The cell membrane serves as a barrier for the molecules entering or leaving the cell. It is very easy to think that there is a remarkable difference between the internal and external environment of the cell and a plasma membrane serves as a boundary between the two. The presence of a membrane in the cell was visualized for the first tin by C. Nageli and C. Cramer in the year 1855 and they named it as cell membrane. Its presence was later confirmed by E. Overton in the year 1899 and he assumed that it was a double layer of lipid molecules. Since then efforts are being made to elucidate the ultra-structure of this membrane. It was given the name plasma-lemma by J. Flower in1931. In eukaryotic cells, in addition to plasma membrane intracellular membranes are also present which surround the vacuole and organelles. Plasma membranes and sub-cellular membranes are group together as biological membranes or bio-membranes.
Cell Membrane is composed of 60-80% proteins, 20-40% lipids and 1-5% carbohydrates. Proteins provide mechanical support to the membrane and act as channels for various metabolic activities. Phospholipids, cholesterol and galactolipids are the main lipids found in plasma membranes but phospholipid is the only lipid found in internal membranes.
There are some proteins which are definite to membranes and these are called membrane receptors. These receptors mediate the flow of materials and information into the cell. They attach the specific molecules reaching the cell surface and help in their flow into the cell.
Unit Cell Membrane Concept
As per J. F. Danielli and H. Davson in the year1935, the plasma membrane has a trilaminar structure, i.e., a lipid layer is surrounded on either side by a protein layer .The proteins are usually represented in globular form. Studies by Electron microscopic also supported the presence of three layers in the cell membrane. There is an outer and an inner dark osmiophilic layer separated by a light osmiophobic layer. The osmiophobic layer is about 30 Å and osmiophilic layer about 25 Å thick. . All unit membranes consist of a bimolecular layer of phospholipids covered by a layer of proteins. Phospholipids are oriented in such a way that their fatty acid groups face towards each other and water soluble ends outwards.
Micellar Cell Membrane Theory
According to the micellar theory of Hilleir and Hoffman in the year 1953, the plasma membrane consists of a mosaic of closely packed globular subunits (micelles) having a central core of lipid molecules with hydrophilic polar end. Each lipid micelle measures 40-70 Å in diameter. A Single layer of globular proteins is present on both side of the lipid micelle. The spaces between the globular micelles are represented by water filled pores which measure about 4Åin diameter. The pores are surrounded partly by the polar groups of micelles and partly by the polar groups of protein molecules.
Fluid Mosaic Model of Cell Membrane
According to Fluid Mosaic Model of Cell Membrane, proposed by S. J. Singer and G. Nicholson in the year 1972, the lipids and included proteins in a membrane are disposed in a sort of mosaic pattern. The lipids are arranged as a thin double layer while the proteins can be compared to icebergs floating in a sea of the phospholipid double layer. The proteins may be of peripheral and integral types. The previous remains associated with membrane surface, while the second enters the lipid double layer and makes an integral part of the cell membrane.
Biological cell membranes are thus semi-fluids so that lipids and intrinsic proteins can make movements within the double layer but are held in their positions by means of non-covalent interactions. The lipid molecules may display intra-molecular movements or may rotate about their axis or may show flip-flop movements. The fluid-mosaic model explains the dynamic and functional properties of the cell membrane in contrast with the static picture of the cell membrane in the Danielli-Davson model. The proteins are disturbed with the enzymatic activity of the membrane, transport of molecules, and a receptor function. The lipid double layer provides permeability barricade. While the IDanielli-Davson model assumes hydrophilic bonding between lipids and proteins.