In passive transport, how does increasing a cell membrane’s surface area (while keeping the concentration gradient and membrane permeability constant) affect the rate of diffusion across the membrane?

• A. It increases diffusion only for water (osmosis) but not for solutes.
• B. It has no effect on diffusion rate because diffusion depends only on the concentration gradient.
• C. It increases the rate of diffusion because more membrane area is available for particles to cross per unit time.
• D. It decreases the rate of diffusion because particles must travel farther across a larger membrane.

Answer: (C)

Verified step by step guidance

1. Recall Fick's first law of diffusion, which describes the rate of diffusion across a membrane: \(\text{Rate of diffusion} = P \times A \times (C_1 - C_2)\), where \(P\) is the membrane permeability, \(A\) is the surface area of the membrane, and \((C_1 - C_2)\) is the concentration gradient across the membrane.
2. Understand that in this problem, the concentration gradient \((C_1 - C_2)\) and membrane permeability \(P\) are held constant, so the only variable changing is the surface area \(A\).
3. Since the rate of diffusion is directly proportional to the surface area \(A\), increasing \(A\) means there is more membrane available for particles to cross, which should increase the overall diffusion rate.
4. Note that this relationship applies to all particles diffusing passively, including both water (osmosis) and solutes, because the equation does not distinguish between them.
5. Therefore, increasing the membrane surface area increases the rate of diffusion because it provides more space for particles to move across per unit time, without affecting the distance particles must travel.