A bag-like membrane system of channels, flattened sacs and tubes within eukaryotic cells that stores and transports proteins within cells to the Golgi apparatus, as well as the various enzymes required for the synthesis of fats. Proteins that are to become part of a cell membrane are translated and folded in the ER before being transported. Thus, in short, ER is a transport system within a cell. It terms of these functions, it is important to distinguish between rough ER (RER) and smooth ER (SER). RER is that part of the ER covered by ribsomes, hence its name (see figure below). It has a number of functions, including the manufacture and transport of proteins that go to form cell membranes and the modification and folding of proteins. SES is ribosome-free ER, and once again it has a range of functions, such as the synthesis of various lipids, fatty acids and steroids (e.g., hormones), as well as having an important role in carbohydrate metabolism and calcium storage. It also detoxifies poisons, and consequently yhr liver has an abundance of SER. The ER found in muscle fibbers is called sarcoplasmic reticulum.
