Editing Endomembrane system (section)

==== Functions of the smooth ER ====
In the great majority of cells, smooth ER regions are scarce and are often partly smooth and partly rough. They are sometimes called transitional ER because they contain ER exit sites from which transport vesicles carrying newly synthesized proteins and lipids bud off for transport to the Golgi apparatus. In certain specialized cells, however, the smooth ER is abundant and has additional functions. The smooth ER of these specialized cells functions in diverse metabolic processes, including synthesis of lipids, [[Carbohydrate metabolism|metabolism of carbohydrates]], and detoxification of drugs and poisons.<ref name=Cooper3 /><ref name=Alberts3 />

Enzymes of the smooth ER are vital to the synthesis of lipids, including [[oil]]s, [[phospholipid]]s, and [[steroid]]s. Sex hormones of vertebrates and the steroid hormones secreted by the [[adrenal gland]]s are among the steroids produced by the smooth ER in animal cells. The cells that synthesize these hormones are rich in smooth ER.<ref name=Cooper3 /><ref name=Alberts3 />

[[Liver]] cells are another example of specialized cells that contain an abundance of smooth ER. These cells provide an example of the role of smooth ER in carbohydrate metabolism. Liver cells store carbohydrates in the form of [[glycogen]]. The [[Glycogenolysis|breakdown of glycogen]] eventually leads to the release of [[glucose]] from the liver cells, which is important in the regulation of sugar concentration in the blood. However, the primary product of glycogen breakdown is glucose-1-phosphate. This is converted to glucose-6-phosphate and then an enzyme of the liver cell's smooth ER removes the phosphate from the glucose, so that it can then leave the cell.<ref name=Cooper3 /><ref name=Alberts3 />

Enzymes of the smooth ER can also help detoxify drugs and poisons. Detoxification usually involves the addition of a hydroxyl group to a drug, making the drug more soluble and thus easier to purge from the body. One extensively studied detoxification reaction is carried out by the [[cytochrome P450]] family of enzymes, which catalyze oxidation reactions on water-insoluble drugs or metabolites that would otherwise accumulate to toxic levels in cell membrane.<ref name=Cooper3 /><ref name=Alberts3 />

In muscle cells, a specialized smooth ER ([[sarcoplasmic reticulum]]) forms a membranous compartment (cisternal space) into which [[calcium]] ions are pumped. When a muscle cell becomes stimulated by a nerve impulse, calcium goes back across this membrane into the cytosol and generates the contraction of the muscle cell.<ref name=Cooper3 /><ref name=Alberts3 />