membrane transporter

A membrane transporter is a protein ferry. The fatty cell membrane blocks many molecules from crossing on their own, so transporters grab specific substances and carry them through to the other side. Unlike an open channel, a transporter physically changes shape with each cargo it moves, like a revolving door that admits one passenger at a time.

Transporters move nutrients, ions, neurotransmitters and drugs across membranes, sometimes using cellular energy to pump substances against their concentration gradient. Drugs interact with them in two ways: a drug can inhibit a transporter to change the concentration of a natural substance, or it can hitch a ride on a transporter to enter cells it could not otherwise reach.

Many important medicines block transporters. Antidepressants inhibit the serotonin or noradrenaline transporters to raise neurotransmitter levels in the synapse, and certain diabetes drugs block a glucose transporter in the kidney. A clinically vital subset are efflux transporters such as P-glycoprotein, which pump drugs back out of cells and can limit absorption, restrict brain entry and drive drug resistance in tumours.