Synthetic lipid bilayer, coloured atomic force micrograph (AFM). The membrane consists of a dual layer of phospholipids. Each phospholipid consists of a phosphate group head and a fatty acid tail. 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. Lipid bilayer membranes are common to all living cells. Magnification: x51,000 when printed at 10 centimetres wide.