In biology and chemistry, a lipid bilayer is a membrane or zone of a membrane composed only of lipid molecules. The lipid bilayer is the foundation of all biological membranes, and is a prerequisite of cell-based life.
Within a critical range of concentrations, certain kinds of lipids alone in a test tube of water will self-organize to form a "bilayer". The bilayer 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 only a 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 type, include micelles, monolayers and vesicles. Cholesterol molecules in the bilayer assist in regulating fluidity.