It is a membranous network of tube like structures extending from nuclear membrane to plasma membrane.
It is absent in prokaryotic cells and matured RBCs of mammals.
There are two types of endoplasmic reticulum:
(i) Rough Endoplasmic Reticulum (RER): Here ribosomes are present on the surface for the synthesis of proteins.
(ii) Smooth Endoplasmic Reticulum (SER): Here ribosomes are absent and is meant for secreting lipids.
It gives internal support to cell.
It helps in transport of various substances from nuclear membrane to plasma membrane or vice versa.
RER helps in synthesis and transportation of proteins.
SER helps in synthesis and transportation of lipids.