<P> 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 water - insoluble drugs or metabolites that would otherwise accumulate to toxic levels in cell membrane . </P> <P> Muscle cells have another specialized function of smooth ER . The ER membrane pumps calcium ions from the cytosol into the cisternal space . When a muscle cell becomes stimulated by a nerve impulse, calcium goes back across the ER membrane into the cytosol and generates the contraction of the muscle cell . </P> <P> Many types of cells export proteins produced by ribosomes attached to the rough ER . The ribosomes assemble amino acids into protein units, which are carried into the rough ER for further adjustments . These proteins may be either transmembrane proteins, which become embedded in the membrane of the endoplasmic reticulum, or water - soluble proteins, which are able to pass through the membrane into the lumen . Those that reach the inside of the endoplasmic reticulum are folded into the correct three - dimensional conformation . Chemicals, such as carbohydrates or sugars, are added, then the endoplasmic reticulum either transports the completed proteins, called secretory proteins, to areas of the cell where they are needed, or they are sent to the Golgi apparatus for further processing and modification . </P> <P> Once secretory proteins are formed, the ER membrane separates them from the proteins that will remain in the cytosol . Secretory proteins depart from the ER enfolded in the membranes of vesicles that bud like bubbles from the transitional ER . These vesicles in transit to another part of the cell are called transport vesicles . An alternative mechanism for transport of lipids and proteins out of the ER are through lipid transfer proteins at regions called membrane contact sites where the ER becomes closely and stably associated with the membranes of other organelles, such as the plasma membrane, Golgi or lysosomes . </P>

Is rough er part of the endomembrane system