In this article we will discuss about the chemical composition and functions of plasma membrane.
Chemical Composition of Plasma Membrane:
The membrane is mainly composed of lipids, proteins and carbohydrates. Water makes about 29% of total weight. Robertson (1959) proposed that plasma membrane is three-layered structure where proteins form the outer and inner layers of membrane that encloses lipids to form a unit membrane.
i. Lipid:
The lipids identified in the plasma membrane consist of cholesterol, phospholipids and galactolipids. The phospholipids include phosphatidylcholine, phosphatidylethanolamine and sphingomyelin. The phospholipids are found to be associated with the outer protein shell in the plasma membrane. Glycerol and fatty acid constitute lipid molecules.
In the membrane the lipid molecules consist of two parts —a head and two tails. The head is composed of glycerol and is hydrophilic where as the tails are composed of fatty acids that are hydrophobic. The head and tail are usually designated as polar and nonpolar end respectively. The proteins, in the membrane, are present in two layers and the lipids occur in between them.
The lipid molecules are oriented in two (bimolecular) layers with their hydrophilic polar ends directed towards protein and the hydrophobic nonpolar ends face each other. Hydrogen bonds, ionic linkages or electrostatic forces bind the protein and lipid components.
ii. Protein:
In the membrane it is present as enzyme protein, carrier protein and structural protein. The enzyme proteins have catalytic activity. The carrier proteins help to transport materials in and out of the cell across membrane. The structural proteins play an important role to form the structure of membrane.
iii. Carbohydrates:
They occur in the form of glycolipids and glycoproteins. Both of these forms are confined exclusively to the external membrane surface. Bell (1962) is of opinion that polysaccharides confer some stability to the lipoprotein complex in the membrane.
Origin of Plasma Membrane:
Bell suggested that the phragmosomes, those appear during middle lamella formation and are composed of polysaccharides, help in the formation of new plasma membrane on reaction with lipoprotein.
Morphology of Plasma Membrane:
The plasma membrane is not readily visible under light microscope. Electron microscope shows that the plasma membrane is 6 nm to 10 nm thick. It consists of three layers—two outer dense layer, each about 2 nm thick and a middle less dense area of 3.5 nm across, for a total thickness of 7.5 nm.
On the basis of these findings Robertson (1959) proposed the unit membrane structure to the lipoprotein organization of plasma membrane. According to this concept the outer dense layers represent the protein and the enclosed less dense are the lipids. Very small pores, none of which are of greater diameter than 5 nm, interrupt the continuity of the membrane.
There are different models to explain the structure of plasma membrane. The unit membrane model of Robertson consisting of protein-lipid-protein layer is previously mentioned. The fluid mosaic model of the membrane as proposed by Singer and Nicolson (1972) is now generally accepted. This model assumes that the membrane is a semifluid structure where lipids and integral proteins are present.
They are arranged in a mosaic manner, i.e. the integral proteins are embedded to a greater or lesser extent in the continuous bilayers of phospholipids. The integral proteins are amphipathic molecules, i.e. within the same molecule hydrophilic and hydrophobic groups occur. The hydrophilic groups protrude from the surface while the hydrophobic groups remain embedded in the lipids (Fig. 1.12).
Schematic Representation of Molecular Organization of the Plasma Membrane
Functions of Plasma Membrane:
(i) It is selectively permeable membrane;
(ii) Its principal role is to regulate the flow of materials in and out of the cell;
(iii) Carrier proteins in the membrane are involved in the transport of certain materials across the plasma membrane;
(iv) In some cases it protects the inner cytoplasmic inclusions; and
(v) In some cases it perceives the chemical stimulus.