The endoplasmic reticulum (ER) is an organelle in eukaryotic cells. The name reticulum comes from latin word reticulum meaning network. Thus, ER forms a network of membrane-enclosed sacs and those membranes are actually continuous with the outer nuclear membrane. There are two types of ER - rough and smooth. Rough ER (or granular ER) has ribosomes bound to the surface of its membrane. Since ribosomes are involved in protein synthesis, rough ER, rough ER plays an important role in the assembly of proteins.