The nuclear membrane is continuous with an extensive membrane system known as the endoplasmic reticulum (ER, see Figure 1 and Figure 11), which extends through much of the cytoplasm and is the site of many activities, including the synthesis of lipids in the smooth endoplasmic reticulum and the synthesis of lysosomal proteins, cell membrane proteins and secreted proteins in the rough endoplasmic reticulum. Importantly, the interior of the ER is separate from the cytosol. The membranes of the ER form tubes and sacs, and depending on the plane in which a section cuts through the cell, the ER may therefore have the appearance of circular or elongated tubes, or parallel systems of membrane, or any shape in between, as illustrated in Figure 17.
Some parts of the endoplasmic reticulum appear smooth when viewed by EM and are hence named smooth endoplasmic reticulum (SER). Smooth endoplasmic reticulum is involved in the production of phospholipids and steroids and also in the detoxification of substances such as drugs or ingested chemicals. Ingestion of large amounts of a toxic substance, such as a pesticide, results in an increase in the amount of SER in cells. Normally, however, most of the endoplasmic reticulum appears granular, being dotted with many ribosomes (Figure 17). This part of the endoplasmic reticulum is given the name rough endoplasmic reticulum (RER), and is the site where lysosomal proteins (mainly digestive enzymes), membrane proteins, and proteins that are destined for export from the cell are synthesised and processed.
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.
