Every cell is enclosed by a plasma membrane .
In addition, the cytoplasm of eukaryotic cells contains a variety of organelles, each bounded by an internal membrane . All biological membranes are assemblies of lipid and protein molecules held together by non-covalent interactions. The lipids are amphipathic , ie they have a hydrophilic polar 'head' and a hydrophobic non-polar 'tail', and they therefore spontaneously form bilayers in aqueous solution. The membrane is fluid, with individual lipid molecules able to diffuse freely within the bilayer. It may therefore be thought of as a '2-dimensional solvent' for the proteins.