Lipid membrane. Computer illustration showing the phospholipid membrane (centre) that surrounds cells (bottom). Cell membranes are composed of a bilayer of phospholipids that have hydrophilic ('water-loving') heads (spherical) and hydrophobic ('water-hating') tails. This causes them to align themselves with their tails pointing towards each other (away from any water), leaving the heads on the outside of the bilayer.