Later chemical analysis revealed the membrane contains phospholipids. In 1972, Singer and Nicolson introduced the currently accepted fluid-mosaic model of membrane structure. Plasma membrane is phospholipid bilayer in which protein molecules are partially or wholly embedded.
Membrane structure has two components, lipids and proteins.
Lipids are arranged into a bilayer. Most plasma membrane lipids are phospholipids, which spontaneously arrange themselves into a bilayer. At body temperature, the phospholipid bilayer has consistency of olive oil. The greater the concentration of unsaturated fatty acid residues, the more fluid the bilayer. However, phospholipid molecules rarely flip-flop from one layer to the other.