Increasing the concentration of a drug undergoing facilitated diffusion will increase the rate of absorption.

• A. True
• B. False
• C. Depends on cell type
• D. Cannot determine

Answer: (B)

Facilitated diffusion is carried out by specific transport proteins, and the number of these carriers is finite. As a result, the transport process is saturable: at low drug concentrations the rate increases with concentration, but once the transporters become occupied, the rate approaches a maximum (Vmax) and additional increases in luminal drug concentration no longer speed up absorption. This is why simply increasing the drug amount in the lumen does not keep boosting absorption indefinitely.

In the gut, the observed rate depends on how many transporters are available and their affinity for the drug. If you push the concentration high enough to reach saturation, you won’t get a higher absorption rate, even though more drug is present. That’s the essential reason the statement is not generally true, though the exact point of saturation can vary with tissue type and transporter characteristics.