Which enzyme uses a proton gradient to synthesize ATP?

• A. ATP synthase
• B. Rubisco
• C. Photosystem II
• D. NADP reductase

Answer: (A)

Chemiosmosis is the process of using a proton gradient to drive ATP production. Protons are pumped across a membrane, creating a high concentration on one side. When these protons flow back through ATP synthase, the energy of their movement powers the rotor of the enzyme, causing the catalytic F1 portion to convert ADP and inorganic phosphate into ATP. This direct coupling of a proton motive force to ATP synthesis is what ATP synthase does.

Rubisco fixes carbon dioxide in the Calvin cycle, not ATP production. Photosystem II starts the light reactions and helps establish the proton gradient by splitting water and pumping protons, but it does not synthesize ATP itself. NADP reductase reduces NADP+ to NADPH, providing reducing power rather than producing ATP.