Lipid bilayer membrane. Molecular model simulation of a lipid bilayer membrane consisting of POPC lipids (green), surrounded by water molecules (red and white). The POPC lipids consist of a hydrophobic hydrocarbon chain (green) and a hydrophilic phosphate end (pink and red). In water, the hydrophobic ends orientate away from water molecules, while the hydrophilic ends bond with the water molecules. Thus the lipids assemble in opposing pairs to form the bilayer structure seen here and in cell membranes. This simulation involves 128 POPC lipids and 2460 water molecules. POPC is also known as 1-palmitoyl 2-oleoyl phosphatidylcholine. The atoms are: carbon (green), phosphorus (pink), oxygen (red), hydrogen (white). Only the water hydrogens are shown.