Advanced Placement (AP) Biology Practice Exam

Which enzyme is responsible for synthesizing ATP using a proton gradient?

1. Cyclic AMP

2. ATP Synthase

3. Rubisco

4. Hexokinase

The correct answer is: ATP Synthase

ATP Synthase is the enzyme responsible for synthesizing ATP using a proton gradient. This process occurs during oxidative phosphorylation in cellular respiration and also during photophosphorylation in photosynthesis. The enzyme is embedded in the inner mitochondrial membrane (or thylakoid membrane in chloroplasts) and utilizes the energy stored in a proton gradient created by the electron transport chain. As protons flow down their concentration gradient through ATP Synthase, the enzyme catalyzes the phosphorylation of ADP to form ATP. In contrast, cyclic AMP is a signaling molecule that acts as a secondary messenger in various biological processes but is not involved in ATP synthesis. Rubisco is an enzyme that plays a critical role in the Calvin cycle for carbon fixation in photosynthesis, while hexokinase is an important enzyme in glycolysis that phosphorylates glucose. Neither of these enzymes is directly involved in utilizing a proton gradient to generate ATP. Thus, ATP Synthase is the correct enzyme for this process, highlighting its crucial role in energy production within the cell.