Endoplasmic Reticulum

The endoplasmic reticulum (ER, endo meaning “inner,” reticular meaning “net”) was an unknown constituent of cells until early in the 1950s when the electron microscope brought it into view. It is now known that the endoplasmic reticulum is present in all eukaryotic cells. It appears as a system of paired, parallel membranes running through the cytoplasm and taking the form of flattened tubes or bags. The bags are called cisternae. It has been suggested that the endoplasmic reticulum divides the cytoplasm into compartments and that it may be likened to a mass of soap bubbles continually changing form and position. There are two known kinds of endoplasmic reticulum: rough and smooth. Rough endoplasmic reticulum is so-called because it has ribosomes on its outer surface. (Ribosomes are involved with protein synthesis and secretions.) Smooth endoplasmic reticulum has no ribosomes and may be involved in the production of carbohydrate. The endoplasmic reticulum, particularly the smooth form, may be associated with plasmodesmata, strands of cytoplasm that run through cell walls, creating the appearance of communicating linkages between cells. Evidence also indicates that the endoplasmic reticulum is contiguous with the outer nuclear membrane.