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 is a prerequisite for cell-based organisms.

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 amphipathic molecules.

The bilayer is composed of two opposing layers of lipid molecules arranged so that their hydrocarbon tails face one another and form an oily core, while their electrically 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 ions and other hydrophilic molecules.

Model lipid bilayers

Within a critical range of concentrations, certain kinds of lipids alone in a test tube of water will self-organize to form a "bilayer", often referred to as bilayer lipid membranes (BLMs). Such membranes can be used in research, for instance on their electrical behavior (using the patch clamp technique, for example).

Model bilayers can be produced using one of three methods:

Over an aperture between two aqueous solutions, where it is called a black BLM. 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.
Over an electrode patterned on a substrate, where it is called a supported BLM or s-BLM. 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.)
Over a polymer cushion. This more recent technique allows a combination of the first two methods. Electrodes are patterned over a surface and a polymer (typically a hydrogel of cellulose gel) is coated on top of these electrodes. This polymer will act as a stabilizer for the membrane and also a spacer from the solid substrate.

Other lipid structures

Lipids can assume self-organized structures other than bilayers, depending on their concentration and type:

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Structure and function

Model lipid bilayers

Other lipid structures

See also