Ion channels are present in the membranes that surround all biological cells. By conducting and controlling the flow of ions, these pore-forming enzymes help establish the small negative voltage that all cells possess at rest (see cell potential). An ion channel is an integral membrane protein or more typically an assembly of several proteins. Such "multi-subunit" assemblies usually involve a circular arrangement of identical or related proteins closely packed around a water-filled pore. Ions move through the pore single file--nearly as fast as the ions move through free fluid. Access to the pore is governed by "gates," which may be opened or closed by chemical or electrical signals, or mechanical force, depending on the variety of channel.
Transmembrane proteins of excitable cells that allow a flux of ions to pass only under defined circumstances. Channels may be either voltage-gated, such as the sodium channel of neurons or ligand-gated such as the acetylcholine receptor of cholinergic synapses. Channels tend to be relatively ion specific and allow fluxes of typically 1000 ions/msec. They are thus much faster at moving ions across a membrane than transporters and pumps.