The basic structural component of biological cell membranes are bilayer-forming lipids. In these amphiphilic molecules a hydrophilic group is connected to one or two hydrophobic hydrocarbon chains. When dissolved into water they spontaneously assemble into a variety of structures. In Nature lipid bilayers form the outer plasma membrane of cells as well as the walls of the different cellular compartments and organelles, such as the endoplasmic reticulum, the Golgi apparatus, and the nucleus.