The endoplasmic reticulum, or ER, is an organelle inside cells. It is found in both plant and animal cells and is a very important manufacturing site for fat and proteins. There are two types of endoplasmic reticulum -- smooth endoplasmic reticulum and rough endoplasmic reticulum. The two types of ER are separate entities and are not joined together.
Structurally, the endoplasmic reticulum is an extensive membrane network of cisternae, or sac-like structures. The ER membrane forms a continuous sheet enclosing an internal space called the ER lumen. The narrow ER lumen is continuous with the lumen of the nuclear membrane -- the double-layered membrane surrounding the nucleus of the cell, so that the ER system appears as an outgrowth of the nuclear membrane. The membrane of the ER on its external surface can carry granular structures known as ribosomes. The ER regions carrying ribosomes are known as rough endoplasmic reticulum, and those devoid of ribosomes are known as smooth endoplasmic reticulum.
Endoplasmic reticulum has a high lipid content. There is more lipid relative to proteins in the smooth endoplasmic reticulum than in the rough endoplasmic reticulum. The latter also contains less cholesterol and sphingomyelin, a type of lipid found in animal cell membranes.
Rough Endoplasmic Reticulum
The ribosomes attached to the membrane of rough endoplasmic reticulum play an important role in the synthesis and packaging of proteins. The newly synthesized proteins are discharged and penetrate into the cisternae, where these proteins may be utilized within the cell or may be exported outside the cell to the site of their utility. During the transport of these proteins, three types of membrane -- ER, Golgi membrane and plasma membrane -- interact, connecting and disconnecting.
Smooth Endoplasmic Reticulum
The functions of smooth endoplasmic reticulum vary in different cell types. More commonly, it is engaged in the synthesis of lipids and steroids. Smooth ER also serves as a storage space for enzymes that detoxify lipid-soluble drugs and other harmful compounds. Smooth endoplasmic reticulum is also involved in the storage of calcium in muscle cells. At the time of need, such as during muscle contraction, this stored calcium is released into the cytosol -- the liquid inside the muscle cells.