Describe the types and functions of endoplasmic reticulum.
Endoplasmic reticulum is of following two types:

A. Smooth endoplasmic reticulum - does not have ribosome.
B. Rough endoplasmic reticulum - has ribosomes attached to it. 

A. Functions of smooth endoplasmic reticulum :

1. It is especially prominent in cells which are not involved in the synthesis of lipids.

2. It is involved in the synthesis of Golgi complex, lysosmoes and microbodies.

B. Functions of rough endoplasmic reticulum:

1. It is primarily involved in protein synthesis owing to the presence of ribosomes.

2. Packaging. The proteins manufactured in ribosomes are packed in RER. They are processed and packed in vesicles which are pinched off from RER.

3. Formation of glycoproteins. The carbohydrates and proteins are linked in RER to form glycoproteins.

4. Helps in the formation of  nuclear envelops.