A cell is a tiny structure, which cannot be seen with naked eyes. A membrane surrounds cell, which is commonly known as the cell membrane. The function of a cell membrane is to protect the cell from its surrounding. Let us see the discovery of cell membrane from the beginning.
During the eighteenth century, Charles Overton suggested that cell membrane, the outermost covering of cell is composed of lipids. In his observations, he found that the materials, which are soluble in lipids enter the cell, but the materials which were water-soluble do not enter into the cell. In 1925, Evert Gorter and Francois Grendel said that the cell membrane consists of two layers which are sandwiched together.
Later in the year 1935, James Danielli and Hugh Davson stated that it is not just a two layer membrane but some protein molecules are found above the layers. This theory was also known as the Lamellar theory or sandwich theory.
- They found that the phospholipid molecules were perpendicular to the surface.
- These protein molecules are globular.
- Protein and carbohydrate molecules are linked together by Vander Waal forces which are very weak.
Demerits of Lamellar theory:
- In this theory, there was no information about the movement of water and water soluble materials around the membrane.
- They did not explain the phenomenon of active transport.
- They assumed that the cell membrane was very stable.
Later on, David Robertson came up with the Robertson model. He based his model on the observation of a red blood cell. He found that the cell membrane structure is a three layer structure in which there are two dark staining layers and one light staining layer in between. The outer layers were considered as protein layers and the middle layer as lipid bi-layer. He also said that proteins are not necessarily globular, but they can be extended. So he proposed a hypothesis that says cell membrane have a phospholipid bilayer, which is sandwiched between the two protein layers.
He also stated that the membranes of various particles in the cell can be of two types: Unit Membrane and Multiple Membrane. The Golgi, the Endoplasmic Reticulum, and the Lysosomes all have a unit membrane, whereas Mitochondria and Plastids have multiple unit membranes.
This was just the brief layout about cell membranes. To know more about it and to visualize the structure of cell membrane watch the above video.