Advanced Placement (AP) Biology Practice Exam

What is defined as the difference in proton concentration across a membrane?

1. Proton Gradient

2. Electrochemical Gradient

3. Membrane Potential

4. Concentration Gradient

The correct answer is: Proton Gradient

The answer identifies the concept known as the proton gradient, which specifically refers to the difference in proton (H⁺) ion concentration across a biological membrane. This gradient is crucial in various biological processes, particularly in cellular respiration and photosynthesis where it drives ATP synthesis. In cellular respiration, as protons are pumped across the inner mitochondrial membrane, they create a higher concentration of protons in the intermembrane space compared to the mitochondrial matrix. The resulting proton gradient creates potential energy that can be harnessed by ATP synthase to produce ATP when protons flow back into the matrix. This phenomenon is integral to the functioning of the electron transport chain, where the transfer of electrons leads to the movement of protons across the membrane, establishing the gradient necessary for ATP generation. While the other options describe related concepts, they are distinct from the specific definition of a proton gradient. An electrochemical gradient incorporates both the concentration gradient of the ions and the membrane potential, which is not limited to protons alone. Membrane potential refers to the electrical charge difference across the membrane, and a concentration gradient can refer to any type of solute, not just protons.