The lipid bilayer is formed primarily of phospholipids, which, in a watery solution, orient with their hydrophilic (water-loving) heads toward the outside and theirhydrophobic (water-hating) tails to the inside. Other lipids, the sterols, are present in small numbers. (In animals, the representative sterol is cholesterol; in plants,stigmasterol).
Embedded in this bilayer are proteins that function in three cellular activities: transport, reception, and communication. These proteins are structured with two different configurations: Some extend through the lipid layer, with their hydrophobic portions amongst the lipid tails and their hydrophilic portions protruding on either side of the lipid layer. These are the transmembrane proteins. The second group, the peripheral proteins, are mostly glycoproteins (proteins with short carbohydrate side chains) attached to the projecting ends of the transmembrane proteins. Some of the proteins are not attached and appear to float; others, the integral proteins, are firmly embedded and don't move.