Endoplasmic Reticulum

Within cells are complex membranous structures that serve to separate some of the products of the cell from the synthetic machineray that produces them. Often them membranes of these structures are lined with ribosomes on their outer surfaces, giving them a rough appearance. These parts are called the rough endoplasmic reticulum to contrast them with the smooth endoplasmic reticulum where there are no attached ribosomes.