carrier-mediated transport

Carrier-mediated transport is when a drug crosses a cell membrane with the help of a dedicated protein 'ferry' embedded in the membrane, rather than just diffusing through on its own. Think of a specialized doorman who only lets in molecules of a certain shape and physically carries them across — quite different from molecules slipping through a gap unaided.

These membrane proteins (transporters) normally exist to move the body's own nutrients and chemicals — sugars, amino acids, vitamins — and some drugs that resemble those natural substances hitch a ride on them. Because the carriers can use energy or an ion gradient, some can move a drug 'uphill' against its concentration gradient (active transport), something passive diffusion can never do. This lets the body absorb, secrete, or concentrate certain drugs far beyond what simple diffusion would allow.

Two features set carrier transport apart from passive diffusion. First, it is selective — only molecules that fit the carrier are moved, so chemically similar drugs can compete for the same transporter. Second, it is saturable — there are a finite number of carriers, so once they are all busy the rate cannot rise further even if concentration keeps climbing.

Carrier-mediated transport works both ways. Uptake transporters bring drugs into cells (and aid absorption), while efflux transporters such as P-glycoprotein pump drugs back out, limiting absorption and protecting tissues. Competition and saturation at these carriers underlie many drug interactions and dosing surprises.