Which enzyme uses a proton gradient to synthesize ATP during cellular respiration?

• A. NADH dehydrogenase
• B. ATP synthase
• C. Cytochrome c oxidase
• D. Phosphatase

Answer: (B)

The key idea is that ATP production in cellular respiration is driven by a proton gradient across the inner mitochondrial membrane. The enzyme that uses this gradient to make ATP from ADP and inorganic phosphate is ATP synthase. Protons flow back through ATP synthase, causing its rotary engine to drive the phosphorylation reaction, turning ADP and Pi into ATP. This step, powered by the proton motive force created by the electron transport chain, is the essence of oxidative phosphorylation.

Other components of the electron transport chain, like NADH dehydrogenase and cytochrome c oxidase, help establish and maintain the gradient by pumping protons, but they don’t convert ADP to ATP themselves. A phosphatase, in contrast, removes phosphate groups and does not synthesize ATP in this context.