Fluid mosaic model of plasma membrane:

In 1972 Singer and Nicolson suggested a model, called fluid mosaic model, to explain the ultrastructure of the plasma membrane of the cell. It can be described as 'a number of protein icebergs floating in a sea of lipids'. This is the most accepted model as it describes the properties as well as the organisation of plasma membrane. According to this model, plasma membrane is made up of a bilayer (two molecules thick layer) of phospholipids.

The following are the two types of proteins that float in the layer of phospholipid fluid:

(i) Intrinsic proteins: It completely covers the lipid bilayer.

(ii) Extrinsic proteins: It occurs either on the outer surface or on the inner surface of the lipid membrane.

Proteins and lipids provide flexibility to the plasma membrane that helps it in the movement of molecules in and out of the cell.