The living outer boundary of cytoplasm of a cell is the plasma membrane. Though a number of models have been proposed to explain the ultra-structure of plasma membrane, the fluid mosaic model proposed by Singer and Nicolson (1974) is widely accepted. According to this model, the plasma membrane is formed of proteins iceberg in a sea of lipids.
In the lipoproteinaceous, trilaminar membrane the phospholipid bilayer is fluidly in nature in which proteins present globular and of two types. Those lie outside the phospholipid bilayer are called extrinsic or peripheral proteins and those embedded completely or partially in the bilayer are called intrinsic or integral proteins.
Some intrinsic proteins are very large and project on both surface of phospholipids. These proteins are called tunnel or channel proteins and are responsible for passage of water-soluble substances. Besides the phospholipids and proteins, some carbohydrate molecules are also present on outer surface on the membrane.