The cell has a different internal environment from its outside. A distinct internal environment is possible by a thin limiting membrane called cell membrane.
It is so thin that it can not be seen with the light microscope. But in some cells, it is covered by thicker protective layers that can be seen with a light microscope. For example, the plant cells have cellulose walls that cover and protect the plasma membrane. Besides, all eukaryotic organisms possess various organelles, which are surrounded by intra-cellular membrane systems.
Lipids and proteins together constitute the major components of membranes. Some of the proteins and lipids, however, may have oligosaccharides, covalently attached to them. The sugar containing sequences of these glycoproteins and glycolipids also play a role in determining the identity of cells.
Membranes from various cells of an organism may differ widely, both in relative amounts and type of their constituent proteins and lipids.
The variations provide a basis for wide range of physiological activities displayed by different membranes. Again, as pointed earlier, eukaryotic cells contain intra-cellular membrane systems along with plasma membrane. Like the plasma
membrane, the internal membranes form closed compartments and have structural features li that of the plasma membrane.
The common structural organisation of all biological membran has been referred to as the unit membrane structure (J.D.Robertson, 1965).
The basic framework of all the membranes is a continuous double layer of lipid molecii in which protein molecules are dispersed. Lipids and proteins are held together by non-covali bonds, while oligosaccharides are attached to the lipid and protein molecules by covalentbom
There is a considerable variation in the lipid, protein and carbohydrate content aim different cell membranes depicts the ratio of the three components in several dive types of membranes. Cell membranes are asymmetric, dynamic and fluid in nature. Membr, fluidity is important for many of its functions.
(a) Membrane Lipids:
The major lipids in the cell membrane are phospholipids, cholesterol and glycolip Cholesterol is abundant in the plasma membrane of mammals. Plant membranes have little or cholesterol. Phytosterols are present in plant membranes. It is absent in cell membranes of mo of the prokaryotes. However, all the membranes have phospholipids.
The phospholipids are amphipathic in nature. Each phospholipid molecule ha: hydrophilic (polar) head and a hydrophobic (non-polar) tail. These molecules, in the aqueo medium, spontaneously associate in a way that their hydrophilic heads interact with water why hydrophobic tails remain away from it and in contact with eachother. The lipid molecules exchar
positions horizontally (laterally) with their neighbours in the same layer. Some, occasionally, move from one monolayer to the other. This has been termed as vertical (flip-flop) exchange.
(B) Membrane Proteins:
Membrane proteins are also amphipathic. Their hydrophobic regions are embeded in the lipid bi layer, where they interact with the hydrophobic tails of the lipid molecules. The hydrophilic regions of the protein molecules are exposed to water on one or both sides of the membrane.
Based on how easily they may be isolated from the membrane, proteins may be classified as peripheral or extrinsic and integral or intrinsic. Generally, peripheral proteins are exposed to one side of the bilayer.
They do not interact with the hydrophobic core of the phospholipid bilayer, but weakly bind to the hydrophilic regions.
The integral proteins are usually globular and they normally extend in the interior of the lipid bilayer. It directly interacts with hydrophobic regions of the bilayer. The hydrophilic regions of integral proteins are generally exposed to the cytoplasm and external aqueous phase outside the cell. The proteins that extend across the membrane are called transmembrane proteins.
Proteins, in the lipid bilayer of the cell membrane, carry out all the functions of the membrane such as transport of molecules across the membrane, receiving signals from hormones and establishing cell shape.
(c) Membrane Carbohydrate:
Carbohydrates (oligosccharides) in plasma membrane occur as glycol proteins and glycolipids most of the membrane carbohydrates are bound to protein molecules. A single glycoprotein may have many oligosaccharide side chains whereas a glycolipid molecule has only one. Glycoproteins are absent from all prokaryotic membranes. Carbohydrate chains of all the cell membranes are located extensively on the exoplasmic surface, i.e. outside the cell, while the membranes of the cellular organelles, they are exposed to the lumen of the membrane-bounded compartments. Examples are mitochondrial membranes, chloroplast lamellae and several intracellular membranes.
Although the functions of membrane carbohydrates are yet to be established, they seem to be involved in cell communication and recognition processes.
Structural organisation of cell membrabe:
In 1902, Overton proposed that the plasma membrane is composed of a thin lay lipid molecule, because substances only soluble in lipid could pass through the plasma mem’ easily. Later in 1926, Gorter and Grendell described the plasma membrane as a lipid bil Working on erythrocyte memberane, they observed that the lipid component, if spread monolayer, covered about twice the area of the membrane.
James Danielli and Hugh Davson in 1935 proposed the lipid bilayer model. Acco to this model, the plasma membrane is composed of a lipid bilayer which is coated on Bo surfaces by continuous monolayers of hydrated globular proteins. According to their mod– phospholipid molecules are at right angles to the plane of the protein layer. Their unch hydrophilic heads are associtated with ionised groups of the protein, present in the fonno sheets. The hydrophilic and hydrophobic properties of the membrane make it selec permeable.
In early 1960, David Robertson came forward with a modified version of Danielli-D model, called unit membrane model based on the information obtained by electron micro and functions of the membranes.
He proposed that the plasma membrane was of uniform thick with two protein (outer and inner) coats and middle phospholipid bi-layer. This triple layer membrane structure can be applied to the membranes of intracellular organelles. The lipid b is a permeability barrier.
Phospholipids are the predominant lipids in all biological memb they are amphipathic in nature. The cell membrane consists of a single unit membrane, which double membrane coverings of mitochondria and chloroplasts consist of two unit membe one above the other.
The grana of the chloropla’st are interpreted to be multilayered struck with unit memberanes closely packed and stacked one above the other.
John Singer and Garth Nicolson modified this model and called it fluid mosaic According to them, the protein molecules are distributed in the lipid bilayer in a mosaic p The lipid bilayer exists in a relatively fluid state, as some of the protein molecules are emb in and float within the bilayer, while other proteins penetrate the bilayer and protrude from both of its surfaces.
The lipid molecules exhibit two types of movements: horizontal (la and vertical (flip-flop). In lateral movement, the molecules change positions within the layer, while they move across the layers in the flip-flop movement.