The endoplasmic reticulum is an organelle found in eukaryotes consisting of a continuation of the nuclear membrane in cisternae that are held in shape by various cytoskeletal systems. These membranes enclose a region from the cytoplasm and so can form vesicles that can eventually leave the cell via exocytosis.
The ER consists of the rough and smooth endoplasmic reticulum, with the former being known as rough due to the presence of ribosomes (the molecular machinery responsible for protein synthesis). Once a protein that is either to be excreted from the cell or in a membrane of some form is produced the ER is responsible for sorting this protein to the correct location. If a protein is to be excreted then the synthesised protein is brought through the ER membrane fully and so can be used to form a vesicle to be transported out of the cell; however a membrane protein is only partly passed through the membrane and so once a vesicle buds it can fuse withe the correct organelle. The smooth ER has some roles that differ from the rough ER such as glycosylation, drug detoxification, disulphide bond rearrangement. Targeting is achieved by the ER examining signal peptides usually found on the N-terminal of a polypeptide and so it is easy for the ER to choose a destination.
Once the ER has finished with the protein a vesicle can bud off from it and hence a protein can be passed on to the golgi apparatus, a similar membrane system that is physically separate from the ER.
The sarcoplasmic reticulum is a special form of ER found in muscle cells that is responsible for the storage of calcium ions that are very important in muscle contraction as if there was no separation muscles would be continually contracted.
The endoplasmic reticulum is very important in life as it is responsible for modification and targeting of proteins and so if we were to lack it then proteins would be in the wrong places with a waste of metabolic energy as well as more serious consequences. For example if an ion channel were embedded in the worng membrane the ionic gradient could potentially be broke n down and so, in the case of mitochondria the proton gradient could be reduced so ATP production would be stopped.