<P> The ER is involved in cotranslational sorting of proteins . A polypeptide which contains an ER signal sequence is recognised by a signal recognition protein which halts the production of the protein . the SRP transports the polypeptide to the ER membrane where its released in through a membrane pore and translation resumes . </P> <P> There are two distinct, though connected, regions of ER that differ in structure and function: smooth ER and rough ER . The rough endoplasmic reticulum is so named because the cytoplasmic surface is covered with ribosomes, giving it a bumpy appearance when viewed through an electron microscope . The smooth ER appears smooth since its cytoplasmic surface lacks ribosomes . </P> <P> 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, metabolism of carbohydrates, and detoxification of drugs and poisons . </P> <P> Enzymes of the smooth ER are vital to the synthesis of lipids, including oils, phospholipids, and steroids . Sex hormones of vertebrates and the steroid hormones secreted by the adrenal glands are among the steroids produced by the smooth ER in animal cells . The cells that synthesize these hormones are rich in smooth ER . </P>

The endomembrane system contains all of the following except