Facilitated Diffusion: Carrier Proteins and Ion Channels
Glucose, sodium ions and choride ions are just a few examples of molecules and ions that must efficently get across the plasma membrane but to which the lipid bilayer of the membrane is virtually impermiable. Their transport must therefore be "facilitated" by proteins that span the membrane and provide an alternative route or bypass. Facilitated diffusion is the name given this process. It is similar to simple diffusion in the sense that it does not require expenditure of metabolic energy and transport is again down an electrochemical gradient.
Two major groups of integral membrane proteins are involved in facilitated diffusion:
- Carrier proteins(also known as permeases or transporters) bind a specific type of solute and are thereby induced to undergo a series of conformational changes which has the effect of carrying the solute to the other side of the membrane. The carrier then discharges the solute and, through another conformational change, reorients in the membrane to its original state. Typically, a given carrier will transport only a small group of related molecules.
Some important and illustrative groups of transporters are:
- Certain of the hexose transporters, which transport glucose and similar monosaccharides into and out of cells
- Band 3, the anion transporter, which facilitates transport of bicarbonate and chloride ions
- Ion Channels do not really bind the solute, but are like hydrophilic pores through the membrane that open and allow certain types of solutes, usually inorganic ions, to pass through. In general, channels are quite specific for the type of solute they will transport and transport through channels is quite a bit faster than by carrier proteins. Additionally, many channels contain a "gate" which is functions to control the channel's permiability. When the gate is open, the channel transports, and when the gate is closed, the channel is closed. Such gates can be controlled either by voltage across the membrane (voltage-gated channels) or have a binding site for a ligand which, when bound, causes the channels to open (ligand-gated channels).
Ion channels allow currents the be carried across the membrane and are thus of particular importance in the physiology of excitable cells like neurons and muscle cells.
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Last updated on May 21, 1997