STRUCTURE AND FUNCTIONS OF CELL MEMBRANE (PLASMA MEMBRANE)
The cytoplasm of the animal cell is bounded by a thin limiting membrane called "Cell membrane".
- Nageli and Cramer called this membrane as cell membrane in 1855.
- In 1931 "Plowe" named this as plasmalemma.
- Modem cell biologists prefer to call it plasma membrane (According to De Robertis).
- Danielli & Davson in 1952 proposed molecular model of plasma membrane.
- Robertson proposed unit membrane concept in 1960.
1) Material used for study of plasma membrane:
- Human R.B.C. are kept in dilute solution (Hypotonic). Then it swells. It breakes and protoplasm will go out (Haemolysis). The remaining membrane is called Ghost. It is used for plasma membrane studies.
- Liver cell or the membrane surrounding the nerve fibre are used as a material for the study of plasma membrane.
2) Physical nature of plasma membrane:
Danielli described the nature of plasma membrane. Robertson by using electron microscope and X-ray diffraction methods proposed the structure of plasma membrane. The plasma membrane shows 3 layers. (Trilaminar).
- Outer layer is made by proteins....20 A°
- Middle layer is bimolecular lipid layer....35A°
- Inner layer is made by proteins ...20A°
3) Membrane model:
The membrane models are many. This model represents the molcular srganisation of the membrane.
- Danielli Davsons model: It shows thin lipid layer with protein absorbed on both the sides. The lipid contains polar heads pointing out sides and ion poiar parts run transversly. In 1954 Danielli modified this model and gave a new model.
- Robertson unit membrane concept: Robertson in 1959 described trilaminar structure of plasma membrane. It contains an outer and inner protein layers in between them a bimolecular lipid layer is present.
1) The unit membrane is 75A° thick.
2) The Outer and inner protein layers are 20A° thick.
3) In between them the lipid layer will show 35A° units.
4) The polar hydrophyllic ends of the lipid layer will face the proteins. Where as the hydrophobic ends of the lipids are away from the proteins.
- Fluid Mosaic model: In 1972 "Singer" & "Nicholson" proposed this model.
- Cell membrane is a mosaic of lipids and proteins.
- Lipids are arranged in a bilayer way. It forms the structural frame work of plasma membrane.
- Protein molecules are arranged in 2 ways.
1) Extrinsic proteins: These proteins are located adjacent to the outer and inner surfaces of the lipid layer.
2) Intrinsic proteins: These proteins will penetrate into the lipid layers partially or totally. They are called integral or intrinsic proteins.
This fluid mosaic model is accepted universally. The differential distribution of protein in the various regions of membrane is known as "Membrane asymmetry".
4) Chemical composition of plasma membrane:
Plasma membrane Shows proteins, lipids and small percent of carbohydrates.
a) Lipids: The lipids of the plasma membrane are phospholipid, lecithin cholesterol glycolipids. the distribution of phospholipids in the bilayer of plasma membrane is highly asymmetrical The phospholipid will show 2 ends.
- Hydrophobic end: It is water hating, non polar end.
- Hydrophilic end: It is water loving part. It is called polar end. It is present near the proteins.
Hydrophobic end contains fatty acids and Hydrophilic end contains a phosphate group.
b) Proteins: The peripheral proteins or extrinsic proteins are free from lipids. They are loosely attached to the membrane. The integral proteins or intrinsic proteins will penetrate into the lipid layer. These proteins will give mechanical strength to the plasma membrane. They regulate cell activities.
c) Carbohydrates: Very small percentage of carbohydrates are present. Hexoses, Hexosamine, Fructose etc., are present. They may be attached to lipids glycolipids; and they are attached to proteins... Glycoproteins. These glycolipids and glycoproteins are present on the external surface.
d) Enzymes: More than 30 enzymes are isolated from plasma membrane. They are phosphotase, ATP ase, RNA ase etc.
5) Pores in plasma membrane:
Plasma membrane shows some pores. The diameter is 35 Nm (nanometer). There are several models to describe the structure of pore.
- Structural pore: These are permanent openings.
- Dyanamic pores: They form during the intake of material.
- Protein channel pores: These are small channels of specific proteins through which some ions can diffuse.
- Modification of plasma membrane: To perform specialised functions plasma membrane will show some modifications.
- They are (a) Microvilli, (b) Inter digitations, (c) Desomosomes, tight junctions etc.
a) Microvilli: In certain parts the plasma membrane will show minute infloldings they are called microvilli. They show -1 mili micron diameter and 6 milli micron length. In some cases the villi are connected with endoplasmic reticulum. These microvilli are more in intestinal mucosa cells. The microvilli show cytoplasm with micro filaments. The microvilli will increase the absorptive surface of the intestinal epithelium.
b) Inter digitations: At some places the plasma membrane of adjacent cells will develop into finger like projections they are called inter digitations.
c) Desmosomes: The plasma membranes of adjacent cells become thicker in certain regions. On these thick areas fine, filaments are present. They are called tonbfilaments or tonofibrils. Such parts are called desmosomes. Desmosome is concerned with cell adhesion and maintenance of cell shape.
d) Terminal bar: It is a desmosome without tonofibrils. It is called terminal bar.
e) Zonula acculdens: These are special area of adjacent cells where the 2 plasma membranes fuse. These tight junctions are present below the apical boarder. It gives mechanical attachment between cells.
7) Properties of plasma membrane:
The substances which either enter the cell or leave the cell should pass through plasma membrane. The plasma membrane will' show permeability. It shows the following properties.
i) Osmosis: The diffusion of water molecular from low concentrated solution to high concentrated solution through plasma membrane is called osmosis.
ii) Passive transport: The movement of molecules from higher concentration to lower concentration without the expenditure of .energy, through plasma membrane is called passive transport.
- Endocytosis: It is the process of by which large particles of food are engulfed. 1) Pinocytosis or cell drinking : Ingestion of liquid through plasma membrane is called pinocytosis.
- Micro pinocytosis is observed at submicroscopic level. When a liquid come nearer to plasma membrane, it forms a vesicles. It is called pinoc vesicle. After some time it is pinched off from the plasma membrane and becomes pinocytic vesicle in the cytoplasm
- Phagocytosis: The process of taking solid food or solid material through the cell membrane into the cell is called phagocytosis. Phagocytosis means to eat. It is first discovered by "Metknikoff
- Exocytosis: It is also called cell vomiting . The process of sending out products from the cell to the out side is called exocytosis or emeiocytosis.
- Active transport: If molecules or ions move against the concentrate gradient through plasma membrane, it is called active transport. For this energy is required. Now a days it is proved that carrier system is seen in the plasma membrane and it is responsible for active transport.
8) Origin of plasma membrane:
- It is believed that it develop de novo.
- It is formed by the assembly of lipids & proteins.
- It is believed that it develops from other membrane systems.
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