The endoplasmic reticulum consist of flattened single membrane vesicles. These have the same lipid bilayer structure but thinner than the cell membrane (about 7mm). The endoplasmic reticulum is of two types; rough (RER) and smooth (SER). Only the rough type has small granules known as ribosomes (Fig. 1.6).
RER is concerned with protein synthesis while SER is concerned with lipids and glycoprotein synthesis. The cisternae (enclosed spaces) of the endoplasmic reticulum play a role in the exchange of material between the cell and the extra cellular fluid. The exchange of material takes place by the process of pinocytosis.