Tuesday 7 October 2003
Definition: Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from sphingosine instead of glycerol.
The first phospholipid identified as such in biological tissues was lecithin, or phosphatidylcholine, in the egg yolk, by Theodore Nicolas Gobley, a French chemist and pharmacist, in 1847.
The structure of the phospholipid molecule generally consists of hydrophobic tails and a hydrophilic head. It is usually found with cholesterol molecules which are found in-between the spaces of the phospholipid.
Phospholipids are a class of lipids formed from four components: fatty acids, a negatively-charged phosphate group, an alcohol and a backbone.
Phospholipids with a glycerol backbone are known as glycerophospholipids or phosphoglycerides. There is only one type of phospholipid with a sphingosine backbone; sphingomyelin.
Due to its polar nature, the head of a phospholipid is hydrophilic (attracted to water); the nonpolar tails are hydrophobic (not attracted to water).
When placed in water, phospholipids form a bilayer, where the hydrophobic tails line up against each other, forming a membrane with hydrophilic heads on both sides extending out into the water.
This allows it to form liposomes spontaneously, or small lipid vesicles, which can then be used to transport materials into living organisms and study diffusion rates into or out of a cell membrane.
This membrane is partially permeable, very flexible, and has fluid properties, in which embedded proteins and phospholipid molecules are constantly moving laterally across the membrane because of the forces generated by their vibrations.
Such movement can be described by the Fluid Mosaic Model, which describes the membrane as a "mosaic" of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are then free to diffuse laterally through the lipid matrix and migrate over the membrane.