Plasma membrane

PLASMA MEMBRANE

          Every animal cell is bounded by a living, extremely, a thin delicate membrane called a cell membrane. And in plant cells, this membrane occurs just below the cell wall. The cell membrane is the outermost layer of the cell that separates cytoplasm from the external environment. This is also known as "Plasma membrane" or "plasmalemma".

               The term plasma membrane was first coined by "Nageli". This plasma membrane defines the shape and size of the cell, encloses the cytoplasm, and protects it from the other external environment. This membrane serves as a boundary for the cytoplasm i.e, a fluid-like content present in it. It is a biological membrane ranging from 6 to 8 nm and constitutes a layer of lipids and proteins. It acts as a protective barrier.

           

                            

 This membrane is a selectively permeable membrane as it controls the exit and entry of materials. This allows the transport of small molecules such as water, ions, oxygen, carbon dioxide, ethanol, glucose, etc. It also maintains a constant internal environment called as homeostasis. Various processes occur here such as osmosis, diffusion, and active transport which is carried out by special proteins called "transport proteins". For bulk transport of large particles, it performs endocytosis and exocytosis. The permeability depends mainly on the electric charge and polarity of the molecule.

Structure of Plasma membrane:

        One of the widely accepted models is of "Singer and Nicolson" i.e Fluid Mosaic model. According to this model, the membrane is interrupted by some proteins and lipids. The lipid is in the form of fluid and the proteins are embedded here and there in lipid in a mosaic pattern. Hence the name fluid mosaic model. The protein molecules are globular and are of two types namely peripheral or extrinsic proteins and integral or intrinsic proteins. The proteins which are attached to the polar surface of the lipid called as "Extrinsic proteins" whereas the others which penetrate the bilayer called as "intrinsic proteins". The peripheral proteins are arranged on the surface and loosely bound to the lipids. The integral proteins are deeply embedded and tightly bound to the lipid molecules. These proteins and lipids consist of chains of sugars or oligosaccharides called as glycoproteins and glycolipids respectively. 

    

                                

                Each lipid molecule has a hydrophobic tail and hydrophilic head. The hydrophilic heads face outwards and the hydrophobic tails of the two layers face each other. The proteins are deeply embedded and tightly bound to the lipids. These lipids and proteins move freely in the lipid bilayer.

                               

Functions of the Plasma membrane:

1.Shape: It provides a definite shape to semifluid contents of the cell.

2.Mechanical barrier: It protects the cell from any external injuries and protects the contents of the cell.

3.Exchange of materials: It regulates the exchange of materials in and out of the cell and also exhibits the nature of selective permeability.

4.Cell continuity: The adjacent cells of the plasma membrane continue to form the plasmodesmata and other cell junctions.

5.Biogenesis of cell organelles: Many other cell organelles like endoplasmic reticulum, nuclear membrane, etc develop from this membrane.

6.Endocytosis: The ability of the cell to engulf the food and other foreign materials from the external environment.

7.Osmosis: The movement of molecules across the semipermeable membrane is called as osmosis. When the water molecules enter the cell, it is called as endosmosis and when they leave the cell, it is called as exosmosis.

8.Diffusion: The movement of molecules from a region of higher to lower concentration in a membrane is called as diffusion or passive transport.

9.Active transport: The movement of molecules from a region of lower to higher concentration in a membrane is called as active transport.

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