<P> The endoplasmic reticulum (ER) is a type of organelle found in eukaryotic cells that forms an interconnected network of flattened, membrane - enclosed sacs or tube - like structures known as cisternae . The membranes of the ER are continuous with the outer nuclear membrane . The endoplasmic reticulum occurs in most types of eukaryotic cells, but is absent from red blood cells and spermatozoa . There are two types of endoplasmic reticulum: rough and smooth . The outer (cytosolic) face of the rough endoplasmic reticulum is studded with ribosomes that are the sites of protein synthesis . The rough endoplasmic reticulum is especially prominent in cells such as hepatocytes . The smooth endoplasmic reticulum lacks ribosomes and functions in lipid manufacture and metabolism, the production of steroid hormones, and detoxification . The smooth ER is especially abundant in mammalian liver and gonad cells . The lacy membranes of the endoplasmic reticulum were first seen in 1945 using electron microscopy . </P> <P> The lacy membranes of the endoplasmic reticulum were first seen in 1945 by Keith R. Porter, Albert Claude, Brody Meskers and Ernest F. Fullam, using electron microscopy . The word reticulum, which means "network", was applied to describe this fabric of membranes . </P> <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>

What describes a function of the endoplasmic reticulum within a cell