In 1959, scientist Robertson proposed the unit membrane model of cell membrane structure. According to him, the molecular structure of all biological membranes is similar. It consists of three levels. The three levels together are called units or units. It is composed of phospholipid bilayers. Each lipid molecule has two parts. head and tail Molecules are polar and water absorbing (hydrophilic). The tail is non-polar and hydrophobic. Protein is grown in its place. A phospholipid bilayer with proteins is called a unit membrane. Its structure is protein-lipid-protein (P-L-P) with a total thickness of 75 Å. The outer layer is 20 Å thick and the inner layer is 35 Å thick.