The outer membrane of the nuclear envelope is physically continuous with the endoplasmic reticulum (ER), an organelle bounded by a single membrane (Fig. 5.20). The ER is a conspicuous feature of many eukaryotic cells, accounting in some cases for as much as half of the total amount of membrane. The ER produces and transports many of the lipids and proteins used inside and outside the cell, including all transmembrane proteins, as well as proteins destined for the Golgi apparatus, lysosomes, or export out of the cell. The ER is also the site of production of most of the lipids that make up the various internal and external cell membranes.

Unlike the nucleus, which is a single spherical structure in the cell, the ER consists of a complex network of interconnected tubules and flattened sacs. The interior of the ER is continuous throughout and is called the lumen. As shown in Fig. 5.20, the ER has an almost mazelike appearance when sliced and viewed in cross section. Its membrane is extensively convoluted, allowing a large amount of membrane surface area to fit within the cell.

When viewed through an electron microscope, ER membranes have two different appearances (Fig. 5.20). Some look rough because they are studded with ribosomes. This portion of the ER is referred to as rough endoplasmic reticulum (RER). The rough ER synthesizes transmembrane proteins, proteins that end up in the interior of organelles, and proteins destined for secretion. As a result, cells that secrete large quantities of protein have extensive rough ER, including cells of the gut that secrete digestive enzymes and cells of the pancreas that produce insulin. All cells have at least some rough ER for the production of transmembrane and organelle proteins.

There is a small amount of ER membrane in most cells that appears smooth because it lacks ribosomes. This portion of the ER is therefore called smooth endoplasmic reticulum (SER) (Fig. 5.20). Smooth ER is the site of fatty acid and phospholipid biosynthesis. Thus, this type of ER predominates in cells specialized for the production of lipids. For example, cells that synthesize steroid hormones have a well-developed SER that produces large quantities of cholesterol. Enzymes within the SER convert cholesterol into steroid hormones.