The Endoplasmic Reticulum (ER) forms an interconnected network of tubules, vesicles and sac-like structures called cisternae within the cells. It is a part of the endomembrane system which consists of the plasma membrane of the cell, the nuclear envelope, etc. apart from the ER. It is held together by the cell’s cytoskeleton. The ER comes in two different varieties, the rough Endoplasmic Reticulum (rER) and the smooth Endoplasmic Reticulum (sER).
The rough Endoplasmic Reticulum is connected to the outer layer of the nuclear envelope. However, it is not continuous with the cisternae of the Golgi complex. The transport vesicles shuttles carrying proteins, still travel between the rER and the Golgi complex. rER are especially found in secretory cells, as they produce proteins which these cells must secrete into out via exocytosis so as to be used by other tissues and organs of our body. This type of ER is called as rough due to the Ribosomes being studded on it, giving it a rough appearance. The exact functions of rER, however, depend largely on the type of cells they are in. Overall though, the rER is important in protein translation, folding and transport of transmembrane proteins and other integral membrane proteins as well as those to be secreted out, like the digestive enzymes for instance. They also have a role in sequestration of calcium, and the production and storage of glycogen, steroids and other macromolecules. The rER also produces lysosomal proteins and membrane lipids.
The next type of ER is the smooth Endoplasmic Reticulum. The sER is a network of agranular connecting tubes connected to the nuclear envelope. It is pretty similar to the rER, except that sER does not have Ribosomes attached to it. The sER consists of tubules and vesicles that branch out to form a network. The sER tubules are generally about 150 nano meters in diameter, though they vary in different cells. In some cells, there are dialated areas like those of the sacks in rER, while others do not have them. The sER has functions in several metabolic processes, including lipid and steroid synthesis, carbohydrate metabolism, regulation of calcium concentration in the cell, drug detoxification, attaching receptors to membrane bound proteins and steroid metabolism. The sER also contains an enzyme called glucose-6-phosphatase which converts the glucose-6-phosphate to glucose. This is a very important step in Gluconeogenesis, which is producing glucose internally within the cell, rather than obtaining it from the food consumed. The sER is also a site where certain drugs are modified by microsomal enzymes like the Cytochrome P450 enzymes. Hence, it plays a vital role in drug metabolism as well.
Reference: Introduction to Anatomy & Physiology, by Tortora and other lecture notes