CELL MEMBRANE | कोशिका झिल्ली

0
68
FLUID MOSAIC MODEL

 INTRODUCTION OF CELL MEMBRANE

•Plasma membrane encloses every type of cell , both prokaryotic and eukaryoti iic cells .

• It physically separate the cytoplasm from the surrounding cellular environment .

Advertisement

• It is a ultra-thin ( 5-8 nm ) , elastic , living , dynamic and selectively permeable barrier .

• It control the entry and exit of molecules / ion concentration between the anterior and exterior of the cell .

 

HISTORY OF CELL MEMBRANE

• The term ‘ cell membrane ‘ was coined by C.Nageli and C.Cramer in 1855 .

• In 1900-1920 , further studies on plasma membrane state that lipid must be a phospholipid .

• In 1925 , E

• Gorter and G.Grendel proposed that plasma membrane is composed of phospholipid bilayer.

 

 

DEFENATION OF CELL MEMBRANE

An outermost envelope surrounding the cell that separates and protects the cell from the external environment and provides a connecting system between the cell and it’s environment it is call Plasma membrane or cell membrane.

 

 

COMPOSITION:-

LIPID – 40%

PROTEIN – 52%

CARBOHYDRATES – 8%

 

The cell membrane is composed mainly of protein , lipid and a small percentage of oligosaccharides that may be attached to either lipid ( Glycolipid ) or the protein ( Gylcoprotein ) .

There is a wide variation in the lipid protein ratio between different cell membranes.

 

 

STRUCTURE OF PLASMA MEMBRANE

W

CELL MEMBRANE OR PLASMA MEMBRANE

 

 

 

 

 

 

 

MODLES OF CELL MEMBRANE

 

DANIELLI AND DAVISON MODEL

Io

DANIELLI AND DAVISON MODELS

• In 1935 , Danielli and Davison studied triglyceride lipid bilayers over a water surface.

• They found that they arranged themselves with the polar heads facing outwards .

• It always formed droplets ( oil in water ) and the surface was much higher than that of cells .

• They modle also known as Sandwich model.

 

 

 

ROBERTSON MODEL 

ROBERTSON MODEL

• In 1965 , Robertson noted the structure of members seen in the electron micrographs .

• He saw no spaces for pores in the electron micrographs .

• He hypothesized that the railroad track apperance came from the binding of osmium tetroxide to proteins and polar group of lipids .

• Unit membrane hypothesis .

 

 

FLUID MOSAIC MODEL 

FLUID MOSAIC MODEL

Fluid mosaic model proposed by S.J. Singer and G.L Nicolson in 1972 .

 

 

Fluid Meaning :- A substance that can flow .

 

Fluid property of member is due to lipid molecules.

Evidence – It is proved by D.Fruge and M.Edidin (1970) .

 

 

Mosaic :- It means no fix pattern .

 

Mosaic property is due to no fix pattern or arrangement of proteins.

Evidence – Freeze – Fracture electron microscopy of plasma membrane by Barton (1968)

 

Hence ,

Due to fluid property of lipid and mosaic property of proteins this model name is given that is ” FLUID MOSAIC ” .

 

 

Singer and Nicolson give name Fluid Mosaic model because it describes properties as well as organization of plasma membrane .

 

The phospholipid molecules and the molecules of cholesterol are linked together . This keeps the cell membrane intact and cohesive . This is why it is called fluid mosaic model .

 

 

According to this model the molecular arrangement of the plasma membrane resembles on moving sea of lipids that contains of mosaic of many different proteins of polar molecules and ions into and out of the cell phospholipids are arrange in a bilayer in a bilayer such that the polar portion . Face outward and the non polar protions face inward .

 

 

On the basis of position of globular protien around lipid bilayer :-

 

1) Peripheral / Extrinsic Protein :-

It attached in peripheri means it attached to polar surface of lipid .

 

2) Integral / Intrinsic Protein :-

Integral protein are partially or fully penetrate lipid bilayer it is also known as transmembrane protein.

Some pours are form by integral protein and are form by integral protein and are called tunnel protein .

 

 

CARBOHYDRATES

Are found in the form of Glycoproteins and Glycolipid.

 

 

Glycoproteins:-

If any chain of oligosaccharides are attached in the surface of protein they are called Glycoproteins .

 

 

 

Glycolipid:-

If any chain of oligosaccharides are attached in the surface of the lipid

they are called Glycolipid .

 

 

TYPES OF MOVEMENT OF LIPID MOLECULES:-

1) FLIP-FLOP OR TRANSBILAYER MOVEMENT :-

One lipid monolayer migrate towards another lipid monolayer and this movement is called flip-flop movement . It occur very rarely in cell membrane.

 

How in some membranes where the lipid actively synthesis .

eg:- In smooth endoplasmic reticulum has active lipid synthesis and therefore , that have flip- flop movement very rapidly this lipid synthesis take place by some membrane bound enzymes and that enzymes are known as phospholipid trans locator or flipphase .

 

 

 

2) LATERAL DIFFUSION:-

Lipid change it place with his side lipid is known as rapid lateral diffusion -10^7 times in a second occur .

 

 

ROLE OF UNSATURTE FATS IN THE INCREASE FLUDITY:-

Double bonds in unsaturated hydrocarbon chain together .

 

 

FUNCTIONS :-

•It protects the cell contents form the surrounding environment.

•It provides a shape to the cell .

•It is semi p

ermeable and allows transport of certain substances in to and out of the cell.

•It helps in forming cell junctions .

 

LEAVE A REPLY

Please enter your comment!
Please enter your name here