The term was originally used by Nageli and Cramer (1855) for the membranous covering of the protoplast. Plasmalemma or plasma membrane was discovered by Schwann (1838). Membranes also occur inside the cytoplasm of eucaryotic cells as covering of several organelles like nucleus, mitochondria, plastids, lysosomes, Golgi bodies etc. Vacuoles are separated from cytoplasm by a membrane called tonoplast.
All membranes whether external or internal are now called biomembranes. Average thickness is 75 Angstrom. Biomembranes are selectively permeable membranes.
Composition:- Chemically biomembrane consists of lipids (20-79%), proteins (20-70%), carbohydrates (1-5%) and water.The important lipids of the membrane are phospholipids, sterols, glycolipids, sphingolipids.
The lipid molecules are amphiatic or amphipathic, they posses both polar hydroplilic (water loving) and nonpolar hydrophobic (water repelling) ends. The hydrophilic region is in the form of head and hydrophobic tails usually occurs towards the center of the membrane. It results in the formation of lipid bilayer.
Sveral types of models have been put forward to explain the structure of biomembrane.
Lamellar Models :- They are the early molecular models of biomembrane. according to these models biomembranes are stable layered structures.
Danielli and Davson Model :- This model can be described in the following points:-
1) According to this model the plasma membrane consists of a biomolecular layer of phospholipids which is sandwiched between the two layers of proteins.
2) The molecules of phospholipids are arranged in the two parallel chains.
3) The polar hydrophilic heads of the phospholipid molecules are facing towards outer surface while hydrophilic tails lie toward the centre.
4) The protein layers are asymmetrical. On one side folded beta chain proteins are present while on the other side globular proteins are present.
Drawbacks :- a) this model fail to explain how the water and water soluble materials pass through cell membrane.
b) How the biomembranes are flexible.
c) How active transportation occurs through biomembranes.
All membranes whether external or internal are now called biomembranes. Average thickness is 75 Angstrom. Biomembranes are selectively permeable membranes.
Composition:- Chemically biomembrane consists of lipids (20-79%), proteins (20-70%), carbohydrates (1-5%) and water.The important lipids of the membrane are phospholipids, sterols, glycolipids, sphingolipids.
The lipid molecules are amphiatic or amphipathic, they posses both polar hydroplilic (water loving) and nonpolar hydrophobic (water repelling) ends. The hydrophilic region is in the form of head and hydrophobic tails usually occurs towards the center of the membrane. It results in the formation of lipid bilayer.
Sveral types of models have been put forward to explain the structure of biomembrane.
Lamellar Models :- They are the early molecular models of biomembrane. according to these models biomembranes are stable layered structures.
Danielli and Davson Model :- This model can be described in the following points:-
1) According to this model the plasma membrane consists of a biomolecular layer of phospholipids which is sandwiched between the two layers of proteins.
2) The molecules of phospholipids are arranged in the two parallel chains.
3) The polar hydrophilic heads of the phospholipid molecules are facing towards outer surface while hydrophilic tails lie toward the centre.
4) The protein layers are asymmetrical. On one side folded beta chain proteins are present while on the other side globular proteins are present.
Drawbacks :- a) this model fail to explain how the water and water soluble materials pass through cell membrane.
b) How the biomembranes are flexible.
c) How active transportation occurs through biomembranes.
Robertson Model :- J. David Robertson (1959) modified the model of Danielli and Davson. This model can be described in the following points:-
a) The cell membrane consists of three layers - outer dark, middle light and inner dark. The outer and inner proteins layers and middle one is layer of phospholipids.
b) According to this theory the cell membrane is a unit membrane, consisting of a biomolecular lipid layer between the two protein layers.
Robertson Model of cell membrane
c) Each protin layer is 20 angtsrom in thickness and the bimolecular lipid layer is 35 angstrom.
It is simply the modification of sandwich model therefor it faces the same drawbacks as of sandwich hypothesis.
Mosaic Model :- It is most recent model of biomembrane proposed by Singer and Nicolson in 1972.
According to this model the membrane dors not have a uniform disposition of lipids and proteins. Further the membrane is not solid but is quasifluid.
Fluid-mosaic model postulates that the lipid molecules are present in a viscous bilayer as in lipid layer. Protein molecules occur at place both inside and on the outer side of lipid bilayer -- Proteins icebergs in a sea of lipids. The internal proteins are called intrinsic or integral proteins while the external ones are known as extrinsic or peripheral proteins.The intrinsic proteins account for 70% of the total membrane proteins. Some of the intrinsic proteins run throughout the lipid bilayer. They are called tunnel proteins. The extrinsic proteins are present superficially on the two surfaces of the membrane. Many membranous proteins function as enzymes. Some of them behaves as permeases. Some lipids on the outer side possess small carbohydrate molecules to form glycolipids.
Evidences in support of Fluid Mosaic Model:-
1) The model provides the occurrence of protein particles both on the surface and interior of cell membranes.
2) Fluid mosaic model can explain the presence of different types of permeability of cell membrane.
3) It accounts for dynamic nature of biomembranes.
4) The model explain the passage of both electrolytes and non-electrolytes.
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