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.


                      Contributed By: John B. Ferguson, Sc.B., M.Phil., Ph.D.
                      Professor of Biology, Division of Natural Sciences and
                      Mathematics, Bard College. Director, Distinguished
                      Scientist Lecture Series, Bard Center.
                      Biology, © Microsoft
 

This image is an example of Rough Endoplasmic Reticulum.  Smooth ER has none of the small black dots tha