Endoplasmic Reticulum

Endoplasmic Reticulum (ER), an extensive network of tubes that manufacture, process, and transport materials within nucleated cells. The ER consists of a continuous membrane in the form of branching tubules and flattened sacs that extend throughout the cytoplasm (the cell’s contents outside of the nucleus) and connect to the double membrane that surrounds the nucleus. There are two types of ER: rough and smooth.

The outer surface of rough ER is covered with tiny structures called ribosomes, where protein synthesis occurs. Proteins are created as long polypeptide chains, some of which require modification. These proteins are transported into the rough ER, where enzymes fold and link them into the three dimensional shape that completes their structure. The rough ER also transports proteins either to regions of the cell where they are needed or to the Golgi apparatus, from which they may be exported from the cell. Rough ER is particularly dense in cells that manufacture proteins for export. White blood cells, for example, which produce and secrete antibodies, contain abundant rough ER.

Smooth ER lacks ribosomes and so has a smooth appearance. It is involved in the synthesis of most of the lipids that make up the cell membrane, as well as membranes surrounding other cell structures like mitochondria. It also manufactures carbohydrates, stores carbohydrates and lipids, and detoxifies alcohol and drugs such as morphine and phenobarbitol. Cells that specialize in lipid and carbohydrate metabolism, such as brain and muscle cells, or those that carry out detoxification, such as liver cells , tend to have more smooth ER.

Smooth ER also is involved in the uptake and release of calcium to mediate some types of cellular activity. In skeletal muscle cells, for example, the release of calcium from the smooth ER triggers muscle contraction.