CELL MEMBRANES 》
1. All cells are enclosed by a thin, film-like plasma membrane or plasmalemma.
2. This is also known as cell as cell membrane (Nigeli and Cramer 1855) which is a component of the céll surface.
3. Plasmalemma is another term for plasma membrane.
4. The term 'Plasmalemma' was introduced by Seifriz in 1928 later followed by J.Q. Plower (1931).
5. The plasma membrane and other intracellular membranes surrounding organelles and vacuoles are collectively known as biological membranes.
6. Historically, E. Overton (1895) was first to study the structure or composition of plasma membrane.
7. Overton postulated that cell membrane is composed of a continuous layer of lipid material.
8. E. Gorter and F. Grendel (1925) studied RBCs of a variety of mammals and proposed that the cell membrane is formed of a bimolecular layer of lipid sheet.
● MODELS OF PLASMA MEMBRANE:-
Three important models explaining the ultrastructure of plasma or cell membrane are:
Three major types of cells in a multicelluiar organism are:
(i) Danielii-Davson Model
(ii) Robertsonian Unit Membrane concept
(iii) Fluid Mosaic Model
● DANIELLI-DAVSON MODEL:-
1. J.F. Danielli and H. Davson (1935) suggested a lipoprotein bilayer model of cell membrane based upon surface tension studies.
2. Danielli and Davson proposed that the plasma membrane is made up of three layers: a bimolecular layer of lipid sandwiched between two layers of proteins.
3. A lipoid layer of undefined thickness is sandwiched between two layers of hydrated globular proteins.
4. Danielli and Davson model is the oldest model on the structure of plasma membrane.
● UNIT MEMBRANE CONCEPT:-
5. Based upon electron microscopy on myelin, Robertson in 1959 proposed his famous 'unit membrane concept'.
6. According to Robertson the thickness of lipoid bimolecular layer is 3.5 nm, each protein layer 2.0 nm, making up a total thickness of 7.5 nm (75 A).
7. Robertson called it as a 'unit membrane' because the pattern of molecular organization was same for all mem- branes.
8. One of the major weaknesses of Robertson's model was its failure to explain permeability and transport properties of the membrane.
● FLUID MOSAIC MODEL:-
9. The most universally accepted 'fluid mosalc model' of structure of plasma membrane was proposed in 1972 by: S. Jonathan Singer of university of California and Garth Nicolson of the Salk Institute.
1O Singer and Nicolson took the help of freeze-fracture techniques in electron microscopy.
11. According to fluid mosaic model, the membrane contains a bimolecular lipid layer, the surface of which is interrupted by proteins.
12. Some proteins are attached at polar surface of lipids called the peripheral or extrinsic proteins and the other proteinswhich penetrate the bilayer or span membrane entirely are called the integral or intrinsic proteins.
13. The proteins on the outer side may bear chains of sugar forming glycoproteins.
14. The sugar chains are believed to be involved in a variety of physiological phenomena including the adhesion of cells to their neighbours.
15. Membrane lipid is primarily phospholipid, some quan- tities of neutral lipids may also be present.
16. Some of the lipid at the outer surface is complexed with carbohydrate to form glycolipid.
17. The molecular arrangement of plasma membrane resem- bles a moving sea of lipids that contains a "mosaic" of many different proteins.
18. The proteins may float freely like icebergs.
19. The lipids act as a barrier to the entry or exit of charged polar substances.
20 Some proteins in the plasma membrane act as "gatekeepers" that regulate the traffic of molecules and ions into and out of the cell.
21. The fluid mosaic model is now the most reasonable explanation of the structure of plasma membrane,
22. Selective permeability of plasma membrane can be explained with the help of fluid mosaic model.
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