The endoplasmic reticulum is a complex of cellular membranes—including channels and sacs—that are found in the cell’s cytoplasm. It is the largest and most extensive system of the internal membranes—a collection of membrane tubes and channels in the cytoplasm—whose main function is the transport of proteins throughout both plant and animal cells.

E.R., as it is abbreviated, is divided into the sandpaper-looking rough E.R. that houses ribosomes (that manufacture proteins) and smooth E.R. that helps with such processes as the synthesis of steroid hormones and other lipids, connecting the Golgi apparatus to the rough E.R. and helping to carry out certain detoxification of drugs and other components that may be toxic or harmful to the cells.