Endoplasmic reticulum (ER) is a complex system of membranes present in cytoplasm. It is a network of lipoprotein membranes in the form of flattened sacs, tubules, and vesicles, enclosing a space called intracisternal space. Two types of ER are seen: (a) smooth ER without any attached particles and (b) rough (granular) ER with spherical particles called ribosomes. The structure of endoplasmic reticulum is as shown in Figure 1.5. ER is actively involved in the synthesis of proteins, lipids, and polysaccharides and the transport of these molecules to the site of storage or utilisation. It also acts as a precursor of the membranes like the nuclear membrane, cell membrane, and Golgi bodies. Glucose 6–phosphatase acts as a marker enzyme.