Editing Endomembrane system (section)

=== Endoplasmic reticulum ===
[[Image:nucleus ER golgi.svg|thumb|350px|'''1'''&nbsp;[[Cell nucleus|Nucleus]]{{nbsp}}
'''2'''&nbsp;[[Nuclear pore]]{{nbsp}}
'''3'''&nbsp;Rough endoplasmic reticulum (RER){{nbsp}}
'''4'''&nbsp;Smooth endoplasmic reticulum (SER){{nbsp}}
'''5'''&nbsp;[[Ribosome]] on the rough ER{{nbsp}}
'''6'''&nbsp;[[Protein]]s that are transported{{nbsp}}
'''7'''&nbsp;Transport [[Vesicle (biology)|vesicle]]{{nbsp}}
'''8'''&nbsp;[[Golgi apparatus]]{{nbsp}}
'''9'''&nbsp;Cis face of the Golgi apparatus{{nbsp}}
'''10'''&nbsp;Trans face of the Golgi apparatus{{nbsp}}
'''11'''&nbsp;Cisternae of the Golgi apparatus]]

{{main|Endoplasmic reticulum}}

The [[endoplasmic reticulum]] (ER) is a membranous synthesis and transport organelle that is an extension of the nuclear envelope. More than half the total membrane in eukaryotic cells is accounted for by the ER. The ER is made up of flattened sacs and branching tubules that are thought to interconnect, so that the ER membrane forms a continuous sheet enclosing a single internal space. This highly convoluted space is called the ER lumen and is also referred to as the ER [[cisterna]]l space. The lumen takes up about ten percent of the entire cell volume. The endoplasmic reticulum membrane allows molecules to be selectively transferred between the lumen and the cytoplasm, and since it is connected to the nuclear envelope, it provides a channel between the nucleus and the cytoplasm.<ref name=Cooper3>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=endoplasmic%20reticulum&rid=cooper.section.1466|title=The Endoplasmic Reticulum|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref>

The ER has a central role in producing, processing, and transporting [[Biochemistry|biochemical compounds]] for use inside and outside of the cell. Its membrane is the site of production of all the transmembrane proteins and lipids for many of the cell's organelles, including the ER itself, the Golgi apparatus, lysosomes, [[endosome]]s, secretory vesicles, and the plasma membrane. Furthermore, almost all of the proteins that will exit the cell, plus those destined for the lumen of the ER, Golgi apparatus, or lysosomes, are originally delivered to the ER lumen. Consequently, many of the proteins found in the cisternal space of the endoplasmic reticulum lumen are there only temporarily as they pass on their way to other locations. Other proteins, however, constantly remain in the lumen and are known as endoplasmic reticulum resident proteins. These special proteins contain a specialized retention signal made up of a specific sequence of [[amino acids]] that enables them to be retained by the organelle. An example of an important endoplasmic reticulum resident protein is the [[Chaperone (protein)#Human chaperone proteins|chaperone protein]] known as [[Binding immunoglobulin protein|BiP]] which identifies other proteins that have been improperly built or processed and keeps them from being sent to their final destinations.<ref name="Bertolotti et al.">{{cite journal | vauthors = Bertolotti A, Zhang Y, Hendershot LM, Harding HP, Ron D | title = Dynamic interaction of BiP and ER stress transducers in the unfolded-protein response | journal = Nature Cell Biology | volume = 2 | issue = 6 | pages = 326–32 | date = June 2000 | pmid = 10854322 | doi = 10.1038/35014014 | s2cid = 22684712 }}</ref>

The ER is involved in cotranslational sorting of proteins. A polypeptide which contains an ER signal sequence is recognised by the [[signal recognition particle]] which halts the production of the protein. The SRP transports the nascent protein to the ER membrane where it is released through a membrane channel and translation resumes.<ref>{{Cite book|title=Biology |year=2011 |url=https://archive.org/details/biologythedition00maso |url-access=limited |publisher=McGraw Hill education|pages=[https://archive.org/details/biologythedition00maso/page/n117 89]|isbn=9780078936494 }}</ref>
[[File:0313 Endoplasmic Reticulum b en.png|thumb|243x243px|By using [[electron microscope]], ribosomes ("particles") on the [[rough endoplasmic reticulum]] can be observed ]]
There are two distinct, though connected, regions of ER that differ in structure and function: smooth ER and rough ER. The rough endoplasmic reticulum is so named because the cytoplasmic surface is covered with ribosomes, giving it a bumpy appearance when viewed through an [[electron microscope]]. The smooth ER appears smooth since its cytoplasmic surface lacks ribosomes.<ref name=Alberts3>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=endoplasmic%20reticulum&rid=mboc4.section.2202#2204|title=Membrane-bound Ribosomes Define the Rough ER|access-date=2008-12-09|last=Alberts|first=Walter | name-list-style = vanc |year=2002|work= Molecular Biology of the Cell 4th edition|publisher=Garland Science|display-authors=etal}}</ref>

==== Functions of the smooth ER ====
In the great majority of cells, smooth ER regions are scarce and are often partly smooth and partly rough. They are sometimes called transitional ER because they contain ER exit sites from which transport vesicles carrying newly synthesized proteins and lipids bud off for transport to the Golgi apparatus. In certain specialized cells, however, the smooth ER is abundant and has additional functions. The smooth ER of these specialized cells functions in diverse metabolic processes, including synthesis of lipids, [[Carbohydrate metabolism|metabolism of carbohydrates]], and detoxification of drugs and poisons.<ref name=Cooper3 /><ref name=Alberts3 />

Enzymes of the smooth ER are vital to the synthesis of lipids, including [[oil]]s, [[phospholipid]]s, and [[steroid]]s. Sex hormones of vertebrates and the steroid hormones secreted by the [[adrenal gland]]s are among the steroids produced by the smooth ER in animal cells. The cells that synthesize these hormones are rich in smooth ER.<ref name=Cooper3 /><ref name=Alberts3 />

[[Liver]] cells are another example of specialized cells that contain an abundance of smooth ER. These cells provide an example of the role of smooth ER in carbohydrate metabolism. Liver cells store carbohydrates in the form of [[glycogen]]. The [[Glycogenolysis|breakdown of glycogen]] eventually leads to the release of [[glucose]] from the liver cells, which is important in the regulation of sugar concentration in the blood. However, the primary product of glycogen breakdown is glucose-1-phosphate. This is converted to glucose-6-phosphate and then an enzyme of the liver cell's smooth ER removes the phosphate from the glucose, so that it can then leave the cell.<ref name=Cooper3 /><ref name=Alberts3 />

Enzymes of the smooth ER can also help detoxify drugs and poisons. Detoxification usually involves the addition of a hydroxyl group to a drug, making the drug more soluble and thus easier to purge from the body. One extensively studied detoxification reaction is carried out by the [[cytochrome P450]] family of enzymes, which catalyze oxidation reactions on water-insoluble drugs or metabolites that would otherwise accumulate to toxic levels in cell membrane.<ref name=Cooper3 /><ref name=Alberts3 />

In muscle cells, a specialized smooth ER ([[sarcoplasmic reticulum]]) forms a membranous compartment (cisternal space) into which [[calcium]] ions are pumped. When a muscle cell becomes stimulated by a nerve impulse, calcium goes back across this membrane into the cytosol and generates the contraction of the muscle cell.<ref name=Cooper3 /><ref name=Alberts3 />

==== Functions of the rough ER ====
Many types of cells export proteins produced by ribosomes attached to the rough ER. The ribosomes assemble [[amino acids]] into protein units, which are carried into the rough ER for further adjustments. These proteins may be either [[transmembrane proteins]], which become embedded in the membrane of the endoplasmic reticulum, or water-soluble proteins, which are able to pass through the membrane into the lumen. Those that reach the inside of the endoplasmic reticulum are folded into the correct three-dimensional conformation. Chemicals, such as carbohydrates or sugars, are added, then the endoplasmic reticulum either transports the completed proteins, called secretory proteins, to areas of the cell where they are needed, or they are sent to the Golgi apparatus for further processing and modification.<ref name=Cooper3 /><ref name=Alberts3 />

Once secretory proteins are formed, the ER membrane separates them from the proteins that will remain in the cytosol. Secretory proteins depart from the ER enfolded in the membranes of vesicles that bud like bubbles from the transitional ER. These vesicles in transit to another part of the cell are called [[Vesicular transport protein|transport vesicles]].<ref name=Cooper3 /><ref name=Alberts3 /> An alternative mechanism for transport of lipids and proteins out of the ER are through lipid transfer proteins at regions called [[membrane contact site]]s where the ER becomes closely and stably associated with the membranes of other organelles, such as the plasma membrane, Golgi or lysosomes.<ref>{{cite journal | vauthors = Levine T, Loewen C | title = Inter-organelle membrane contact sites: through a glass, darkly | journal = Current Opinion in Cell Biology | volume = 18 | issue = 4 | pages = 371–8 | date = August 2006 | pmid = 16806880 | doi = 10.1016/j.ceb.2006.06.011 }}</ref>

In addition to making secretory proteins, the rough ER makes membranes that grows in place from the addition of proteins and phospholipids. As [[polypeptides]] intended to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane itself and are kept there by their [[hydrophobic]] portions. The rough ER also produces its own membrane phospholipids; enzymes built into the ER membrane assemble phospholipids. The ER membrane expands and can be transferred by transport vesicles to other components of the endomembrane system.<ref name=Cooper3 /><ref name=Alberts3 />