Structure ,types and function of plasma membrane class 11th

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Plasma membrane/ cell membrane/ biomembrane

Each cells of plant and animal  are covered by a thin membrane is called plasma membrane. It is composed of protein lipid and carbohydrates 20-40% part made up of protein it is of three layers structural protein made up backbone of the membrane and enzymatic protein made the specific function of the membrane and carrier protein help to the movement of substance  accross the plasma membrane. Lipid made 60-80% part of plasma membrane . Carbohydrate made 1-5 of plasma membrane.

Sandwich / Danielli Davson model

According to this model biomembrane is made up of two layers of protein and the layers of lipid present in between the protein. layer it just looked like a sandwich lipid is of two polar in rounded hydrophilic end or water loving  while other non polar or hydrophobic tail part or water hating end.

1. Outermost layer of protein is 20 A o thick.

2. Middle layer of phospholipid is 35 Ao thick.

3. Innermost layer of protein is about 20 A o thick.

Fluid mosaic model of plasma membrane

1. This model of plasma membrane explain by singer and nicolson in 1972.

2. According to this model protein is penetrated into lipids.

3. Plasma membrane contain two types of protein extrinsic and intrinsic protein

4. Intrinsic protein are embedded in lipid layer and it is difficult to remove it from plasma membrane.

5. This model shows the fluidity of lipid molecules and scattering of protein molecules like mosaic so it is called fluid mosaic model.

6. This model is most widely accepted model of plasma membrane.

7. Phospholipid present in this model having two ends hydrophilic head and hydrophobic tail.

8. The cell membrane can be described as protein Iceberg in in sea of lipids.

Micellar theory

1.This theory was explained by hilleir ans Hoffman in 1953.

2. According to this theory plasma membrane consists of number of global subunit called Micelles.

3. They are arranged in mosaic fashion.

4. In this model the fatty acid molecules are completely enclosed by water

5. The hydrophilic ends of fatty acid molecules face towards outside and their hydrophobic towards inside and a layer of protein is found on either side

6. The central space in Micell is  in the form of pores.

Plasma membrane functions

1. It forms of protective covering around the Protoplasm and cell organelles.

2. It maintains The Identity of cell organelles .

3. It helps in absorption of molecules

4. It helps in excretion or removal of water product on the body.

5. It helps in Ionic exchange.

6. It helps in cellular Movement in some organisation example- amoeba

7. It provides shape to the cells and cell organelles.

8. It regulates excretion and secretion.

Modification of plasma membrane 

The plasma membrane is some times associated with certain specialised structure in order to perform specific function . The structure are as follows

1. Microvilli

    These are finger like projections which are found on the outer surface of plasma membrane of animal cells

2. Desmosomes

  These are Micro thread like structure on the area of plasma membrane where adjacent cells come in contact with each other.

3. Interdigitation

   It is also found between two adjacent cells it consists of ridges and furrows 

On the both sides of membrane it helps in maintaining close connection between two cells.

4. Tight junctions

   These are formed when plasma membrane of two adjacent cells are joined together in such way that intercellular space is not left between them permeability of Ions is maximum

5. Terminal bars 

   These are also known as zonula adherens and it is similar to desmosomes.

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