Endoplasmic reticulum

The endoplasmic reticulum (ER) is a feature of all eukaryotic cells, thats main functions are its role in lipid and protein biosynthesis and as a store for calcium within the cell. It is a network of internal membranes called cisternae which are continuous with the outer membrane . The ER can be rough (rough endoplasmic reticulum) in which ribosomes coat the surface of the ER  which synthesize proteins in the ER. In mammalian cells, synthesis of proteins is co-translational, which means proteins are captured from the cytosol by the ER and imported into it before they are fully synthesized . Cells that produce a lot of specific proteins to be secreted have an abundance of rough ER . Modifications of proteins to be secreted such as glycosylation or formation of disulphide bonds take place in the lumen of the ER . The ER can also be smooth (smooth endoplasmic reticulum) which lack a coating of ribosomes. Transport vesicles bud off from the smooth ER, which carry the newly synthesized proteins and lipids and transport their contents to the Golgi apparatus . Here they are modified further, before being transported to their final destination . As the ER is also a store of calcium, the ER contains a calcium pump which transports calcium ions from the cytosol into the ER lumen . Cells which require rapid responses to extracellular signals, such as muscle cells, require transport of calcium ions into the cytosol from the ER (which is the uptaken back into the ER.) This is why muscle cells have many modified smooth ER, named the sarcoplasmic reticulum, so that calcium is available to trigger myofibril contraction during muscle contraction .