Cell membrane ion channel. Computer artwork of an ion channel (orange) in a lipid bilayer plasma membrane. The ion channel is a pore-forming transmembrane protein that allows selected ions (yellow) to diffuse in and out of the cell. The ions move down their electrochemical gradient changing the electrical potential of the cell. This mechanism is particularly important in nerve and muscle cells, where it is responsible for the transmission of nerve impulses and contraction of fibres respectively. The membrane of the cell consists of a dual layer of phospholipids (green), with the hydrophobic lipid tails in the centre and the hydrophilic phosphate groups on either side.