Editing Endoplasmic reticulum (section)

===Smooth endoplasmic reticulum===
[[File:0313 Endoplasmic Reticulum c labeled.png|thumb|left|250px|Electron micrograph showing smooth ER (arrow) in mouse tissue, at 110,510× magnification]]

In most cells the smooth endoplasmic reticulum (abbreviated '''SER''') is scarce. Instead there are areas where the ER is partly smooth and partly rough, this area is called the transitional ER. The transitional ER gets its name because it contains ER exit sites. These are areas where the transport vesicles which contain lipids and proteins made in the ER, detach from the ER and start moving to the [[Golgi apparatus]]. Specialized cells can have a lot of smooth endoplasmic reticulum and in these cells the smooth ER has many functions.<ref name="Alberts-2002" /> It synthesizes [[lipids]], [[phospholipids]],<ref>{{cite journal |last1=Kannan |first1=Muthukumar |last2=Lahiri |first2=Sujoy |last3=Liu |first3=Li-Ka |last4=Choudhary |first4=Vineet |last5=Prinz |first5=William A. |title=Phosphatidylserine synthesis at membrane contact sites promotes its transport out of the ER |journal=Journal of Lipid Research |date=March 2017 |volume=58 |issue=3 |pages=553–562 |doi=10.1194/jlr.M072959  |doi-access=free |pmid=28119445 |pmc=5335585 }}</ref><ref>{{cite journal |last1=Kannan |first1=Muthukumar |last2=Riekhof |first2=Wayne R. |last3=Voelker |first3=Dennis R. |title=Transport of Phosphatidylserine from the Endoplasmic Reticulum to the Site of Phosphatidylserine Decarboxylase2 in Yeast: Phosphatidylserine Transport to the Locus of Psd2p |journal=Traffic |date=February 2015 |volume=16 |issue=2 |pages=123–134 |doi=10.1111/tra.12236 |pmid=25355612 |s2cid=34302 |doi-access= }}</ref><ref>{{cite journal |last1=Friedman |first1=Jonathan R. |last2=Kannan |first2=Muthukumar |last3=Toulmay |first3=Alexandre |last4=Jan |first4=Calvin H. |last5=Weissman |first5=Jonathan S. |last6=Prinz |first6=William A. |last7=Nunnari |first7=Jodi |title=Lipid Homeostasis Is Maintained by Dual Targeting of the Mitochondrial PE Biosynthesis Enzyme to the ER |journal=Developmental Cell |date=January 2018 |volume=44 |issue=2 |pages=261–270.e6 |doi=10.1016/j.devcel.2017.11.023 |pmid=29290583 |pmc=5975648 }}</ref> and [[steroids]]. Cells which secrete these products, such as those in the [[testes]], [[ovaries]], and [[sebaceous gland]]s have an abundance of smooth endoplasmic reticulum.<ref>{{cite news|title=Functions of Smooth ER|publisher=University of Minnesota Duluth}}<!--|access-date=16 December 2012--></ref> It also carries out the metabolism of carbohydrates, detoxification of natural metabolism products and of alcohol and drugs, attachment of receptors on cell membrane proteins, and [[steroid metabolism]].<ref>{{cite journal | vauthors = Maxfield FR, Wüstner D | title = Intracellular cholesterol transport | journal = The Journal of Clinical Investigation | volume = 110 | issue = 7 | pages = 891–8 | date = October 2002 | pmid = 12370264 | pmc = 151159 | doi = 10.1172/JCI16500 }}</ref> In muscle cells, it regulates [[Calcium in biology|calcium ion]] concentration. Smooth endoplasmic reticulum is found in a variety of cell types (both animal and plant), and it serves different functions in each. The smooth endoplasmic reticulum also contains the enzyme [[glucose-6-phosphatase]], which converts [[glucose-6-phosphate]] to glucose, a step in [[gluconeogenesis]]. It is connected to the [[nuclear envelope]] and consists of tubules that are located near the cell periphery. These tubes sometimes branch forming a network that is reticular in appearance.<ref name="Shibata-2006" /> In some cells, there are dilated areas like the sacs of rough endoplasmic reticulum. The network of smooth endoplasmic reticulum allows for an increased surface area to be devoted to the action or storage of key enzymes and the products of these enzymes.{{cn|date=December 2023}}

==== Sarcoplasmic reticulum ====
[[File:Blausen 0801 SkeletalMuscle.png|thumb|left|250px|[[Skeletal muscle]] fiber, with sarcoplasmic reticulum colored in blue]]
{{main|Sarcoplasmic reticulum}}
{{See also |Calcium-induced calcium release}}

The sarcoplasmic reticulum (SR), from the Greek σάρξ ''sarx'' ("flesh"), is smooth ER found in [[muscle cell]]s.  The only structural difference between this organelle and the smooth endoplasmic reticulum is the composition 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 endoplasmic reticulum synthesizes molecules, while the sarcoplasmic reticulum stores calcium ions and pumps them out into the sarcoplasm when the muscle fiber is stimulated.<ref name="Toyoshima-2000">{{cite journal | vauthors = Toyoshima C, Nakasako M, Nomura H, Ogawa H | title = Crystal structure of the calcium pump of sarcoplasmic reticulum at 2.6 A resolution | journal = Nature | volume = 405 | issue = 6787 | pages = 647–55 | date = June 2000 | pmid = 10864315 | doi = 10.1038/35015017 | bibcode = 2000Natur.405..647T | s2cid = 4316039 }}</ref><ref>{{cite book | first = Steven R | last = Goodman | name-list-style = vanc | title = Medical Cell Biology | edition = 3rd|publisher=Academic Press|pages=69|url=https://books.google.com/books?id=tRbCHk9easQC&q=smooth+er+stores+calcium+ions+muscle+cells&pg=PA69 | isbn = 9780080919317 | date = 2007-11-26 }}</ref> After their release from the sarcoplasmic reticulum, calcium ions interact with contractile proteins that utilize ATP to shorten the muscle fiber. The sarcoplasmic reticulum plays a major role in [[excitation-contraction coupling]].<ref>{{cite book |title = Fundamentals of Anatomy and Physiology |first1 = Frederick |last1 = Martini |first2 = Judi |last2 = Nath |first3 = Edwin |last3 = Bartholomew | name-list-style = vanc |edition = 10th |isbn = 978-0321909077 |year = 2014|publisher = Pearson }}</ref>