Biological Membrane 

                As mentioned previously, biological membranes are selectively permeable. They allow nonpolar compounds but deny polar compounds.

             They are 5-8 nm thick. The mobility of lipids and protein molecules within membranes led to the discovery of fluid mosaic model which provided the basis for the structure of biological membranes.

Fluid Mosaic Model 

               ➤It was first proposed by Singer and Nicolson in 1972, in order to have a clear-cut idea on the structure of biological membranes.

               ➤Phospholipid is the main component of the membrane. It consists of two long tails that are made up of Fatty acids. They are hydrophobic in nature and form an oily Barrier to most water – soluble substances while the head if the molecule is hydrophilic and consists of a charged phosphate group, that binds to a charged nitrogen- containing group.

               ➤The phospholipid is a bilayer in which the central core is occupied by the hydrophobic tails while the surfaces are formed by hydrophilic heads. The bilayer faces both the internal and the external environment. The Proteins are seeming to float like icebergs and others extend through the bilayer. Some are also attached to both the inner and outer surfaces. Proteins that are bound to the carbohydrate chains are called as glycoproteins, and in the other hand, lipids that are bound to the carbohydrate chains are called as glycolipids.

                ➤As we read in the above paragraph, the phosphate ends of the bilayer tends to move towards the surface of the membrane, while the fatty acid chains move inwards. Proteins that runs through the membrane forms pores or channels that makes the membrane selectively permeable. The Proteins that are found inside are usually enzymes while that are embedded on the outer surface, helps in identifying themselves as a particular organism.

                ➤It is often mentioned as “hydrophobic interactions” while talking about phospholipid bilayer. The term describes the clusters of lipid molecules that are found in the hydrophobic molecular surfaces in an aqueous environment. It is clear that the molecules do not interact while they simply find an environment where the clustering reduces the amount of hydrophobic surface exposed to water and thus minimizes the number of molecules in the shell at the lipid-water interface.

               ➤When amphipathic lipids (having both hydrophilic and hydrophobic parts) are mixed with water, different types of lipid aggregates are being obtained. They are

(1)  Micelles

(2)  Bilayer

(3)  Vesicle.

                ➤Micelles are lipid aggregates that contain a number of amphipathic molecules ranging from a few dozen to a thousand. The individual units are wedge- shaped and the hydrophobic regions are found in the interior secluded from the aqueous environment. The cross section of head is greater than that of the side chains.

                ➤Next comes the bilayer, which is a compilation of two lipid monolayers. They are two – dimensional. In this case, the cross-sectional area and the side chains should be similar. This is often seen in glycerophospholipids and sphingolipids. The hydrophobic portion of both the layers interact with each other and since they are found at the edges, they are in contact with water which makes the lipid aggregate unstable and forms a sphere. The hydrophilic region may be in contact with the water or the hydrophilic regions that are found on either side of the monolayer. The spherical surface of vesicles helps in the excluded environment for the hydrophobic region, and thus achieving the maximum stability in their aqueous environment.

                ➤Three types of Membrane Proteins differ in the nature of their Association with the Membrane

                ➤Integral Membrane Proteins may be monotopic or polytopic, i.e., interacting with one if the bilayer or having polypeptide chains that runs throughout the membrane respectively. These membranes are embedded within the bilayer and the hydrophobic effect can be outwitted by agents such as detergents and organic solvents and also denaturants. On the other hand, peripheral membrane proteins are found to interact with the membrane through electrostatic interactions and Hydrogen bonding with the hydrophilic domains with the integral proteins and membrane lipids.

              ➤Amphitropic proteins are in contact with the membrane and sometimes, not in contact. This is due to a regulatory process such as reverse palmitoylation.

                          By Madhuvantii. N