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

• A. It has no effect on the rate because diffusion depends only on the concentration gradient.
• B. It increases the rate because more membrane area is available for molecules to cross per unit time.
• C. It decreases the rate because molecules must spread out over a larger area before crossing.
• D. It increases the rate only for water (osmosis) but not for solutes.

Answer: (B)

Verified step by step guidance

1. Recall that passive transport, specifically diffusion, is the movement of molecules from an area of higher concentration to an area of lower concentration without the use of energy.
2. Understand that the rate of diffusion across a membrane depends on several factors, including the concentration gradient, diffusion distance, and the surface area of the membrane.
3. Recognize that increasing the surface area of the membrane provides more space for molecules to pass through simultaneously, which can increase the overall rate of diffusion.
4. Since the concentration gradient and diffusion distance are held constant, the only variable changing is the surface area, so the rate of diffusion will increase proportionally with the increase in surface area.
5. Therefore, increasing the cell's surface area increases the rate of diffusion because more membrane area is available for molecules to cross per unit time.