It is the outer limiting membrane of both prokaryotic and eukaryotic cells. It is an ultra thin, elastic, living membrane. Plasma membrane is a dy-namic and selective transport barrier.
Since the plasma membrane is ultra thin, it could be observed only under electron microscope. Structure of the membrane is studied by isolating the same from the cell and conducting biochemical investigations.
In 1895 Overton suggested that the membrane is made of fatty sub-stances. Other workers later concluded that two layers of lipid were present in the cell membrane. According to a model proposed by Danielli and Davson in 1935, the lipid bilayer of the membrane was coated on either side with protein.
In 1960, Robertson using electronmicrographs proposed a unit membrane hypothesis. According to this hypothesis the two outer layers of proteinare about 2 nm thick and appear densely granular. They enclose a clear central area of about 3.5 nm wide consisting of lipids. The lipids are mainlyphospholipid molecules.
Singer and Nicholson (1972) have proposed a fluid mosaic model for the plasma membrane. The fluid mosaic membrane is a dynamic structure. In this structure much of the protein molecules float about. Some of them are anchored to the organelles within the cell. Lipid molecules also move about. 'Fluid mosaic model' is applied to all biological membranes in general.
The cell membrane controls the passage of materials both into and out of the cell. It regulates the passage of water and dissolved substances. Water passes through the membrane by Osmosis. Water soluble substances cross the membrane by diffusion or by active transport. Many water soluble sol-utes are transported by carrier proteins. Lipid soluble compounds pass more quickly by dissolving in the phospholipid layer.