Why is the plasma membrane called a selectively permeable membrane?

The membrane that surrounds the organelles of the cell, by creating a barrier between cell organelles and the external environment is called the plasma membrane. It is also called a boundary of the cell.

Selectively Permeable Membrane

The plasma membrane is called a selectively permeable membrane as it permits the movement of only certain molecules in and out of the cells. Not all molecules are free to diffuse. If plasma membrane ruptures or breaks down then molecules of some substances will freely move in and out of the cells.

As the plasma membrane acts as a mechanical barrier, the exchange of material from its surroundings through osmosis or diffusion in a cell won’t take place. Consequently, the cell would die due to the disappearance of the protoplasmic material. It allows hydrophobic molecules and small polar molecules diffuse through the lipid layer, but does not allow ions and large polar molecules cannot diffuse through the membrane.

Proteins In Plasma Membrane

There are two types of proteins molecules are found in the plasma membrane:
(i) Intrinsic proteins -They completely cover the lipid bilayer.’

(ii) Extrinsic proteins – These occur either on the outer surface or on the inner surface of the lipid layer.

Functions Of Plasma Membrane

  • The plasma membrane forms a barrier between the cell organelles from the outside environment.
  • It allows only certain molecules to pass through it.
  • It facilitates communication and signalling between the cells.


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