The cell membrane is a thin elastic transparent semi-permeable membrane covering all cells plants and animals. It is also called plasma membrane or plasmalemma. It is chemically composed of lipid, protein, and a small amount of carbohydrate.
Different models have been proposed to describe the structure of cell membrane. The most widely accepted model is fluid mosaic model given by Singer and Nicholson.
According to this model, the structure of cell membrane is just like "protein iceberg" in a sea of lipid.
There is present of phospholipid bi-layer which is fluid in nature and in which lipid molecules change their position either within the same layer or between the layers.
The proteins are globular in shape and are of two types i.e. Extrinsic and intrinsic. The extrinsic protein or peripheral protein lie on the surface of the lipid layer, but the intrinsic protein is embedded either partially or completely in the lipid layer.
Phospholipid molecules are polar molecules having a hydrophilic head and hydrophobic tail.
The heads are directed towards opposite sides and tails face each other.
On the outer surface of the cell membrane, there is a long chain of carbohydrate which either attached to lipid or protein forming glycolipid, glycoprotein respectively.
It gives protection to the inner content of the cell.
It regulates the flow of materials in and out of the cell through the processes like osmosis, diffusion(passive transports) which does not require energy.
It also helps in active transport which requires energy in the form of ATP.
It also helps in the processes like endocytosis and exocytosis.
In some organisms like the amoeba, it helps in locomotion through pseudopodial movement.
Endocytosis and Exocytosis
Endocytosis is the process of intaking of large sized food materials by the activity of cell membrane. Exocytosis is the process of throwing waste materials out of the cell by the activity of cell membrane. It is commonly called cell vomiting process.
Phagocytosis and Pinocytosis
Phagocytosis is the process of intaking of solid food by the activity of cell membrane. It is commonly called as a cell eating process.
Pinocytosis is the process of intaking of liquid food by the activity of cell membrane. It is commonly called as a cell drinking process.