In carrier-mediated transport, what does saturation imply compared to diffusion?

• A. Carrier capacity limits rate; diffusion depends on gradient
• B. Rate increases indefinitely with substrate
• C. Both are driven solely by gradient
• D. Diffusion saturates while active transport does not

Answer: (A)

Carrier-mediated transport relies on proteins that have a limited number of binding sites. As substrate concentration increases, more carriers become occupied until all are working, causing the transport rate to reach a maximum—this is saturation. Diffusion, in contrast, moves molecules down a concentration gradient through the membrane and does not depend on a finite set of carriers, so its rate tracks the gradient rather than hitting a hard ceiling due to carrier occupancy. That’s why the correct statement is that carrier capacity limits the rate while diffusion depends on the gradient. The other options contradict the saturable nature of carrier-mediated transport or misstate what drives diffusion versus active transport.