In a mature cell a network is found in the cytoplasm. This is called endoplasmic reticulum. Endoplasmic reticulum is of two types – smooth and rough. ER having ribosome on its body is called rough endoplasmic reticulum.
Structure: They are bounded by double-layered semipermiable membrane. Normally they are branched but may occur in parallel. They are continuous with the nuclear membrane and the cell membrane. Chemically the membrane is made by lipoprotein. Small granular bodies may be present on it.
Function: They form the skeleton of protoplasm. Protein synthesis occurs in rough endoplasmic reticulum. Lipid, in alternative opinion, different hormones, glycogens etc. are synthesized in the smooth endoplasmic reticulum. They play the role of internal carrier of lipid and protein.