Summary of Membrane Transport Processes
Other than lipid-soluble molecules (steroids, O2
, etc.) and some very small polar molecules (water, urea, ethanol, glycerol) (see Lipid Bilayer Permeability
), the passage of ions and most polar molecules across biological membranes requires the presence of integral membrane proteins that function as transport proteins
. Transport proteins are referred to as transporters
or, less commonly, carriers
, and may be active transporters
or passive transporters
. Figure 1 shown below summarizes the major pathways by which molecules/ions can cross biological membranes.
Figure 1. Summary of Membrane Transport Processes.
Only a limited number of molecules can cross biological membranes without the aid of transport proteins. The majority of biologically relevant molecules and ions utilize membrane transport proteins to cross the membrane. Transport systems may be passive or active. Passive transport proteins may be channels or facilitative transporters. Active transporters may be primary active transporters (pumps) or secondary active transporters (cotransporters and exchangers). See text for details. ATP, adenosine triphosphate; ADP, adenosine diphosphate, Pi, inorganic phosphate.
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+
/iodide symporter, Na+
/phosphate cotransporter, and transporters for amino acids and neurotransmitters. Examples of (secondary active) exchangers include the Na+
exchanger, and Cl-
/bicarbonate exchanger. See Secondary Active Transport
for additional details.
Posted: Friday, January 21, 2011
Last updated: Saturday, November 17, 2012