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The endoplasmic reticulum (ER) is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae. Rough endoplasmic reticulum are involved in the synthesis of proteins and is also a membrane factory for the cell, while smooth endoplasmic reticula are involved in the synthesis of lipids, including oils, phospholipids and steroids, metabolism of carbohydrates, regulation of calcium concentration and detoxification of drugs and poisons. Sarcoplasmic reticula solely regulate calcium levels.
The general structure of the endoplasmic reticulum is a membranous network of cisternae (sac-like structures) held together by the cytoskeleton. The phospholipid membrane encloses a space, the cisternal space (or lumen), which is continuous with the perinuclear space but separate from the cytosol. The functions of the endoplasmic reticulum vary greatly depending on its cell type, cell function, and cell needs. The ER can even modify to change over time in response to cell needs. The three most common varieties are called rough endoplasmic reticulum, smooth endoplasmic reticulum, and sarcoplasmic reticulum.
The quantity of RER and SER in a cell can slowly interchange from one type to the other, depending on changing metabolic needs. Transformation can include embedment of new proteins in membrane as well as structural changes. Massive changes may also occur in protein content without noticeable structural changes.
The surface of the rough endoplasmic reticulum (RER) is studded with protein-manufacturing ribosomes giving it a "rough" appearance (hence its name). The binding site of the Ribosome on RER is the translocon. However, the ribosomes bound to the RER at any one time are not a stable part of this organelle's structure as ribosomes are constantly being bound and released from the membrane. A ribosome binds to the ER only when it begins to synthesize a protein destined for the secretory pathway. Here, a ribosome in the cytosol begins synthesizing a protein until a signal recognition particle recognizes the signal peptide of 5-30 hydrophobic amino acids, sometimes preceded by a positively charged amino acid. This signal sequence allows the recognition particle to bind to the ribosome, causing the ribosome to bind to the RER and pass the new protein through the ER membrane. The signal peptide is then cleaved off within the lumen of the ER. Ribosomes at this point may be released back into the cytosol, however non-translating ribosomes are also known to stay associated with translocons. 
The membrane of the RER forms large double membrane sheets that are located near, and continuous with the outer layer of the nuclear envelope.  Although there is no continuous membrane between the RER and the Golgi apparatus, membrane-bound vesicles shuttle proteins between these two compartments. Vesicles are surrounded by coating proteins called COPI and COPII. COPII targets vesicles to the golgi and COPI marks them to be brought back to the RER. The RER works in concert with the Golgi complex to target new proteins to their proper destinations. A second method of transport out of the ER is areas called membrane contact sites, where the membranes of the ER and other organelles are held closely together, allowing the transfer of lipids and other small molecules.
The RER is key in multiple functions:
Disturbances in redox regulation, calcium regulation, glucose deprivation, and viral infection can lead to ER stress. It is a state in which the folding of proteins slows leading to an increase in unfolded proteins. This ER stress is emerging as a potential cause of damage in hypoxia/ischemia, insulin resistance and other disorders.
The smooth endoplasmic reticulum (SER) has functions in several metabolic processes, including synthesis of lipids and steroids, metabolism of carbohydrates, regulation of calcium concentration, drug detoxification, attachment of receptors on cell membrane proteins, and steroid metabolism. It is connected to the nuclear envelope. Smooth endoplasmic reticulum is found in a variety of cell types (both animal and plant), and it serves different functions in each. The Smooth ER also contains the enzyme glucose-6-phosphatase, which converts glucose-6-phosphate to glucose, a step in gluconeogenesis. The SER consists of tubules that are located near the cell periphery. These tubes sometimes branch forming a network that is reticular in appearance. In some cells, there are dilated areas like the sacs of RER. The network of SER allows increased surface area for the action or storage of key enzymes and the products of these enzymes.
The sarcoplasmic reticulum (SR), from the Greek sarx, ("flesh"), is smooth ER found in smooth and striated muscle. The only structural difference between this organelle and the ER is the medley of proteins they have, both bound to their membranes and drifting within the confines of their lumens. This fundamental difference is indicative of their functions: The ER synthesizes molecules, while the SR stores and pumps calcium ions. The SR contains large stores of calcium, which it sequesters and then releases when the muscle cell is stimulated. It plays a major role in excitation-contraction coupling.
The endoplasmic reticulum serves many general functions, including the facilitation of protein folding and the transport of synthesized proteins in sacs called cisternae.
Correct folding of newly-made proteins is made possible by several endoplasmic reticulum chaperone proteins, including protein disulfide isomerase (PDI), ERp29, the Hsp70 family member Grp78, calnexin, calreticulin, and the peptidylpropyl isomerase family. Only properly-folded proteins are transported from the rough ER to the Golgi complex.
Secretory proteins, mostly glycoproteins, are moved across the ER membrane. Proteins that are transported by the ER and from there throughout the cell are marked with an address tag called a signal sequence. The N-terminus (one end) of a polypeptide chain (i.e., a protein) contains a few amino acids that work as an address tag, which are removed when the polypeptide reaches its destination. Proteins that are destined for places outside the ER are packed into transport vesicles and moved along the cytoskeleton toward their destination.
The ER is also part of a protein sorting pathway. It is, in essence, the transportation system of the eukaryotic cell. The majority of ER resident proteins are retained in the ER through a retention motif. This motif is composed of four amino acids at the end of the protein sequence. The most common retention sequence is KDEL (lys-asp-glu-leu). However, variation on KDEL does occur and other sequences can also give rise to ER retention. It is not known whether such variation can lead to sub-ER localizations. There are three KDEL receptors in mammalian cells, and they have a very high degree of sequence identity. The functional differences between these receptors remain to be established.