part of part of cytoplasm endoplasmic reticulum Wikipedia:Endoplasmic_reticulum cellular_component GO:0005783 ER The irregular network of unit membranes, visible only by electron microscopy, that occurs in the cytoplasm of many eukaryotic cells. The membranes form a complex meshwork of tubular channels, which are often expanded into slitlike cavities called cisternae. The ER takes two forms, rough (or granular), with ribosomes adhering to the outer surface, and smooth (with no ribosomes attached). NIF_Subcellular:sao1036339110 endomembrane system intracellular membrane-bounded organelle