ENDOPLASMIC RETICULUM

(ER) in biology, a highly convoluted membrane within the cytoplasm of a eukaryotic cell that is important in the biosynthesis of proteins and lipids. The ER usually constitutes more than half of the membrane of the cell and is continuous with the outer membrane of the nuclear envelope. Endoplasmic reticulum has two distinct regions: the rough ER (RER; so-called because of the ribosomes attached to its outer cytoplasmic surface), which synthesizes secretory proteins, phospholipids, and membrane, and the smooth ER (SER), which is not associated with ribosomes and principally transfers products of the RER by budding off transport vesicles. The SER is also involved in the synthesis of lipids and the detoxification of some toxic chemicals. The sarcoplasmic reticulum is a specialized ER that regulates the calcium ion concentration in the cytoplasm of striated muscle cells. See also cell.