Lipid bilayer
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A diagonal molecular slab from the DPPC lipid bilayer simulation1; color scheme: PO4 - green, N(CH3)3 - violet, water - blue, terminal CH3 - yellow, O - red, glycol C - brown, chain C - grey. |
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 absolutly essential for all life on earth.
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.
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.
Lipids can assume
self-organized structures other than bilayers, depending on their concentration and type:
*
Micelles
*
Monolayers
*
Vesicles
*
Biological membrane*
Extracellular matrix*
OTG*
Phospholipid
