1. The problem asks: Which system begins the light-dependent reaction in photosynthesis? 2. The light-dependent reactions occur in the thylakoid membranes of chloroplasts and involve two main photosystems: Photosystem II (PSII) and Photosystem I (PSI). 3. Photosystem II is the first to absorb light and initiate the process by exciting electrons, which then travel through the electron transport chain. 4. ATP synthase is involved later, using the proton gradient created by electron transport to produce ATP, but it does not start the reaction. 5. Therefore, the system that begins the light-dependent reaction is Photosystem II. Final answer: Photosystem II