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Plasma membrane

Plasma membrane
Introduction
  • It is also known as cell membrane. It is the outermost layer in the animal cell but lies beneath the cell wall in plant cell.
  • It is thin, delicate, elastic and capable of limited self repair.
Chemical Composition
Cell membrane is chemically composed of lipids and proteins.
  • 60 – 80% are proteins.
  • 20 – 40% are lipids.
  • Very small quantity of carbohydrate.
Physical Structure
Many biologists contributed in describing the arrangement of lipids and proteins in cell membrane.
There are two important models about it i.e.
(i)     Unit membrane model
(ii)    Fluid mosaic model
(i)     Unit Membrane Model
Generally, it was believed that plasma membrane is composed of lipid bilayer sandwiched between two protein layers. This is called unit membrane model.
(ii)    Fluid Mosaic Model
The modern technology has revealed that lipid bilayer is not sandwiched between two protein layers. Instead proteins are embedded in the lipid bilayer in a mosaic manner.
This model is called fluid mosaic model and is most accepted now. This basic structure is found in all the membranes such as those of mitochondria, chloroplasts etc. 
According to this, cell membrane also contains charged pores through which movement of material takes place both by active and passive transport.

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