Illustration of a section through a cell membrane. This is a semi-permeable membrane that controls what substances leave and enter the cell. It consists of a phospholipid bilayer, with the hydrophobic lipid tails in the centre and the hydrophilic phosphate groups on either side. Various proteins are embedded within the membrane. These can be transmembrane proteins (one at top left) or peripheral surface proteins. Attached to the outer surface proteins are molecular chains such as carbohydrates and glycolipids.