A bilayer is a double layer of closely packed molecules, usually composed of phospholipids in a biological cell membrane. In this structure, the polar head groups of the lipids face the aqueous phase on both sides of the bilayer, and the hydrocarbon, nonpolar, chains oppose each other in the bilayer. The bilayer phase is also known as the lamellar phase or as the liquid disordered phase in lipid polymorphism. The composition of biological cell membranes is usually random leading to the liquid disordered definition.
Techniques to Study
editX-Ray diffraction data have been used to establish the electron density profile across a phospholipid bilayer. The electron density profile usually shows a symmetric plot, mainly due to the fact that the phospholipid bilayer is symmetrical in composition. However, if a membrane protein were introduced into the bilayer, this symmetry would be destroyed. The middle of the bilayer has the least electron density. The midplane of the membrane contains the terminal methyl groups of the phospholipid hydrocarbon chains, which have a considerable amount of rotational freedom. The outside of the bilayer contains the highest degree of electron density. This location corresponds to where the electron rich phospholipid head group resides. As a result, it is possible to obtain the thickness of the bilayer membrane by collecting electron density profiles. Though this is not the exact membrane thickness, due to the bulky head groups of the phospholipids not allowing the membrane to be perfectly parallel with itself, it is an accurate measurement of the overall membrane and bilayer thickness. Also, it is important to note that the thickness of the bilayer changes in direct proportion with the acyl chain length of the phospholipid.
Another aspect of phospholipids that effect the thickness of bilayer membranes is the degree of unsaturation. The thickness of the bilayer is decreased upon an increase in unsaturation. Thus, with the increase in unsaturation, a phospholipid will increase the surface area it occupies and decrease its average length, thereby preserving its total molecular volume.
Packing of Lipids Within the Bilayer
editUnderstanding how bilayers pack together can be obtained by considering the shape of the phospholipid molecule. The packing depends on the cross-sectional area of the head group and how it compares to the cross-sectional area of the hydrocarbon, tail group. Quantitatively, this can be describes as:
Pr = Ah /Ac
Where Ah is the effective cross sectional area of the polar head group, and Ac is the cross sectional area of the hydrocarbon chain region, and Pr is the packing ratio. A lipid bilayer should have a Pr ≈ 1. This would indicate a cylindrical shape, where the cross-sectional areas of both the head and tail groups are approximately the same. This would lead to a bilayer packing. When the packing ratio is either greater or less than 1, the packing of lipids change, which give rise to the hexagonal I, hexagonal II and the cubic phases of lipids.
Gennis, R. B. (1989). Biomembranes: Molecular structure and function. Michigan: Springer-Verlag.</references>
Yeagle, P. (1993). The membranes of cells (2nd ed.). Michigan: Academic Press. </references>
Yeagle, P. (2005). The structure of biological membranes (2nd ed.). United States: CRC Press. </references>