Smooth and Rough Endoplasmic Reticulum

Within the cytoplasm of all cells is a 3-dimensional maze of connecting and branching channels that is made by a continuous membrane. This membrane is called the endoplasmic reticulum (ER). ER can be classified as rough ER when ribosomes are attached to the cytosolic side (side touching the cytoplasm) of the membrane or they can be classified as smooth ER when no ribosomes are present. Rough ER is prominent in cells that are synthesizing proteins for export. Some examples of such proteins are the digestive enzymes, hormones, structural proteins or antibodies. The main purpose of rough ER is the separation of proteins designed for export from the cell or for intracellular use. Proteins are also modified within the ER by the addition of carbohydrate, removal of a signal sequence and other post-translational modifications. Phospholipid synthesis and assembly of multichain proteins are other important occurences in the ER.

Smooth ER lacks attached ribosomes and often has a more tubular appearance than rough ER. The two types of ER are continuous in some sections. Smooth ER bestows on the cell the ability to perform a variety of very specialized functions. ER is necessary for steroid synthesis, metabolism and detoxification of substances in the liver, phospholipid synthesis, and excitation-contraction coupling in skeletal muscle.