<Li> Initial glycosylation as assembly continues . This is N - linked (O - linking occurs in the Golgi). <Ul> <Li> N - linked glycosylation: If the protein is properly folded, Oligosaccharyltransferase recognizes the AA sequence N X S or N X T (with the S / T residue phosphorylated) and adds a 14 - sugar backbone (2 - N - acetylglucosamine, 9 - branching mannose, and 3 - glucose at the end) to the side - chain nitrogen of Asn . </Li> </Ul> </Li> <Ul> <Li> N - linked glycosylation: If the protein is properly folded, Oligosaccharyltransferase recognizes the AA sequence N X S or N X T (with the S / T residue phosphorylated) and adds a 14 - sugar backbone (2 - N - acetylglucosamine, 9 - branching mannose, and 3 - glucose at the end) to the side - chain nitrogen of Asn . </Li> </Ul> <Li> N - linked glycosylation: If the protein is properly folded, Oligosaccharyltransferase recognizes the AA sequence N X S or N X T (with the S / T residue phosphorylated) and adds a 14 - sugar backbone (2 - N - acetylglucosamine, 9 - branching mannose, and 3 - glucose at the end) to the side - chain nitrogen of Asn . </Li> <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>

Where is the endoplasmic reticulum located in an animal cell