In the context of a concentration gradient across a membrane, it is energetically favorable for protons to flow in which direction?

• A. From an area of low proton concentration to an area of high proton concentration
• B. Equally in both directions regardless of concentration
• C. Only when energy is supplied from an external source
• D. From an area of high proton concentration to an area of low proton concentration

Answer: (D)

Verified step by step guidance

1. Understand that a concentration gradient refers to the difference in the concentration of a substance (in this case, protons) across a membrane.
2. Recall that particles tend to move from an area of higher concentration to an area of lower concentration to reach equilibrium, a process known as diffusion.
3. Recognize that this movement down the concentration gradient is energetically favorable because it increases entropy and does not require external energy.
4. Apply this concept to protons: they will naturally flow from an area of high proton concentration to an area of low proton concentration across the membrane.
5. Conclude that movement from low to high concentration would require energy input, so it is not energetically favorable without external energy.