Summary of Membrane Transport Processes

Transport systems may be passive or active. Passive transport does not require direct energy expenditure. It utilizes already existing concentration gradients. Passive transport may be through the lipid bilayer (simple diffusion), mediated by a channel (passive diffusion), or mediated by a facilitative transporter (facilitated diffusion or uniport), but movement is always down an electrochemical gradient. Active transport may be primary or secondary. In both cases, direct energy expenditure is required. In primary active transport, adenosine triphosphate (ATP) is hydrolyzed by the very protein performing transport in order to provide the free energy needed for transport against an electrochemical gradient. Thus, primary active transporters are ATPase proteins. Primary active transporters are also referred to as pumps. In contrast, secondary active transporters use the energy stored in the concentration gradient of a driving ion (typically, but not always, Na⁺ or H⁺). Secondary active transporters couple the movement of the driving ion down its electrochemical gradient to the movement of another molecule or ion against a concentration gradient. When the driving ion and the driven molecule (or ion) move in the same direction, transport is referred to as cotransport or symport. When the driving ion and the driven molecule (or ion) move in opposite directions, transport is referred to exchange or antiport.

Examples of channel proteins include water channels (aquaporins), a variety of voltage- and ligand-gated ion channels, mechanosensitive channels, and others.

An example of a facilitative transporter is the ubiquitous glucose transporter (GLUT) found in the plasma membrane of virtually all body cells.

Examples of primary active transporters (i.e., pumps) include the ubiquitous Na⁺/K⁺/ATPase, as well as H⁺/ATPase, H⁺/K⁺/ATPase, and Ca²⁺/ATPase.

Examples of (secondary active) cotransporters include the Na⁺/glucose cotransporter, Na⁺/K⁺/2Cl^- cotransporter, Na⁺/iodide symporter, Na⁺/phosphate cotransporter, and transporters for amino acids and neurotransmitters. Examples of (secondary active) exchangers include the Na⁺/H⁺ exchanger, Na⁺/Ca²⁺ exchanger, and Cl^-/bicarbonate exchanger. See Secondary Active Transport for additional details.

For a complete of list of membrane transport proteins found in the human genome, please visit the Human Genome Organization (https://www.hugo-international.org/) nomenclature page (https://www.genenames.org/).