Endoplasmic reticulum

The cytoplasmic matrix of a eukaryotic cell contains an irregular network of branching and fusing membranous tubules which are about 40 to 70 nm in diameter, along with a number of flattened sacs called cisternae. This network of tubules and cisternae is endoplasmic reticulum (ER). The membranes of the ER are continuous with that of the outer nuclear membrane. ER is studded with ribosome on its outer surface and is called rough or granular ER (RER or GER). RER is involved in protein synthesis. Cells that synthesizes large quantity of lipids and less protein molecules have ER that lacks ribosomes; such ER are known as smooth or agranular ER (SER or AER).

The major function of ER is to transport proteins, lipids and other materials through cell. ER also helps in the folding of the protein molecules in its sac cisternae and transport of the synthesized and correctly folded protein in vesicles to the Golgi apparatus. ER associated enzymes and ribosomes helps in the synthesis of lipids and proteins. ER also stands as a key location for cell membrane synthesis.