The endoplasmic reticulum (ER) is a system of membrane-enclosed sacs and tubules in the cell. Their lumens are probably all interconnected, and their membranes are continuous with the outer membrane of the nuclear envelope. All the materials within the system are separated from the cytosol by a membrane. The endoplasmic reticulum is the site where the cell manufactures most of the membranes of the cell (plasma membrane, Golgi apparatus, lysosomes, nuclear envelope), lipids (including lipids for membranes, e.g., of the mitochondria, that are not made by the ER), and transmembrane proteins and secreted proteins. The ER comes in two versions: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).
Rough Endoplasmic Reticulum (RER)
The RER is typically arranged as interconnecting stacks of disc-like sacs. The cytosolic surface of the RER is studded with ribosomes engaged in protein synthesis. As the messenger RNA is translated by the ribosome, the growing polypeptide chain is inserted into the membrane of the RER. Proteins destined to be secreted by the cell or shipped into the lumen of certain other organelles like the Golgi apparatus and lysosomes pass all the way through into the lumen of the RER. Transmembrane proteins destined for the plasma membrane or the membrane of those organelles are retained within the membrane of the RER.
Figure 3.5.1: This electron micrograph (courtesy of Keith Porter) shows the RER in a bat pancreas cell. The clearer areas are the lumens.
In either case, the portion of the protein within the lumen of the RER is subject to extensive glycosylation (primarily N-linked) in Figure 3.5.1. The RER takes up a large proportion of the cytoplasm of cells specialized for protein synthesis such as cells secreting digestive enzymes (e.g. the pancreas cell above) and antibody-secreting plasma cells
Smooth Endoplasmic Reticulum (SER)
The SER differs from the RER in lacking attached ribosomes and usually being tubular rather than disc-like. A major function of the SER is the synthesis of lipids from which various cell membranes are made and like steroids, they are secreted from the cell. The SER represents only a small portion of the ER is most cells, e.g. serving as transport vesicles for the transport of protein to the Golgi apparatus. However, it is a prominent constituent of some cells, especially the cells of the adrenal cortex (which secrete steroid hormones), the cells of the liver (hepatocytes) where it synthesis lipids for secretion of lipoproteins, and the sarcoplasmic reticulum of muscle cells is SER.