facilitatedtransport

Facilitated transport, also known as facilitated diffusion, is a passive membrane transport process in which substances cross the cell membrane with the help of specific membrane proteins. It enables large, polar, or charged molecules that cannot readily diffuse through the lipid bilayer to move down their electrochemical gradient without direct use of cellular energy.

The mechanism involves two main classes of transport proteins: channels and carriers. Channel proteins form hydrophilic aqueous pores that allow selective passage of ions or water, often driven by charge and electrochemical gradients. Carrier proteins bind their substrate on one side of the membrane, undergo conformational changes, and release it on the other side; they are typically saturable and exhibit substrate-specificity, functioning in a manner analogous to enzyme kinetics.

Key characteristics include dependence on existing concentration or electrochemical gradients, lack of direct ATP use, and potential for regulation and inhibition. Transport rates are limited by the number of available transporters and can display saturation at high substrate concentrations. Specificity and regulation allow cells to control uptake and distribution of essential nutrients and ions.

Examples of facilitated transport include glucose uptake into many cells via GLUT transporters and ion or water movement through various channels such as chloride channels or aquaporins. In physiology, facilitated diffusion supports nutrient acquisition, ion homeostasis, and fluid balance. It is distinct from active transport, which moves substances against their gradients using energy input, often via pumps. Transporter malfunctions can contribute to disease states, including channelopathies that affect nerve signaling and muscle function.