carriermediated

Carrier-mediated transport, or carrier-mediated uptake, refers to the movement of solutes across biological membranes via specific transmembrane proteins that bind the solute and undergo conformational changes to shuttle it across the lipid bilayer. This mechanism is selective for particular substrates and can be either passive or energy-requiring.

Two main modes characterize carrier-mediated transport: facilitated diffusion, where substrates move down their electrochemical gradient without direct energy use; and active transport, which uses energy to move substrates against their gradient. Primary active transport uses ATP directly; secondary active transport couples transport to ion gradients, such as Na+ or H+ gradients, generated by pumps elsewhere. Carriers can thus support both energy-dependent and energy-independent transport depending on the context.

Carrier-mediated transport shows saturable kinetics; as substrate concentration increases, the rate approaches a maximum (Vmax) and depends on Michaelis-Menten constants (Km). Transporters display specificity and stereospecificity; different substrates may compete for the same transporter, and inhibitors or drugs can block transport. The process is linked to the expression level and regulation of specific transporter proteins.

Common examples include glucose transporters (GLUT1–GLUT4) that mediate facilitated diffusion of glucose, and Na+-dependent glucose cotransporters (SGLT1, SGLT2). Other solutes are handled by solute carrier (SLC) family transporters, including organic anion and cation transporters (OATs, OCTs). Drug absorption, distribution, and excretion frequently rely on carrier-mediated processes, and transporter dysfunction can contribute to disease.

Carrier-mediated transport is contrasted with channel-mediated transport, which forms pores for ions and is typically faster and less saturable. Understanding carrier-mediated mechanisms is important in physiology, pharmacology, and disease, influencing nutrient uptake, drug delivery, and resistance mechanisms.