A system of membranes that extends throughout the cytoplasm of plant and animal cells forming interconnected tubes and flattened sacs (cisternae). It is part of the endomembrane system, which links many parts of the cell including the plasma and nuclear membranes (see cisterna). The ER is the site of protein synthesis and modification, and also the synthesis of lipids for new membranes. Rough ER (RER) has ribosomes attached to its surface, giving it a ‘rough’ appearance under the microscope. Here, newly synthesized proteins enter the interior lumen where they are sorted, chemically modified and folded to attain their tertiary structure, and tagged with a localization signal (‘address’) that determines their destination. The proteins are then enclosed in vesicles, which pinch off from the surface of the RER to be transported to the Golgi apparatus for further processing. Membrane proteins and lipids are made in the RER and are integrated into the ER membrane in situ to produce new membrane. Smooth ER (SER), which is continuous with RER, lacks ribosomes; it is the site of important metabolic reactions, including further protein modification of proteins, synthesis of phospholipids, fatty acids, and steroids, and (in certain animal cells) the breakdown of glycogen. It also chemically modifies and detoxifies molecules that enter the organism from outside, and is especially prominent in liver cells. SER stores calcium ions, release of which into the cytosol acts as a signal to trigger different responses. In muscle cells SER is adapted to form the sarcoplasmic reticulum.