carrier-mediated transport

Movement which occurs across membranes, such as the blood–brain barrier and the gastrointestinal mucosa. Inherent in the mechanism is a rapidly reversible reaction between the substance being transported and components of the membrane. The membrane component is the ‘carrier’. The mechanism is also characterized by (1) being saturable; (2) like substances being able to compete for the services of the carrier so that competitive inhibition occurs.
There are two forms of carrier-mediated transport, active transport and facilitated diffusion. The rapid transfer of drug metabolites into urine is by active transport. Entry of glucose into most cells is by facilitated diffusion but its passage across the gastrointestinal mucosa is by active transport. Active transport requires a direct expenditure of energy, whereas facilitated diffusion is not energy dependent. Active transport can move substances against a concentration gradient, facilitated diffusion cannot.