Figure 17 (a) Electron micrograph showing rough endoplasmic reticulum (RER) in a rat liver cell. The granular appearance is due to ribosomes that are attached to the endoplasmic reticulum membrane. (b) Schematic diagram of rough endoplasmic reticulum and smooth endoplasmic reticulum; note that the appearance can be as elongated or spherical tubes, or something in between.