Cell plasma membrane. Computer model of a lipid bilayer plasma membrane. The membrane consists of a dual layer of phospholipids. Each phospholipid consists of a phosphate group head (blue) and a fatty acid tail (white). The tails are hydrophobic and so face inwards, forming an oily core. The hydrophilic heads form the outer layers. This structure forms spontaneously whenever water is present. The oily core means that only small hydrophobic solutes can pass through it. Other ions and molecules are transported through the membrane in protein channels and transporters. Lipid bilayer membranes are common to all living cells.