The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. They are formed as an extension of the nuclear membrane and fold out towards the cytoplasm. The two functions of the ER take place in separate areas: the rough ER and the smooth ER, respectively.
The rough endoplasmic reticulum (RER) can be found has ribosomes along its surface, and the proteins they create are either secreted or incorporated into membranes in the cell. The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. SER functions include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storing calcium ions.