What best describes ATP synthase?

• A. A protein on the thylakoid membrane that allows H+ ions to pass through it, converting ADP to ATP
• B. A pigment capturing light energy
• C. A carrier protein for NADP+
• D. An enzyme that splits water to release oxygen

Answer: (A)

ATP synthase is the protein complex in the thylakoid membrane that uses the proton gradient created by light-driven electron transport to drive ATP formation from ADP and inorganic phosphate. As protons flow through the enzyme, its rotor turns and the catalytic sites synthesize ATP, storing energy in a usable form. This process—chemiosmosis during photosynthesis—is the reason this complex is described as a membrane protein that allows H+ to pass and converts ADP to ATP. The other options describe different components: a light-absorbing pigment is chlorophyll, a carrier for NADP+ is not the role of ATP synthase, and the enzyme that splits water to release oxygen is part of the oxygen-evolving complex in photosystem II.