Carrier-mediated transport

A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane proteins; that is they exist permanently within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport. The two main types of proteins involved in such transport are broadly categorized as either channels or carriers.

A carrier is not open simultaneously to both the extracellular and intracellular environments. Either its inner gate is open, or outer gate is open. In contrast, a channel can be open to both environments at the same time, allowing the solutes it transports to diffuse without interruption. Carriers have binding sites, but pores and channels do not. When a channel is opened, millions of ions can pass through the membrane per second, but only 100 to 1000 molecules typically pass through a carrier molecule in the same time. Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases.

Active transport is the movement of a substance across a membrane against its concentration gradient. This is usually to accumulate high concentrations of molecules that a cell needs, such as glucose or amino acids. When the lipid bilayer is impermeable to the molecule needing transport, active transport is also necessary. If the process uses chemical energy, such as adenosine triphosphate (ATP), it is called primary active transport. Secondary active transport involves the use of an electrochemical gradient, and does not use energy produced in the cell. Unlike channel proteins which only transport substances through membranes passively, carrier proteins can transport ions and molecules either passively through facilitated diffusion, or via secondary active transport. A carrier protein is required to move particles from areas of low concentration to areas of high concentration. These carrier proteins have receptors that bind to a specific molecule (substrate) needing transport. The molecule or ion to be transported (the substrate) must first bind at a binding site at the carrier molecule, with a certain binding affinity. Following binding, and while the binding site is facing the same way, the carrier will capture or occlude (take in and retain) the substrate within its molecular structure and cause an internal translocation so that the opening in the protein now faces the other side of the plasma membrane. The carrier protein substrate is released at that site, according to its binding affinity there.

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