USA Biology Olympiad (USABO) Practice Exam

What effect does carrier-mediated diffusion have on the velocity of substances crossing cellular membranes?

1. It always decreases velocity.

2. It has a constant velocity.

3. It can increase to a maximum value.

4. It cannot exceed passive diffusion rates.

The correct answer is: It can increase to a maximum value.

Carrier-mediated diffusion refers to the process by which substances move across cellular membranes with the help of specific proteins, known as carriers or transporters. This mechanism is crucial for the transport of larger or polar molecules that cannot easily pass through the lipid bilayer of the membrane. As the concentration of the substrate increases, the rate of transport via carrier proteins also increases, but only up to a certain point. This behavior is characterized by a saturation effect. Once all available carrier proteins are occupied, the rate of transport reaches a maximum value and cannot increase any further, even if the concentration of the substrate continues to rise. This saturation is a key feature of carrier-mediated processes and distinguishes them from simple diffusion. Understanding this helps clarify why this option is the correct choice. The rate of transport can indeed increase with the concentration of the substrate until a maximum saturation point is reached, after which the velocity stabilizes and no longer rises with increased substrate concentration. This is a fundamental concept in cell biology regarding how substances are transported across membranes.