Amphipathic lipids such as phospholipids, sphingolipids and sterols are virtually insolub in water. When mixed with water, they arrange themselves spontaneously such that their hydrophili (polar) heads orient towards water molecules and the hydrophobic tails away from the water molecules.
Consequently, microscopic lipid aggregates are formed. Based on the precise natu of the lipid molecule, three types of aggregates are formed: micelle, bilayer and liposome Micelles are spherical structures containing a hundred to several thousand molecules we their hydrophobic tails directing towards the centre excluding water and the hydrophilic heads contact with water.
Micelle formation is favored when the cross section area of the head i greater than that of the tail. A bilayer is formed when the cross sectional areas of both the hea and the tail are equal. Two layers of lipid molecules spread such that the hydrophobic tails o both the layers orient towards each other and the polar heads orient towards water baitlayer a characteristic feature of all biological membranes.
The bilayer sheet alone is relatively unstabl and spontaneously folds back on itself forming a third type of aggregate called lipid vesicle o liposome. Liposomes are sometimes used as vehicles for delivering exogenous nucleic acid into cells in recombinant DNA technology.