used to produce ATP.
Step-by-step explanation:
During the light-dependent reactions of photosynthesis, hydrogen ions (H+) build up in the thylakoid space when water molecules (H2O) are split. This process, known as photolysis, occurs in the reaction center of photosystem II (PSII), also called P680. Here, energy from sunlight excites electrons, causing them to leave the chlorophyll and be transferred through an electron transport chain.
To replace the lost electrons from chlorophyll, water molecules are split, which releases hydrogen ions into the thylakoid space as well as electrons and oxygen atoms. The accumulation of H+ ions contributes to an electrochemical gradient across the thylakoid membrane that is used by ATP synthase to produce ATP through chemiosmosis.