Editing Cytosol (section)

===Ions===
The concentrations of the other [[ion]]s in cytosol are quite different from those in [[extracellular fluid]] and the cytosol also contains much higher amounts of charged macromolecules such as proteins and nucleic acids than the outside of the cell structure.

{| class="wikitable" style="margin:auto; text-align:center;"
|+ Typical ion concentrations in mammalian cytosol and plasma.<ref name="Lodish-1999">{{cite book |author=Lodish, Harvey F. |title=Molecular cell biology |publisher=Scientific American Books |location=New York |year=1999 |isbn=0-7167-3136-3 |oclc=174431482 |url-access=registration |url=https://archive.org/details/molecularcellbio00lodi }}</ref>
|-
! rowspan=2 | Ion
! colspan=2 | [[Molar concentration|Concentration]] (millimolar)
|-
! In cytosol 
! In plasma
|-
| [[Potassium]]
| 139–150<ref>{{Cite journal|title=Potassium physiology|first=S. O.|last=Thier|date=April 25, 1986|journal=The American Journal of Medicine|volume=80|issue=4A|pages=3–7|doi=10.1016/0002-9343(86)90334-7|pmid=3706350}}</ref><ref name="Lote-2012">{{cite book |last=Lote |first=Christopher J. |title= Principles of Renal Physiology, 5th edition|page=12|year=2012|publisher=Springer}}</ref>
| 4
|-
| [[Sodium]]
| 12
| 145
|-
| [[Chloride]]
| 4
| 116
|-
| [[Bicarbonate]]
| 12
| 29
|-
| [[Amino acid]]s in proteins
| 138
| 9
|-
| [[Magnesium]]
| 0.8
| 1.5
|-
| [[Calcium]]
| <0.0002
| 1.8
|}

In contrast to extracellular fluid, cytosol has a high concentration of [[potassium]] ions and a low concentration of [[sodium]] ions.<ref name="Lang-2007">{{cite journal |author=Lang F |title=Mechanisms and significance of cell volume regulation |journal=J Am Coll Nutr |volume=26 |issue=5 Suppl |pages=613S–623S |date=October 2007 |pmid=17921474 |doi=10.1080/07315724.2007.10719667 |s2cid=1798009 }}</ref> This difference in ion concentrations is critical for [[osmoregulation]], since if the ion levels were the same inside a cell as outside, water would enter constantly by [[osmosis]] - since the levels of [[macromolecule]]s inside cells are higher than their levels outside. Instead, sodium ions are expelled and potassium ions taken up by the [[Na⁺/K⁺-ATPase]], potassium ions then flow down their concentration gradient through potassium-selection ion channels, this loss of positive charge creates a negative [[membrane potential]]. To balance this [[potential difference]], negative chloride ions also exit the cell, through selective chloride channels. The loss of sodium and chloride ions compensates for the osmotic effect of the higher concentration of organic molecules inside the cell.<ref name="Lang-2007"/>

Cells can deal with even larger osmotic changes by accumulating [[osmoprotectant]]s such as [[betaines]] or [[trehalose]] in their cytosol.<ref name="Lang-2007"/> Some of these molecules can allow cells to survive being completely dried out and allow an organism to enter a state of suspended animation called [[cryptobiosis]].<ref>{{cite journal |vauthors=Sussich F, Skopec C, Brady J, Cesàro A |title=Reversible dehydration of trehalose and anhydrobiosis: from solution state to an exotic crystal? |journal=Carbohydr. Res. |volume=334 |issue=3 |pages=165–76 |date=August 2001 |pmid=11513823 |doi=10.1016/S0008-6215(01)00189-6}}</ref> In this state the cytosol and osmoprotectants become a glass-like solid that helps stabilize proteins and cell membranes from the damaging effects of desiccation.<ref>{{cite journal |vauthors=Crowe JH, Carpenter JF, Crowe LM |title=The role of vitrification in anhydrobiosis |journal=[[Annu. Rev. Physiol.]] |volume=60 |pages=73–103 |year=1998 |pmid=9558455 |doi=10.1146/annurev.physiol.60.1.73}}</ref>

The low concentration of [[calcium]] in the cytosol allows calcium ions to function as a [[second messenger]] in [[calcium signaling]]. Here, a signal such as a [[hormone]] or an [[action potential]] opens [[calcium channel]] so that calcium floods into the cytosol.<ref>{{cite journal |author=Berridge MJ |title=Elementary and global aspects of calcium signalling |journal=J. Physiol. |volume=499 |issue= Pt 2|pages=291–306 |date=1 March 1997|pmid=9080360 |pmc=1159305 |doi=10.1113/jphysiol.1997.sp021927 |url=http://www.jphysiol.org/cgi/pmidlookup?view=long&pmid=9080360 }}</ref> This sudden increase in cytosolic calcium activates other signalling molecules, such as [[calmodulin]] and [[protein kinase C]].<ref>{{cite journal |vauthors=Kikkawa U, Kishimoto A, Nishizuka Y |title=The protein kinase C family: heterogeneity and its implications |journal=[[Annu. Rev. Biochem.]] |volume=58 |pages=31–44 |year=1989 |pmid=2549852 |doi=10.1146/annurev.bi.58.070189.000335}}</ref> Other ions such as chloride and potassium may also have signaling functions in the cytosol, but these are not well understood.<ref>{{cite journal |vauthors=Orlov SN, Hamet P |title=Intracellular monovalent ions as second messengers |journal=J. Membr. Biol. |volume=210 |issue=3 |pages=161–72 |date=April 2006 |pmid=16909338 |doi=10.1007/s00232-006-0857-9|s2cid=26068558 }}</ref>