Final answer:
The pH of the thylakoid lumen is lower than the pH of the stroma prior to ATP synthesis due to the transport of protons into the lumen during the light-dependent reactions of photosynthesis, which creates an electrochemical gradient used by ATP synthase.
Step-by-step explanation:
The pH of the thylakoid lumen is lower than the pH of the stroma prior to the synthesis of ATP in chloroplasts. During the light-dependent reactions of photosynthesis, energy from sunlight is used in photosystem II to extract electrons from water, which travel through the electron transport chain to photosystem I. Meanwhile, protons are pumped across the thylakoid membrane into the thylakoid lumen, and the splitting of water adds even more protons to the lumen. This accumulation of protons results in a high concentration of H+ ions, leading to a low pH in the thylakoid lumen. In contrast, the reduction of NADP+ to NADPH removes protons from the stroma, leading to a higher pH in this region. The electrochemical gradient created by this difference in proton concentration, with a low pH inside the thylakoid lumen and a high pH in the stroma, is what powers ATP synthase to make ATP.