Lipid bilayer phase behavior

Thus, in a liquid bilayer a given lipid will rapidly exchange locations with its neighbor millions of times a second and will, through the process of a random walk, migrate over long distances.

For a lipid to flip from one leaflet to the other, its hydrated headgroup must cross the hydrophobic core of the bilayer, an energetically unfavorable process.

Liquid phase bilayers can spontaneously heal small voids, much the same way a film of oil on water could flow in to fill a gap.

An example of this effect can be noted in everyday life as butter, which has a large percentage saturated fats, is solid at room temperature while vegetable oil, which is mostly unsaturated, is liquid.

The presence of cholesterol exerts a profound but complicated influence on lipid bilayer properties because of its unique physical characteristics.

[8] This interaction also increases the mechanical rigidity of fluid membrane lipid bilayers[9] and decreases their lateral diffusion coefficient.

[10] In contrast, the addition of cholesterol to gel phase bilayers disrupts local packing order, increasing the diffusion coefficient[10] and decreasing the elastic modulus.

Lipid rafts are cholesterol-enriched gel domains that have been potentially implicated in certain cell signaling processes,[12] but the subject remains controversial, with some researchers doubting even their existence in vivo.

[14] Phase transition temperature of liposomes and biological membranes can be measured using calorimetry, magnetic resonance spectroscopy and other techniques.

Diagram showing the effect of unsaturated lipids on a bilayer. The lipids with an unsaturated tail (blue) disrupt the packing of those with only saturated tails (black). The resulting bilayer has more free space and is consequently more permeable to water and other small molecules.
The chemical structure of cholesterol, which differs greatly from a standard phospholipid.
Example of lipid polymorphism as bilayer (le), reverse spherical micelles (M) and reverse hexagonal cylinders H-II phase (H) in negatively stained transmission electron micrograph of spinach thylakoid lipid-water dispersions.