In diffusion across membranes, how does increasing the concentration gradient affect the transport rate?

• A. It increases
• B. It decreases
• C. It stays the same
• D. It drops to zero

Answer: (A)

The main idea here is that diffusion is driven by the concentration difference across a membrane. The bigger the gap between the two sides, the more molecules will move from the side of higher concentration to the side of lower concentration in a given time. In simple diffusion through a lipid membrane, the flux is proportional to this gradient, so increasing the gradient raises the transport rate. If the system stays in the linear, unsaturated regime with constant membrane permeability and surface area, doubling the gradient roughly doubles the rate. The rate will keep increasing as long as there’s a driving difference; it drops to zero when equilibrium is reached (no net gradient) or reverses if the gradient reverses.