STRUCTURE AND FUNCTIONS OF CELL MEMBRANE (PLASMA 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.
- 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.
- Outer layer is made by proteins....20 A°
- Middle layer is bimolecular lipid layer....35A°
- Inner layer is made by proteins ...20A°
- 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.
- 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.
- 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.
- 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.
- 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.
- Posted by BS Media