A protein on the thylakoid membrane that allows H+ ions to pass, converting ADP to ATP, is known as?

• A. ATP Synthase
• B. Photosystem I
• C. NADP Reductase
• D. Rubisco

Answer: (A)

The protein that makes ATP from ADP by using a proton gradient across the thylakoid membrane is ATP synthase. In the light reactions of photosynthesis, light energy drives electrons through the electron transport chain, pumping protons into the thylakoid lumen and creating a high concentration of H+ there. ATP synthase sits in the membrane like a turbine, and as protons flow back across the membrane into the stroma, their movement spins the enzyme. This mechanical rotation drives the chemical reaction that adds a phosphate group to ADP, forming ATP. This process, chemiosmosis, links the proton motive force to ATP production, which will later power the Calvin cycle.

The other options are important players in photosynthesis but don’t synthesize ATP from the proton gradient. Photosystem I helps generate NADPH by transferring electrons to NADP+. NADP reductase is the enzyme that actually reduces NADP+ to NADPH. Rubisco fixes CO2 during the Calvin cycle. Only ATP synthase converts the flowing protons into ATP.