Endoplasmic Reticulum (abbreviated as ER) is a complex network and is composed of sacs and tubules. It is highly twisted.
1. Rough Endoplasmic Reticulum (RER):
The membrane of this type of ER is in the form of sheets (sacs). They are present near the nucleus and is continuous with the nuclear outer membrane. The surface of the membrane of RER is studded with ribosomes (another type of organelle, described here) which are an important site for protein synthesis. The appearance of the membrane is rough as can be seen in the diagram and hence, is called “RER”
The membrane of this type of ER is mostly in the form of tubules; sometimes it may branch and form reticulate kind of network. They lack the ribosomes; thereby gives the appearance as smooth and hence the name Smooth ER. Also, SER contains an enzyme glucose 6-phosphatase, which is involved in gluconeogenesis.
3. Sarcoplasmic Reticulum (SR):
this is a type of SER, which is present in smooth and striated muscle. The only difference between SER and SR is the variety of proteins present in each.
The main and foremost function of RER is the synthesis and manufacture of proteins.
SER is involved in lipid synthesis and steroid synthesis. It is also involved in drug detoxification and steroid metabolism.
SR is present in muscle cells where it regulates calcium ion concentrations.