Endoplasmic reticulum is a membrane system forming a series of sacs in eukaryotic cells. It has many functions, including the synthesis and transport of proteins. There are two types of endoplasmic reticulum with different structures, as follows:
1) Rough endoplasmic reticulum - its sacs are convoluted but flat, contiguous with the nuclear membrane. Its outer surface is studded with ribosomes, which gives it the rough appearance.
2) Smooth endoplasmic reticulum - its sacs are tube-like structures and they are located near the cell periphery. Sometimes these tubules branch, forming a reticular network wit.