Cell membrane receptors. Computer artwork of a G protein-coupled receptor in a lipid bilayer plasma membrane. These receptors are transmembrane proteins that have seven membrane-spanning domains (purple) connected by extracellular loops. They are activated by the binding of molecules on the surface of the cell to induce cellular responses inside the cell. The activating molecules may be, amongst others, hormones, neurotransmitters or odours. The membrane of the cell consists of a dual layer of phospholipids (brown), with the hydrophobic lipid tails in the centre and the hydrophilic phosphate groups on either side. At bottom left is the nucleus, which contains the cell's genetic information.