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LIPID BILAYER
A lipid bilayer or bilayer lipid membrane 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 so is absolutely essential for all life on earth. Its essential structure was discovered in 1925 by two Dutch physicians, E.Gorter and F.Grendel, whilst comparing the surface area of human erythrocytes with that of the isolated lipids in a Langmuir Blodgett trough. They concluded the membrane was two molecules thick and proposed it was made of a bilayer.
[edit] Structure and function
The structure of a bilayer explains its possible function as a barrier. Lipids are fats, like oil, and 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 the polar head region, and the hydrophobic, or nonpolar tail region. The hydrophilic region is attracted to aqueous water while the hydrophobic region is repelled from it. Since lipid molecules contain regions that are both polar and nonpolar, they are called amphiphilic molecules.
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.
Because of the oily core, a pure lipid bilayer is permeable to small hydrophobic solutes, but has only a very low permeability to inorganic compounds and other ionic molecules.
[edit] Model lipid bilayers
Within a critical concentration range, certain lipids will self-organize in water to form a "bilayer", often referred to as a bilayer lipid membrane (BLM). Such membranes can be used in research, for example electrical recordings at the bilayer by using the patch clamp technique.
Black BLM: a BLM over an aperture between two aqueous solutions. The advantage of this method is the ability to control the constituents of each side of the membrane. The disadvantage of this method is that it causes the membrane to be fairly unstable, and rupture is certain in a matter of hours.
Supported BLM (s-BLM): a BLM covering an electrode patterned on a substrate. This method has the advantage of producing an extremely stable membrane, which in some cases may last several days before rupture. It also allows clean and specific electrical measurements thanks to the large experimental knowledge in electrical engineering for electrode patterning. Unfortunately this method has a major drawback in that it is possible to control the solution only on the side of the membrane that is not in contact with the electrode. (Although studies show that a 1 nanometer-thick water layer forms between the membrane and the electrode, this is of insufficient volume for controlling the solution composition.)
Polymer cushioned BLM: This technique is combination of the black BLM and supported BLM approaches. Electrodes are patterned over a surface and a polymer (typically a cellulose hydrogel) is coated on top of the electrodes. This polymer stabilizes the membrane and acts as a spacer from the solid substrate complex.
[edit] Other lipid structures
Lipids can assume self-organized structures other than bilayers, depending on their concentration and type:bilayers can be produced using one of three methods:
[edit] See also
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