Carrier-mediated transport is saturable.

• A. True
• B. False
• C. Not sure
• D. Depends

Answer: (A)

Carrier-mediated transport is saturable because it relies on a finite number of carrier proteins embedded in the membrane, each with a limited number of binding sites. Substrate molecules must bind to these carriers, the carriers undergo conformational changes, and each carrier can shuttle only a certain number of molecules per unit time (its turnover rate). As substrate concentration increases, more carriers become occupied, so the overall transport rate increases, but only up to the point where all carriers are busy. Beyond that, no additional substrate can be transported faster, so the rate levels off at a maximum velocity (Vmax). This behavior follows Michaelis-Menten kinetics and explains why the process cannot increase indefinitely with substrate concentration. In contrast, simple diffusion does not depend on carrier occupancy and is not inherently saturable; its rate is set by the concentration gradient and membrane properties.