<P> The general structure of the endoplasmic reticulum is a network of membranes called cisternae . These sac - like structures are held together by the cytoskeleton . The phospholipid membrane encloses the cisternal space (or lumen), which is continuous with the perinuclear space but separate from the cytosol . The functions of the endoplasmic reticulum can be summarized as the synthesis and export of proteins and membrane lipids, but varies between ER and cell type and cell function . The quantity of both rough and smooth endoplasmic reticulum in a cell can slowly interchange from one type to the other, depending on the changing metabolic activities of the cell . Transformation can include embedding of new proteins in membrane as well as structural changes . Changes in protein content may occur without noticeable structural changes . </P> <P> The surface of the rough endoplasmic reticulum (often abbreviated RER or Rough ER) (also called granular endoplasmic reticulum) is studded with protein - manufacturing ribosomes giving it a "rough" appearance (hence its name). The binding site of the ribosome on the rough endoplasmic reticulum is the translocon . However, the ribosomes are not a stable part of this organelle's structure as they are constantly being bound and released from the membrane . A ribosome only binds to the RER once a specific protein - nucleic acid complex forms in the cytosol . This special complex forms when a free ribosome begins translating the mRNA of a protein destined for the secretory pathway . The first 5--30 amino acids polymerized encode a signal peptide, a molecular message that is recognized and bound by a signal recognition particle (SRP). Translation pauses and the ribosome complex binds to the RER translocon where translation continues with the nascent (new) protein forming into the RER lumen and / or membrane . The protein is processed in the ER lumen by an enzyme (a signal peptidase), which removes the signal peptide . Ribosomes at this point may be released back into the cytosol; however, non-translating ribosomes are also known to stay associated with translocons . </P> <P> The membrane of the rough endoplasmic reticulum forms large double membrane sheets that are located near, and continuous with, the outer layer of the nuclear envelope . The double membrane sheets are stacked and connected through several right or left - handed helical ramps, the so - called Terasaki ramps, giving rise to a structure resembling a multi-storey car park . Although there is no continuous membrane between the endoplasmic reticulum and the Golgi apparatus, membrane - bound transport vesicles shuttle proteins between these two compartments . Vesicles are surrounded by coating proteins called COPI and COPII . COPII targets vesicles to the Golgi apparatus and COPI marks them to be brought back to the rough endoplasmic reticulum . The rough endoplasmic reticulum works in concert with the Golgi complex to target new proteins to their proper destinations . A second method of transport out of the endoplasmic reticulum involves areas called membrane contact sites, where the membranes of the endoplasmic reticulum and other organelles are held closely together, allowing the transfer of lipids and other small molecules . </P> <P> The rough endoplasmic reticulum is key in multiple functions: </P>

Where is the rough er located in a cell