The structure of a bilayer explains its function as a barrier. Lipids are amphiphilic molecules since they consist of polar head groups and nonpolar acyl tails. The bilayer is composed of two opposing layers of lipid molecules arranged so that their hydrocarbon tails face one another to form an oily core, while their charged heads face the aqueous solutions on either side of the membrane. Thus, the bilayer consists of the hydrophobic core region formed by the acyl chains of the lipids, and membrane interfacial regions that are formed by the polar head groups of lipids. The hydrophilic interfacial regions are saturated with water, whereas the hydrophobic core region contains almost no water.
Because of the oily core, a pure lipid bilayer is permeable to small hydrophobic solutes, but has only a very low permeability to polar inorganic compounds and ionic molecules.