Final answer:
The protons used by the proton pump in the light reactions of photosynthesis come from the water molecules that are split during photolysis at the PSII reaction center, aiding in the formation of a chemiosmotic gradient used to make ATP.
Step-by-step explanation:
The protons moved by the proton pump during the light reactions of photosynthesis primarily come from the splitting of water, a process known as photolysis. When water molecules (H₂O) are split, they produce hydrogen ions (H+), electrons, and oxygen atoms. This occurs in the first part of the light-dependent reactions at the Photosystem II (PSII) reaction center. Energy from sunlight is harnessed to extract electrons from water and pass them through the chloroplast electron transport chain (ETC) to Photosystem I (PSI), where NADP+ is reduced to NADPH. Concurrently, as electrons are being transported, protons are pumped across the thylakoid membrane into the lumen, contributing to the formation of a chemiosmotic gradient. This gradient is used by ATP synthase in the process of chemiosmosis to produce ATP.