<P> In most cells the smooth endoplasmic reticulum (abbreviated SER) is scarce . Instead there are areas where the ER is partly smooth and partly rough, this area is called the transitional ER . The transitional ER gets its name because it contains ER exit sites . These are areas where the transport vesicles that contain lipids and proteins made in the ER, detach from the ER and start moving to the Golgi apparatus . Specialized cells can have a lot of smooth endoplasmic reticulum and in these cells the smooth ER has many functions . It synthesizes lipids, phospholipids, and steroids . Cells which secrete these products, such as those in the testes, ovaries, and sebaceous glands have an abundance of smooth endoplasmic reticulum . It also carries out the metabolism of carbohydrates, detoxification of natural metabolism products and of alcohol and drugs, attachment of receptors on cell membrane proteins, and steroid metabolism . In muscle cells, it regulates calcium ion concentration . Smooth endoplasmic reticulum is found in a variety of cell types (both animal and plant), and it serves different functions in each . The smooth endoplasmic reticulum also contains the enzyme glucose - 6 - phosphatase, which converts glucose - 6 - phosphate to glucose, a step in gluconeogenesis . It is connected to the nuclear envelope and consists of tubules that are located near the cell periphery . These tubes sometimes branch forming a network that is reticular in appearance . In some cells, there are dilated areas like the sacs of rough endoplasmic reticulum . The network of smooth endoplasmic reticulum allows for an increased surface area to be devoted to the action or storage of key enzymes and the products of these enzymes . </P> <P> The sarcoplasmic reticulum (SR), from the Greek σάρξ sarx ("flesh"), is smooth ER found in myocytes . The only structural difference between this organelle and the smooth endoplasmic reticulum is the medley of proteins they have, both bound to their membranes and drifting within the confines of their lumens . This fundamental difference is indicative of their functions: The endoplasmic reticulum synthesizes molecules, while the sarcoplasmic reticulum stores calcium ions and pumps them out into the sarcoplasm when the muscle fiber is stimulated . After their release from the sarcoplasmic reticulum, calcium ions interact with contractile proteins that utilize ATP to shorten the muscle fiber . The sarcoplasmic reticulum plays a major role in excitation - contraction coupling . </P> <P> The endoplasmic reticulum serves many general functions, including the folding of protein molecules in sacs called cisternae and the transport of synthesized proteins in vesicles to the Golgi apparatus . Correct folding of newly made proteins is made possible by several endoplasmic reticulum chaperone proteins, including protein disulfide isomerase (PDI), ERp29, the Hsp70 family member BiP / Grp78, calnexin, calreticulin, and the peptidylpropyl isomerase family . Only properly folded proteins are transported from the rough ER to the Golgi apparatus--unfolded proteins cause an unfolded protein response as a stress response in the ER . Disturbances in redox regulation, calcium regulation, glucose deprivation, and viral infection or the over-expression of proteins can lead to endoplasmic reticulum stress response (ER stress), a state in which the folding of proteins slows, leading to an increase in unfolded proteins . This stress is emerging as a potential cause of damage in hypoxia / ischemia, insulin resistance, and other disorders . </P> <P> Secretory proteins, mostly glycoproteins, are moved across the endoplasmic reticulum membrane . Proteins that are transported by the endoplasmic reticulum throughout the cell are marked with an address tag called a signal sequence . The N - terminus (one end) of a polypeptide chain (i.e., a protein) contains a few amino acids that work as an address tag, which are removed when the polypeptide reaches its destination . Nascent peptides reach the ER via the translocon, a membrane - embedded multiprotein complex . Proteins that are destined for places outside the endoplasmic reticulum are packed into transport vesicles and moved along the cytoskeleton toward their destination . In human fibroblasts, the ER is always co-distributed with microtubules and the depolymerisation of the latter cause its co-aggregation with mitochondria, which are also associated with the ER . </P>

What is the primary function of the endoplasmic reticulum