Lipid bilayer
From Freepedia
In biology and chemistry, a lipid bilayer is a membrane or zone of a membrane composed of lipid molecules (usually phospholipids). The lipid bilayer is a critical component of all biological membranes, including cell membranes, and is a prerequisite for cell-based organisms.
Structure and function
The structure of the lipid bilayer explains its function as a barrier. Lipids are fats, like oil, that are insoluble in water. There are two important regions of a lipid that provide the structure of the lipid bilayer: the hydrophilic region, also called a polar head region, and the hydrophobic, or nonpolar tail region. The hydrophilic region is attracted to aqueous water conditions while the hydrophobic region is repelled from such conditions. Since a lipid molecule contains regions that are both polar and nonpolar, they are called amphipathic molecules.
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.
Other lipid structures
Lipids can assume self-organizing structures other than bilayers, depending on their concentration and type:
