Advanced Placement (AP) Biology Practice Exam

What does a proton gradient refer to?

1. Difference in membrane potential

2. Difference in proton concentration across membranes

3. Variety of proton carriers in the electron transport chain

4. Accumulation of protons in the cytoplasm

The correct answer is: Difference in proton concentration across membranes

A proton gradient refers specifically to the difference in proton concentration across membranes, which creates a potential energy difference used for various biological processes, particularly in cellular respiration and photosynthesis. This gradient occurs when protons (H+ ions) are pumped from one side of a membrane to the other, establishing a higher concentration of protons on one side compared to the other. This difference can be harnessed by ATP synthase to generate ATP as protons flow back across the membrane. The other options focus on related but distinct concepts. The difference in membrane potential is related but involves charges and electrical gradients rather than solely the concentration of protons. A variety of proton carriers refers to specific proteins that transport protons and does not define the gradient itself. Finally, the accumulation of protons in the cytoplasm does not accurately describe the essential characteristic of a proton gradient, which must specifically involve two different regions separated by a membrane. Thus, the correct choice effectively captures the core idea of a proton gradient, emphasizing the concentration difference across a membrane that is crucial for energy transformation in cells.