Phosphatides



Phosphatides or phospholipids are a class of lipids and are a major component of all biological membranes. All phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline. They are a type of molecule. They form a lipid bilayer within a cell membrane.

Amphipathic character
The 'back' of a phospholipid is hydrophilic (attracted to water) and the lipophilic (often called hydrophobic) 'tails' repel water. When placed in water, phospholipids form a variety of structures depending on the specific properties of the phospholipid. In biological systems, the phospholipids often occur with other molecules (e.g., proteins, glycolipids, cholesterol) in a bilayer such as a cell membrane. Lipid bilayers occur when lipophilic tails line up against one another, forming a membrane with hydrophilic heads on both sides facing the water.

This membrane is partially permeable, capable of elastic movement, and has fluid properties, in which embedded proteins (integral or peripheral proteins) and phospholipid molecules are able to move laterally. Such movement can be described by the Fluid Mosaic Model, that 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. Cholesterol contributes to membrane fluidity by hindering the packing together of phospholipids. However, this model has now been superseded, as through the study of lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.