Flexi Says: Early scientists who studied cells knew that cells must be surrounded by a boundary layer, but it took many decades to discover the structure of the plasma membrane. In the 1880s, a scientist named Charles Overton conducted studies to determine whether the boundary layer was polar or nonpolar based on the types of molecules that could cross it. From these studies, Overton concluded that the boundary layer was a lipid and that other lipids were able to freely enter and pass through. In the 1920s, other researchers (Gortner and Grendel) determined that the amount of lipids in the boundary layer was twice what they expected. They proposed that the boundary layer was made up of a double layer of lipids. This became the working model for the cell membrane. Finally, in the 1960s, advances in electron microscopy allowed researchers to determine that the bilayer structure had inner and outer surfaces with different properties than the hydrophobic interior. Furthermore, imaging and other studies revealed the roles of proteins embedded within the cell membrane structure. The fluid mosaic model was published by Singer and Nicolson in 1972 and is consistent with the modern model of the plasma membrane.
