The endoplasmic reticulum, also called ER, is a membrane system of folded sacs and interconnected channels that serves as the site of protein and lipid synthesis. The pleats and folds of the ER provide a large amount of surface area where cellular functions can take place.
The area of the ER where ribosomes are attached is called the rough endoplasmic reticulum. Notice that the rough ER appears to have bumps on it. These bumps are the attached ribosomes that will produce proteins to export to other cells.
There are areas of the ER that do not have ribosomes attached. The area of the ER where no ribosomes are attached is called smooth endoplasmic reticulum. Although the smooth ER has no ribosomes, it does perform important functions for the cell. For example, the smooth ER provides a membrane surface where a variety of complex carbohydrates and lipids, including phospholipids, are synthesized. Smooth ER in the liver detoxifies harmful substances.