Membrane
lipids are a group of compounds (structurally similar to fats and oils) which form the double-layered surface of all cells (
lipid bilayer). The three major classes of
membrane lipids are
phospholipids,
glycolipids, and
cholesterol. Lipids are amphiphilic: they have one end that is soluble in water ('polar') and an ending that is soluble in fat ('nonpolar'). By forming a double layer with the polar ends pointing outwards and the nonpolar ends pointing inwards membrane lipids can form a '
lipid bilayer' which keeps the watery interior of the cell separate from the watery exterior. The arrangements of lipids and various proteins, acting as receptors and channel pores in the membrane, control the entry and exit of other molecules and ions as part of the cell's metabolism. In order to perform physiological functions, membrane proteins are facilitated to rotate and diffuse laterally in two dimensional expanse of
lipid bilayer by the presence of a shell of lipids closely attached to protein surface, called
annular lipid shell.