Cell membrane ion channels. Computer artwork of a section through the membrane of an animal cell, showing transmembrane ion channel proteins (yellow). The membrane of the cell consists of a dual layer of phospholipids (grey), with the hydrophobic lipid tails in the centre and the hydrophilic phosphate groups (spherical) on either side. Spanning the membrane are proteins that act as selective channels to allow the movement of specific molecules and ions (blue) in and out of the cell.