Within a critical range of concentrations, certain kinds of lipids alone in a test tube of water will self-organize to form a "bilayer." It is composed of two opposing layers of lipid molecules arranged so that their hydrocarbon tails face one another to form the oily bilayer core, while their electrically charged or polar heads face the watery or "aqueous" solutions on either side of the membrane.
Because of the oily core, a pure lipid bilayer is permeable to small hydrophobic solutes but has an only very low permeability barrier to inorganic ions and other hydrophilic molecules. Water itself is an exception to this rule, and crosses freely.
Other self-organizing structures that lipids assume, depending on their concentration and the type of lipid, include micelles, monolayers and vesicles. Cholesterol molecules, in the bilayer, assist in regulating fluidity.
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