View source for Magnesium in biology

{{Short description|Use of Magnesium by organisms}}
{{Use dmy dates|date=November 2024}}
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[[File:Комплекс MgАТФ 2-.png|thumb|[[Magnesium]]–[[adenosine triphosphate]] ionic mixture, what is often just called adenosine triphosphate colloquially in biology]]

[[Magnesium]] is an essential element in biological systems. Magnesium occurs typically as the Mg<sup>2+</sup> ion. It is an essential mineral [[nutrient]] (i.e., element) for life<ref>{{cite book |title=Van Nostrand's Scientific Encyclopedia |publisher=John Wiley & Sons, Inc. |year=2006 |isbn=978-0471743989 |chapter=Magnesium (In Biological Systems) |doi=10.1002/0471743984.vse4741}}</ref><ref name="Leroy 1926">{{cite journal| last=Leroy| first=J.| year=1926| title=Necessite du magnesium pour la croissance de la souris| journal=Comptes Rendus des Séances de la Société de Biologie| volume=94| pages= 431–433}}</ref><ref name="Lusk 1968">{{cite journal| vauthors = Lusk JE, Williams RJ, Kennedy EP | year=1968| title=Magnesium and the growth of Escherichia coli| journal=Journal of Biological Chemistry| volume=243| pages=2618–2624| pmid=4968384| issue=10 | doi=10.1016/S0021-9258(18)93417-4| doi-access=free }}</ref><ref name="Marschner 1995">{{cite book| last=Marschner| first= H.| year=1995| title=Mineral Nutrition in Higher Plants| location=San Diego| publisher= Academic Press| isbn=978-0-12-473542-2}}</ref> and is present in every [[cell (biology)|cell]] type in every organism. For example, [[adenosine triphosphate]] (ATP), the main source of energy in cells, must bind to a magnesium ion in order to be biologically active. What is called ATP is often actually Mg-ATP.<ref name="cancerweb.ncl.ac.uk">{{cite web|url=http://cancerweb.ncl.ac.uk/cgi-bin/omd?magnesium|title=Definition: magnesium from Online Medical Dictionary|date=25 December 2007|access-date=17 January 2018|archive-url=https://web.archive.org/web/20071225194451/http://cancerweb.ncl.ac.uk/cgi-bin/omd?magnesium|archive-date=2007-12-25}}</ref> As such, magnesium plays a role in the stability of all [[polyphosphate]] compounds in the cells, including those associated with the synthesis of [[DNA]] and [[RNA]].{{cn|date=November 2024}}

[[Image:Chlorophyll-a-3D-vdW.png|thumb|Space-filling model of the [[chlorophyll a]] molecule, with the magnesium ion (bright-green) visible at the center of the [[chlorin]] group]]

Over 300 [[enzyme]]s require the presence of magnesium ions for their catalytic action, including ''all'' enzymes utilizing or synthesizing ATP, or those that use other [[nucleotide]]s to synthesize DNA and RNA.<ref>{{Cite book |url=https://www.nap.edu/read/5776/chapter/8 |title=Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride |publisher=National Academy Press |year=1997 |location=Washington, DC |pages=190–191 |doi=10.17226/5776 |pmid=23115811 |isbn=978-0-309-06403-3 }}</ref>

In plants, magnesium is necessary for synthesis of [[chlorophyll]] and [[photosynthesis]].{{cn|date=November 2024}}

==Function==
{{unreferenced section|date=November 2024}}

A balance of magnesium is vital to the well-being of all organisms. Magnesium is a relatively abundant ion in Earth's crust and mantle and is highly bioavailable in the [[hydrosphere]]. This availability, in combination with a useful and very unusual chemistry, may have led to its utilization in evolution as an ion for signaling, enzyme activation, and [[catalysis]]. However, the unusual nature of ionic magnesium has also led to a major challenge in the use of the ion in biological systems. Biological membranes are impermeable to magnesium (and other ions), so transport proteins must facilitate the flow of magnesium, both into and out of cells and intracellular compartments.

=== Human health ===
{{Further|Magnesium deficiency}}

Inadequate magnesium intake frequently causes muscle [[spasm]]s, and has been associated with [[cardiovascular disease]], [[diabetes]], [[high blood pressure]], [[anxiety]] disorders, [[migraine]]s, [[osteoporosis]], and [[cerebral infarction]].<ref>{{cite book | vauthors = Romani AM | chapter=Magnesium in Health and Disease |editor=Astrid Sigel |editor2=Helmut Sigel |editor3=Roland K. O. Sigel |title=Interrelations between Essential Metal Ions and Human Diseases |series=Metal Ions in Life Sciences |volume=13 |year=2013 |publisher=Springer |pages=49–79 |doi=10.1007/978-94-007-7500-8_3 | pmid=24470089 | isbn=978-94-007-7499-5 }}</ref><ref name="pmid18332289">{{cite journal |vauthors = Larsson SC, Virtanen MJ, Mars M, Männistö S, Pietinen P, Albanes D, Virtamo J |title=Magnesium, calcium, potassium, and sodium intakes and risk of stroke in male smokers |journal=Archives of Internal Medicine |volume=168 |issue=5 |pages=459–465 |date=March 2008 |pmid=18332289 |doi=10.1001/archinte.168.5.459 |doi-access=free }}</ref> Acute deficiency (see [[hypomagnesemia]]) is rare, and is more common as a drug side-effect (such as chronic alcohol or diuretic use) than from low food intake per se, but it can occur in people fed intravenously for extended periods of time.{{cn|date=November 2024}}

The most common symptom of excess oral magnesium intake is [[diarrhea]]. Supplements based on [[amino acid]] [[chelates]] (such as [[glycinate]], [[lysinate]] etc.) are much better-tolerated by the [[digestive system]] and do not have the [[side-effects]] of the older compounds used, while [[sustained-release]] [[dietary supplements]] prevent the occurrence of diarrhea.{{Citation needed|date=November 2007}} Since the kidneys of adult humans excrete excess magnesium efficiently, oral magnesium poisoning in adults with normal [[renal function]] is very rare. Infants, which have less ability to excrete excess magnesium even when healthy, should not be given magnesium supplements, except under a physician's care.{{cn|date=November 2024}}

[[Magnesium (pharmaceutical preparation)|Pharmaceutical preparations with magnesium]] are used to treat conditions including [[magnesium deficiency (medicine)|magnesium deficiency]] and [[hypomagnesemia]], as well as [[eclampsia]].<ref name="Euser2009">{{Cite journal | vauthors = Euser AG, Cipolla MJ | doi = 10.1161/STROKEAHA.108.527788 | title = Magnesium Sulfate for the Treatment of Eclampsia: A Brief Review | journal = Stroke | volume = 40 | issue = 4 | pages = 1169–1175 | year = 2009 | pmid = 19211496| pmc =2663594 }}</ref> Such preparations are usually in the form of [[magnesium sulfate]] or chloride when given [[parenterally]]. Magnesium is absorbed with reasonable efficiency (30% to 40%) by the body from any soluble magnesium salt, such as the chloride or citrate. Magnesium is similarly absorbed from [[Epsom salts]], although the sulfate in these salts adds to their laxative effect at higher doses. Magnesium absorption from the insoluble oxide and hydroxide salts ([[milk of magnesia]]) is erratic and of poorer efficiency, since it depends on the neutralization and solution of the salt by the acid of the stomach, which may not be (and usually is not) complete.

[[Magnesium orotate]] may be used as adjuvant therapy in patients on optimal treatment for severe [[congestive heart failure]], increasing survival rate and improving clinical symptoms and patient's [[quality of life]].<ref name="pmid18281113">{{cite journal |vauthors=Stepura OB, Martynow AI |title=Magnesium orotate in severe congestive heart failure (MACH) |journal=Int. J. Cardiol. |volume= 131|issue= 2|pages= 293–5|date=February 2008 |pmid=18281113 |doi=10.1016/j.ijcard.2007.11.022 }}</ref>

In 2022, magnesium salts were the 207th most commonly prescribed medication in the United States, with more than 1{{nbsp}}million prescriptions.<ref name="Top 300 of 2022">{{cite web | title=The Top 300 of 2022 | url=https://clincalc.com/DrugStats/Top300Drugs.aspx | website=ClinCalc | access-date=30 August 2024 | archive-date=30 August 2024 | archive-url=https://web.archive.org/web/20240830202410/https://clincalc.com/DrugStats/Top300Drugs.aspx | url-status=live }}</ref><ref>{{cite web | title = Magnesium Salts Drug Usage Statistics, United States, 2013 - 2022 | website = ClinCalc | url = https://clincalc.com/DrugStats/Drugs/MagnesiumSalts | access-date = 30 August 2024 }}</ref>

====Nerve conduction====
Magnesium can affect muscle relaxation through direct action on cell membranes. Mg<sup>2+</sup> ions close certain types of [[NMDA Receptor|calcium channels]], which conduct positively charged [[calcium in biology|calcium ions]] into [[neuron]]s. With an excess of magnesium, more channels will be blocked and nerve cells activity will decrease.<ref>{{Cite journal | doi = 10.1016/j.neuron.2004.11.013| pmid = 15572114| title = Enhancement of synaptic plasticity through chronically reduced Ca2+ flux during uncorrelated activity| journal = Neuron| volume = 44| issue = 5| pages = 835–849| date = December 2004 | vauthors = Slutsky I, Sadeghpour S, Li B, Liu G | doi-access = free | url = http://www.slutskylab.com/wp-content/uploads/2013/04/Neuron-2004.pdf }}</ref><ref>{{Cite journal | doi = 10.1016/j.neuron.2009.12.026| pmid = 20152124| title = Enhancement of learning and memory by elevating brain magnesium| journal = Neuron| volume = 65| issue = 2| pages = 165–177| date = January 2010 | vauthors = Slutsky I, Abumaria N, Wu LJ, Huang C, Zhang L, Li B, Zhao X, Govindarajan A, Zhao MG, Zhuo M, Tonegawa S, Liu G | doi-access = free | hdl = 1721.1/96066 | hdl-access = free | url = http://www.slutskylab.com/wp-content/uploads/2013/04/Mg-Neuron-2010.pdf }}</ref>

====Hypertension====
Intravenous magnesium sulphate is used in treating [[pre-eclampsia]].<ref>{{cite journal |vauthors=Duley L, Gülmezoglu AM, Henderson-Smart DJ, Chou D |title=Magnesium sulphate and other anticonvulsants for women with pre-eclampsia |journal=Cochrane Database Syst Rev |issue=11 |pages=CD000025 |year=2010 |volume=2010 |pmid=21069663 |doi=10.1002/14651858.CD000025.pub2 |pmc=7061250 }}</ref> For other than pregnancy-related hypertension, a meta-analysis of 22 clinical trials with dose ranges of 120 to 973&nbsp;mg/day and a mean dose of 410&nbsp;mg, concluded that magnesium supplementation had a small but statistically significant effect, lowering systolic blood pressure by 3–4&nbsp;mm Hg and diastolic blood pressure by 2–3&nbsp;mm Hg. The effect was larger when the dose was more than 370&nbsp;mg/day.<ref>{{cite journal |vauthors=Kass L, Weekes J, Carpenter L |title=Effect of magnesium supplementation on blood pressure: a meta-analysis |journal=Eur J Clin Nutr |volume=66 |issue=4 |pages=411–8 |year=2012 |pmid=22318649 |doi=10.1038/ejcn.2012.4 |doi-access=free }}</ref>

====Diabetes and glucose tolerance====
Higher dietary intakes of magnesium correspond to lower diabetes incidence.<ref>{{cite journal |vauthors=Fang X, Han H, Li M, Liang C, Fan Z, Aaseth J, He J, Montgomery S, Cao Y |title=Dose-Response Relationship between Dietary Magnesium Intake and Risk of Type 2 Diabetes Mellitus: A Systematic Review and Meta-Regression Analysis of Prospective Cohort Studies |journal=Nutrients |volume=8 |issue=11 |pages= 739|year=2016 |pmid=27869762 |pmc=5133122 |doi=10.3390/nu8110739 |doi-access=free }}</ref> For people with diabetes or at high risk of diabetes, magnesium supplementation lowers fasting glucose.<ref>{{cite journal |vauthors=Veronese N, Watutantrige-Fernando S, Luchini C, Solmi M, Sartore G, Sergi G, Manzato E, Barbagallo M, Maggi S, Stubbs B |title=Effect of magnesium supplementation on glucose metabolism in people with or at risk of diabetes: a systematic review and meta-analysis of double-blind randomized controlled trials |journal=Eur J Clin Nutr |volume=70 |issue=12 |pages=1354–1359 |year=2016 |pmid=27530471 |doi=10.1038/ejcn.2016.154 |hdl=10447/297358 |s2cid=24998868 |hdl-access=free }}</ref>

=== Mitochondria ===
{{Further|Mitochondrion}}
Magnesium is essential as part of the process that generates [[adenosine triphosphate]].<ref>{{Cite journal |last1=Liu |first1=Man |last2=Jeong |first2=Euy-Myoung |last3=Liu |first3=Hong |last4=Xie |first4=An |last5=So |first5=Eui Young |last6=Shi |first6=Guangbin |last7=Jeong |first7=Go Eun |last8=Zhou |first8=Anyu |last9=Dudley |first9=Samuel C. |title=Magnesium supplementation improves diabetic mitochondrial and cardiac diastolic function |journal=JCI Insight |date=2019 |volume=4 |issue=1 |pages=e123182 |doi=10.1172/jci.insight.123182 |issn=2379-3708 |pmc=6485371 |pmid=30626750}}</ref><ref>{{Cite journal |last1=Maggio |first1=Marcello |last2=De Vita |first2=Francesca |last3=Lauretani |first3=Fulvio |last4=Nouvenne |first4=Antonio |last5=Meschi |first5=Tiziana |last6=Ticinesi |first6=Andrea |last7=Dominguez |first7=Ligia J. |last8=Barbagallo |first8=Mario |last9=Dall'Aglio |first9=Elisabetta |last10=Ceda |first10=Gian Paolo |date=2014 |title=The Interplay between Magnesium and Testosterone in Modulating Physical Function in Men |journal=International Journal of Endocrinology |volume=2014 |pages=525249 |doi=10.1155/2014/525249 |doi-access=free |issn=1687-8337 |pmc=3958794 |pmid=24723948}}</ref>

[[Mitochondrion|Mitochondria]] are often referred to as the "powerhouses of the cell" because their primary role is generating energy for cellular processes. They achieve this by breaking down [[Nutrient|nutrients]], primarily [[glucose]], through a series of chemical reactions known as '''[[cellular respiration]]'''. This process ultimately produces adenosine triphosphate (ATP), the cell's main energy currency.

=== Vitamin D ===
{{Further|Vitamin D}}
Magnesium and [[vitamin D]] have a synergistic relationship in the body, meaning they work together to optimize each other's functions:<ref>{{Cite journal |last1=Uwitonze |first1=Anne Marie |last2=Razzaque |first2=Mohammed S. |date=2018-03-01 |title=Role of Magnesium in Vitamin D Activation and Function |url=https://pubmed.ncbi.nlm.nih.gov/29480918/ |journal=The Journal of the American Osteopathic Association |volume=118 |issue=3 |pages=181–189 |doi=10.7556/jaoa.2018.037 |issn=1945-1997 |pmid=29480918}}</ref><ref>{{Cite journal |last1=Dai |first1=Qi |last2=Zhu |first2=Xiangzhu |last3=Manson |first3=JoAnn E |last4=Song |first4=Yiqing |last5=Li |first5=Xingnan |last6=Franke |first6=Adrian A |last7=Costello |first7=Rebecca B |last8=Rosanoff |first8=Andrea |last9=Nian |first9=Hui |last10=Fan |first10=Lei |last11=Murff |first11=Harvey |last12=Ness |first12=Reid M |last13=Seidner |first13=Douglas L |last14=Yu |first14=Chang |last15=Shrubsole |first15=Martha J |date=2018-12-01 |title=Magnesium status and supplementation influence vitamin D status and metabolism: results from a randomized trial |journal=The American Journal of Clinical Nutrition |volume=108 |issue=6 |pages=1249–1258 |doi=10.1093/ajcn/nqy274 |pmid=30541089 |pmc=6693398 |issn=0002-9165}}</ref>

* Magnesium activates vitamin D
* Vitamin D influences magnesium absorption.
* '''Bone health:''' They play crucial roles in [[calcium]] absorption and bone metabolism.
* '''Muscle function:''' They contribute to muscle contraction and relaxation, impacting physical performance and overall well-being.
* '''Immune function:''' They support a healthy immune system and may help reduce inflammation.

Overall, maintaining adequate levels of both magnesium and vitamin D is essential for optimal health and well-being.

=== Testosterone ===
{{Further|Testosterone}}

It is theorized that the process of making testosterone from cholesterol, needs magnesium to function properly.<ref>{{Cite journal |last1=Maggio |first1=Marcello |last2=De Vita |first2=Francesca |last3=Lauretani |first3=Fulvio |last4=Nouvenne |first4=Antonio |last5=Meschi |first5=Tiziana |last6=Ticinesi |first6=Andrea |last7=Dominguez |first7=Ligia J. |last8=Barbagallo |first8=Mario |last9=Dall'Aglio |first9=Elisabetta |last10=Ceda |first10=Gian Paolo |date=2014 |title=The Interplay between Magnesium and Testosterone in Modulating Physical Function in Men |journal=International Journal of Endocrinology |volume=2014 |pages=525249 |doi=10.1155/2014/525249 |doi-access=free |issn=1687-8337 |pmc=3958794 |pmid=24723948}}</ref>

Studies have shown that significant gains in testosterone occur after taking 10 mg magnesium/kg body weight/day.<ref>{{Cite journal |last1=Cinar |first1=Vedat |last2=Polat |first2=Yahya |last3=Baltaci |first3=Abdulkerim Kasim |last4=Mogulkoc |first4=Rasim |date=April 2011 |title=Effects of magnesium supplementation on testosterone levels of athletes and sedentary subjects at rest and after exhaustion |url=https://pubmed.ncbi.nlm.nih.gov/20352370/ |journal=Biological Trace Element Research |volume=140 |issue=1 |pages=18–23 |doi=10.1007/s12011-010-8676-3 |issn=1559-0720 |pmid=20352370|bibcode=2011BTER..140...18C |s2cid=23626641 }}</ref>

==Dietary recommendations==
The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and [[Recommended Dietary Allowances]] (RDAs) for magnesium in 1997. If there is not sufficient information to establish EARs and RDAs, an estimate designated Adequate Intake (AI) is used instead. The current EARs for magnesium for women and men ages 31 and up are 265&nbsp;mg/day and 350&nbsp;mg/day, respectively. The RDAs are 320 and 420&nbsp;mg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy is 350 to 400&nbsp;mg/day depending on age of the woman. RDA for lactation ranges 310 to 360&nbsp;mg/day for same reason. For children ages 1–13 years, the RDA increases with age from 65 to 200&nbsp;mg/day. As for safety, the IOM also sets [[Tolerable upper intake level]]s (ULs) for vitamins and minerals when evidence is sufficient. In the case of magnesium the UL is set at 350&nbsp;mg/day. The UL is specific to magnesium consumed as a dietary supplement, the reason being that too much magnesium consumed at one time can cause diarrhea. The UL does not apply to food-sourced magnesium. Collectively the EARs, RDAs and ULs are referred to as [[Dietary Reference Intake]]s.<ref>[https://www.ncbi.nlm.nih.gov/books/NBK109816/ "Magnesium"], pp.190-249 in "Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride". National Academy Press. 1997.</ref>

{| class="wikitable"
|+ [[Reference Daily Intake]] of magnesium<ref>{{cite web|url=https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/|title=Magnesium|website=[[National Institutes of Health]], Office of Dietary Supplements}} Updated: September 26, 2018</ref>
! Age
! Male
! Female
! Pregnancy
! Lactation
|-
| Birth to 6 months
| 30&nbsp;mg*
| 30&nbsp;mg*
|
|
|-
| 7–12 months
| 75&nbsp;mg*
| 75&nbsp;mg*
|
|
|-
| 1–3 years
| 80&nbsp;mg
| 80&nbsp;mg
|
|
|-
| 4–8 years
| 130&nbsp;mg
| 130&nbsp;mg
|
|
|-
| 9–13 years
| 240&nbsp;mg
| 240&nbsp;mg
|
|
|-
| 14–18 years
| 410&nbsp;mg
| 360&nbsp;mg
| 400&nbsp;mg
| 360&nbsp;mg
|-
| 19–30 years
| 400&nbsp;mg
| 310&nbsp;mg
| 350&nbsp;mg
| 310&nbsp;mg
|-
| 31–50 years
| 420&nbsp;mg
| 320&nbsp;mg
| 360&nbsp;mg
| 320&nbsp;mg
|-
| ≥51 years
| 420&nbsp;mg
| 320&nbsp;mg
|
|
|}
<nowiki>*</nowiki> = Adequate intake

The [[European Food Safety Authority]] (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL are defined the same as in the United States. For women and men ages 18 and older, the AIs are set at 300 and 350&nbsp;mg/day, respectively. AIs for pregnancy and lactation are also 300&nbsp;mg/day. For children ages 1–17 years, the AIs increase with age from 170 to 250&nbsp;mg/day. These AIs are lower than the U.S. RDAs.<ref>{{cite web | title = Overview on Dietary Reference Values for the EU population as derived by the EFSA Panel on Dietetic Products, Nutrition and Allergies| year = 2017| url = https://www.efsa.europa.eu/sites/default/files/assets/DRV_Summary_tables_jan_17.pdf}}</ref> The European Food Safety Authority reviewed the same safety question and set its UL at 250&nbsp;mg/day{{snd}} lower than the U.S. value.<ref>{{citation| title = Tolerable Upper Intake Levels For Vitamins And Minerals| publisher = European Food Safety Authority| year = 2006| url = http://www.efsa.europa.eu/sites/default/files/efsa_rep/blobserver_assets/ndatolerableuil.pdf}}</ref> The magnesium UL is unique in that it is lower than some of the RDAs. It applies to intake from a pharmacological agent or dietary supplement only and does not include intake from food and water.

===Labeling===
For U.S. food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of daily value (%DV). For magnesium labeling purposes, 100% of the daily value was 400&nbsp;mg, but as of May 27, 2016, it was revised to 420&nbsp;mg to bring it into agreement with the RDA.<ref name="FedReg">{{cite web|url=https://www.gpo.gov/fdsys/pkg/FR-2016-05-27/pdf/2016-11867.pdf |title=Federal Register May 27, 2016 Food Labeling: Revision of the Nutrition and Supplement Facts Labels. FR page 33982.}}</ref><ref>{{cite web | title=Daily Value Reference of the Dietary Supplement Label Database (DSLD) | website=Dietary Supplement Label Database (DSLD) | url=https://www.dsld.nlm.nih.gov/dsld/dailyvalue.jsp | access-date=16 May 2020}}</ref> A table of the old and new adult Daily Values is provided at [[Reference Daily Intake]].

== Food sources ==
[[Image:FoodSourcesOfMagnesium.jpg|upright|thumb|Some good sources of magnesium]]
[[Green vegetable]]s such as [[spinach]] provide magnesium because of the abundance of [[chlorophyll]] molecules, which contain the ion. [[Nut (fruit)|Nut]]s (especially [[Brazil nut]]s, [[cashew]]s and [[almond]]s), [[seed]]s (e.g., [[pumpkin seed]]s), [[dark chocolate]], roasted [[soybean]]s, [[bran]], and some [[whole grain]]s are also good sources of magnesium.<ref name="foods">{{cite web|url=http://www.healthaliciousness.com/articles/foods-high-in-magnesium.php|title=Top 10 Foods Highest in Magnesium + Printable One Page Sheet|website=HealthAliciousNess|access-date=17 January 2018}}</ref>

Although many foods contain magnesium, it is usually found in low levels. As with most nutrients, daily needs for magnesium are unlikely to be met by one serving of any single food. Eating a wide variety of fruits, vegetables, and grains will help ensure adequate intake of magnesium.

Because magnesium readily dissolves in water, refined foods, which are often processed or cooked in water and dried, in general, are poor sources of the nutrient. For example, [[whole-wheat bread]] has twice as much magnesium as white bread because the magnesium-rich germ and bran are removed when white flour is processed. The table of food sources of magnesium suggests many dietary sources of magnesium.

[[Hard water|"Hard" water]] can also provide magnesium, but [[Soft water|"soft" water]] contains less of the ion. Dietary surveys do not assess magnesium intake from water, which may lead to underestimating total magnesium intake and its variability.

Too much magnesium may make it difficult for the body to absorb [[calcium]]. Not enough magnesium can lead to [[hypomagnesemia]] as described above, with irregular heartbeats, high blood pressure (a sign in humans but not some experimental animals such as rodents), insomnia, and muscle spasms ([[fasciculation]]). However, as noted, symptoms of low magnesium from pure dietary deficiency are thought to be rarely encountered.

Following are some foods and the amount of magnesium in them:<ref>{{cite web|url=http://nutritiondata.self.com|title=SELF Nutrition Data - Food Facts, Information & Calorie Calculator|website=nutritiondata.self.com}}</ref>
{{colbegin}}
* [[Pumpkin]] seeds, no hulls ({{1/4}} cup) = 303&nbsp;mg
*[[Chia seed]]s, ({{1/4}} cup) = 162&nbsp;mg<ref>{{cite web|url=https://fdc.nal.usda.gov/fdc-app.html#/food-details/170554/nutrients|title=Food Composition Databases Show Foods List12006|website=ndb.nal.usda.gov}}</ref>
* [[Buckwheat]] flour ({{1/2}} cup) = 151&nbsp;mg
* [[Brazil nut]]s ({{1/4}} cup) = 125&nbsp;mg
* Oat bran, raw ({{1/2}} cup) = 110&nbsp;mg
* Cocoa powder ({{1/4}} cup) = 107&nbsp;mg
* [[Halibut]] (3&nbsp;oz) = 103&nbsp;mg
* [[Almonds]] ({{1/4}} cup) = 99&nbsp;mg
* [[Cashews]] ({{1/4}} cup) = 89&nbsp;mg
* Whole wheat flour ({{1/2}} cup) = 83&nbsp;mg
* [[Spinach]], boiled ({{1/2}} cup) = 79&nbsp;mg
* [[Swiss chard]], boiled ({{1/2}} cup) = 75&nbsp;mg
* [[Chocolate]], 70% cocoa (1&nbsp;oz) = 73&nbsp;mg
* [[Tofu]], firm ({{1/2}} cup) = 73&nbsp;mg
* [[Black turtle bean|Black beans]], boiled ({{1/2}} cup) = 60&nbsp;mg
* [[Quinoa]], cooked ({{1/2}} cup) = 59&nbsp;mg
* [[Peanut butter]] (2 tablespoons) = 50&nbsp;mg
* [[Walnuts]] ({{1/4}} cup) = 46&nbsp;mg
* [[Sunflower seed]]s, hulled ({{1/4}} cup) = 41&nbsp;mg
* [[Chickpeas]], boiled ({{1/2}} cup) = 39&nbsp;mg
* [[Kale]], boiled ({{1/2}} cup) = 37&nbsp;mg
* [[Lentils]], boiled ({{1/2}} cup) = 36&nbsp;mg
* [[Oatmeal]], cooked ({{1/2}} cup) = 32&nbsp;mg
* [[Fish sauce]] (1 Tbsp) = 32&nbsp;mg
* [[Milk]], non fat (1 cup) = 27&nbsp;mg
* [[Coffee]], [[espresso]] (1&nbsp;oz) = 24&nbsp;mg
* [[Whole wheat bread]] (1 slice) = 23&nbsp;mg
{{colend}}

==Biological range, distribution, and regulation==
In [[animal]]s, it has been shown that different cell types maintain different concentrations of magnesium.<ref name="Valberg 1965">{{cite journal| vauthors = Valberg LS, Holt JM, Paulson E, Szivek J | year=1965| title=Spectrochemical Analysis of Sodium, Potassium, Calcium, Magnesium, Copper, and Zinc in Normal Human Erythrocytes| journal=Journal of Clinical Investigation| volume=44| pages= 379–389| doi=10.1172/JCI105151| pmid=14271298| pmc=292488| issue=3 }}</ref><ref name="Seiler 1966">{{cite journal | vauthors=Seiler RH, Ramirez O, Brest AN, Moyer JH | year=1966| title=Serum and erythrocytic magnesium levels in congestive heart failure: effect of hydrochlorothiazide| journal=American Journal of Cardiology| volume= 17| pages= 786–791| doi=10.1016/0002-9149(66)90372-9| issue=6 }}</ref><ref name="Walser 1967">{{cite journal| last=Walser| first=M.| year=1967| title=Magnesium metabolism| journal=Ergebnisse der Physiologie Biologischen Chemie und Experimentellen Pharmakologie| volume=59| pages=185–296| doi=10.1007/BF02269144| pmid=4865748| s2cid=43703938}}</ref><ref name="Iyengar 1978">{{cite book| last=Iyengar| first=G.V.|author2=Kollmer WE |author3=Bowen HJ | year=1978| title=The Elemental Composition of Human Tissues and Body Fluids| location=Weinheim, New York| publisher=Verlag Chemie| isbn=978-0-89573-003-9}}</ref> It seems likely that the same is true for [[plants]].<ref name="Stelzer 1990">{{cite journal| last=Stelzer| first=R.|author2=Lehmann H |author3=Krammer D |author4=Luttge U | year=1990| title=X-Ray microprobe analysis of vacuoles of spruce needle mesophyll, endodermis and transfusion parenchyma cells at different seasons of the year| journal=Botanica Acta| volume=103| issue=4| pages=415–423| doi=10.1111/j.1438-8677.1990.tb00183.x}}</ref><ref name="Shaul 1999">{{cite journal | vauthors = Shaul O, Hilgemann DW, de-Almeida-Engler J, Van Montagu M, Inz D, Galili G | date = July 1999 | title=Cloning and characterization of a novel Mg(2+)/H(+) exchanger| journal=EMBO Journal| volume=18| pages=3973–3980| doi=10.1093/emboj/18.14.3973| pmid=10406802| issue=14| pmc=1171473 }}</ref> This suggests that different cell types may regulate influx and efflux of magnesium in different ways based on their unique metabolic needs. Interstitial and systemic concentrations of free magnesium must be delicately maintained by the combined processes of buffering (binding of ions to proteins and other molecules) and muffling (the transport of ions to storage or extracellular spaces<ref name="Thomas 1991">{{cite journal| last=Thomas| first= R.C.|author2=Coles JA |author3=Deitmer JW | year=1991| title= Homeostatic muffling| journal= Nature| volume= 350| pages= 564| doi=10.1038/350564b0| pmid=2017256| issue=6319| bibcode= 1991Natur.350R.564T| s2cid= 4346618| doi-access=free}}</ref>).

In plants, and more recently in animals, magnesium has been recognized as an important signaling ion, both activating and mediating many biochemical reactions. The best example of this is perhaps the regulation of [[carbon]] fixation in [[chloroplast]]s in the [[Calvin cycle]].<ref name="Berkowitz 1993">{{cite journal| last=Berkowitz| first= G. A.|author2=Wu W | year= 1993| title= Magnesium, potassium flux and photosynthesis| journal=Magnesium Research| volume= 6| pages= 257–265| pmid=8292500| issue=3}}</ref><ref name="Shaul 2002">{{cite journal| last=Shaul| first= O. | year=2002| title= Magnesium transport and function in plants: the tip of the iceberg| journal=BioMetals| volume= 15| pages= 309–323| doi=10.1023/A:1016091118585| pmid=12206396| issue=3| s2cid= 32535554 }}</ref>

Magnesium is very important in cellular function. Deficiency of the [[nutrient]] causes disease of the affected organism. In single-cell organisms such as [[bacteria]] and [[yeast]], low levels of magnesium manifests in greatly reduced growth rates. In magnesium transport [[knockout]] strains of bacteria, healthy rates are maintained only with exposure to very high external concentrations of the ion.<ref name="Hmiel 1989">{{cite journal| last=Hmiel| first= S. P.|author2=Snavely MD |author3=Florer JB |author4=Maguire ME |author5=Miller CG | year=1989| title= Magnesium transport in Salmonella typhimurium: genetic characterization and cloning of three magnesium transport loci| journal= Journal of Bacteriology| volume= 171| pages= 4742–4751| pmid=2548998| issue=9| pmc=210275| doi= 10.1128/jb.171.9.4742-4751.1989}}</ref><ref name="MacDiarmid 1998">{{cite journal| last=MacDiarmid| first= C.W.|author2=Gardner RC | year=1998| title= Overexpression of the Saccharomyces cerevisiae magnesium transport system confers resistance to aluminum ion| journal= J. Biol. Chem.| volume= 273| pages= 1727–1732| doi=10.1074/jbc.273.3.1727| pmid=9430719| issue=3| doi-access=free}}</ref> In yeast, [[mitochondria]]l magnesium deficiency also leads to disease.<ref name="Wiesenberger 1992">{{cite journal| last=Wiesenberger| first= G.|author2=Waldherr M |author3=Schweyen RJ | year=1992| title= The nuclear gene MRS2 is essential for the excision of group II introns from yeast mitochondrial transcripts ''in vivo''| journal= J. Biol. Chem.| volume= 267| pages= 6963–6969| pmid=1551905| issue=10| doi= 10.1016/S0021-9258(19)50522-1| doi-access=free}}</ref>

Plants deficient in magnesium show stress responses. The first observable signs of both magnesium starvation and overexposure in plants is a decrease in the rate of [[photosynthesis]]. This is due to the central position of the Mg<sup>2+</sup> ion in the [[chlorophyll]] molecule. The later effects of magnesium deficiency on plants are a significant reduction in growth and reproductive viability.<ref name="Marschner 1995" /> Magnesium can also be toxic to plants, although this is typically seen only in [[drought]] conditions.<ref name="Kaiser 1987">{{cite journal| last=Kaiser| first= W. M. | year=1987| title= Effects of water deficit on photosynthetic capacity| journal= Physiologia Plantarum| volume= 71| issue= 1 | pages= 142–149| doi=10.1111/j.1399-3054.1987.tb04631.x| bibcode= 1987PPlan..71..142K }}</ref><ref name="Rao 1987">{{cite journal| last=Rao| first= I. M.|author2=Sharp RE |author3=Boyer JS | year=1987| title= Leaf Phosphate Status, Photosynthesis, and Carbon Partitioning in Sugar Beet: III. Diurnal Changes in Carbon Partitioning and Carbon Export| journal= Plant Physiology| volume= 92| pages= 29–36| pmid=16667261| issue=1| pmc=1062243| doi=10.1104/pp.92.1.29}}</ref>

[[Image:Chlorophyll-a-3D-vdW.png|thumb|Space-filling model of the chlorophyll a molecule, with the magnesium ion (bright-green) visible at the center of the [[chlorin]] group]]

In animals, magnesium deficiency ([[hypomagnesemia]]) is seen when the environmental availability of magnesium is low. In ruminant animals, particularly vulnerable to magnesium availability in pasture grasses, the condition is known as 'grass tetany'. Hypomagnesemia is identified by a loss of balance due to muscle weakness.<ref name="Grunes 1970">{{cite book| last=Grunes| first= D. L.|author2=Stout PR |author3=Brownwell JR | year= 1970| title= Grass tetany of ruminants| volume= 22| pages= 332–374| doi=10.1016/S0065-2113(08)60272-2| series=Advances in Agronomy| isbn=978-0-12-000722-6}}</ref> A number of genetically attributable hypomagnesemia disorders have also been identified in humans.<ref name="Paunier 1968">{{cite journal| last=Paunier| first= L.|author2=Radde IC |author3=Kooh SW |author4=Conen PE |author5=Fraser D | year=1968| title= Primary hypomagnesemia with secondary hypocalcemia in an infant| journal= Pediatrics| volume= 41| pages= 385–402| pmid=5637791| issue=2}}</ref><ref name="Weber 2000">{{cite journal |vauthors=Weber S, Hoffmann K, Jeck N, Saar K, Boeswald M, Kuwertz-Broeking E, Meij II, Knoers NV, Cochat P, Suláková T, Bonzel KE, Soergel M, Manz F, Schaerer K, Seyberth HW, Reis A, Konrad M | year=2000| title= Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis maps to chromosome 3q27 and is associated with mutations in the PCLN-1 gene| journal= European Journal of Human Genetics| volume= 8| pages= 414–422| doi=10.1038/sj.ejhg.5200475| pmid=10878661| issue=6| doi-access=free}}</ref><ref name="Weber 2001">{{cite journal |vauthors=Weber S, Schneider L, Peters M, Misselwitz J, Rönnefarth G, Böswald M, Bonzel KE, Seeman T, Suláková T, Kuwertz-Bröking E, Gregoric A, Palcoux JB, Tasic V, Manz F, Schärer K, Seyberth HW, Konrad M | year=2001| title= Novel paracellin-1 mutations in 25 families with familial hypomagnesemia with hypercalciuria and nephrocalcinosis| journal= Journal of the American Society of Nephrology| volume= 12| pages= 1872–1881| pmid=11518780| issue=9| doi=10.1681/ASN.V1291872| doi-access=free}}</ref><ref name="Chubanov 2004">{{cite journal |vauthors=Chubanov V, Waldegger S, Mederos y Schnitzler M, Vitzthum H, Sassen MC, Seyberth HW, Konrad M, Gudermann T | year= 2004| title= Disruption of TRPM6/TRPM7 complex formation by a mutation in the TRPM6 gene causes hypomagnesemia with secondary hypocalcemia| journal= Proceedings of the National Academy of Sciences of the United States of America| volume= 101| pages= 2894–2899| doi=10.1073/pnas.0305252101| pmid=14976260| issue=9| pmc=365716| bibcode= 2004PNAS..101.2894C| doi-access= free}}</ref>

Overexposure to magnesium may be toxic to individual cells, though these effects have been difficult to show experimentally.{{citation needed|date=November 2013}} [[Hypermagnesemia]], an overabundance of magnesium in the blood, is usually caused by loss of [[kidney]] function. Healthy animals rapidly excrete excess magnesium in the urine and stool.<ref>Harrison's Principles of Internal Medicine, Online Edition</ref> Urinary magnesium is called ''magnesuria''. Characteristic concentrations of magnesium in model organisms are: in ''E. coli'' 30-100mM (bound), 0.01-1mM (free), in budding yeast 50mM, in mammalian cell 10mM (bound), 0.5mM (free) and in blood plasma 1mM.<ref>{{cite web|url = http://book.bionumbers.org/what-are-the-concentrations-of-different-ions-in-cells/|title =Cell Biology by the Numbers: What are the concentrations of different ions in cells? |website =book.bionumbers.org|first1= Ron|last1= Milo|first2= Rob |last2=Philips|access-date=23 March 2017}}</ref>

==Biological chemistry==
Mg<sup>2+</sup> is the fourth-most-abundant [[metal]] [[ion]] in cells (per [[Mole (unit)|mole]]s) and the most abundant free divalent cation — as a result, it is deeply and intrinsically woven into cellular [[metabolism]]. Indeed, Mg<sup>2+</sup>-dependent [[enzyme]]s appear in virtually every metabolic pathway: Specific binding of Mg<sup>2+</sup> to biological membranes is frequently observed, Mg<sup>2+</sup> is also used as a signalling molecule, and much of nucleic acid biochemistry requires Mg<sup>2+</sup>, including all reactions that require release of energy from ATP.<ref name="Cowan 1995">{{cite book| last=Cowan| first= J. A. | year= 1995| title= Introduction to the biological chemistry of magnesium | editor=J.A. Cowan | location=New York| publisher=VCH}}</ref><ref name="Romani 2002">{{cite journal| last=Romani| first= A. M. P.|author2=Maguire ME | year=2002| title= Hormonal regulation of Mg<sup>2+</sup> transport and homeostasis in eukaryotic cells| journal=BioMetals| volume= 15| pages= 271–283| doi=10.1023/A:1016082900838| pmid=12206393| issue=3| s2cid= 20835803}}</ref><ref name="Shaul 2002"/> In nucleotides, the triple-phosphate moiety of the compound is invariably stabilized by association with Mg<sup>2+</sup> in all enzymatic processes.

===Chlorophyll===
In photosynthetic organisms, Mg<sup>2+</sup> has the additional vital role of being the [[Coordination complex|coordinating ion]] in the [[chlorophyll]] molecule. This role was discovered by [[Richard Willstätter]], who received the Nobel Prize in Chemistry 1915 for the purification and structure of chlorophyll binding with sixth number of carbon

===Enzymes===
The chemistry of the Mg<sup>2+</sup> ion, as applied to enzymes, uses the full range of this ion's unusual reaction chemistry to fulfill a range of functions.<ref name="Cowan 1995"/><ref name="Black 1995a">{{cite journal| last=Black| first= C. B.|author2=Cowan JA | year= 1995| title= Magnesium-dependent enzymes in nucleic acid biochemistry| journal=The Biological Chemistry of Magnesium| editor=J.A. Cowan | location=New York| publisher=VCH}}</ref><ref name="Black 1995b">{{cite journal| last=Black| first= C.B.|author2=Cowan JA | year=1995| title= Magnesium-dependent enzymes in general metabolism| journal=The Biological Chemistry of Magnesium| editor=J.A. Cowan | location=New York| publisher=VCH}}</ref><ref name="Cowan 2002">{{cite journal| last=Cowan| first= J. A. | year= 2002| title= Structural and catalytic chemistry of magnesium-dependent enzymes| journal=BioMetals| volume=15 | pages= 225–235| doi=10.1023/A:1016022730880| pmid=12206389| issue=3| s2cid= 40446313 }}</ref> Mg<sup>2+</sup> interacts with substrates, enzymes, and occasionally both (Mg<sup>2+</sup> may form part of the active site). In general, Mg<sup>2+</sup> interacts with substrates through inner sphere coordination, stabilising anions or reactive intermediates, also including binding to ATP and activating the molecule to nucleophilic attack. When interacting with enzymes and other proteins, Mg<sup>2+</sup> may bind using inner or outer sphere coordination, to either alter the conformation of the enzyme or take part in the chemistry of the catalytic reaction. In either case, because Mg<sup>2+</sup> is only rarely fully dehydrated during ligand binding, it may be a water molecule associated with the Mg<sup>2+</sup> that is important rather than the ion itself. The [[Lewis acids and bases|Lewis acidity]] of Mg<sup>2+</sup> ([[Acid dissociation constant|p''K''<sub>a</sub>]] 11.4) is used to allow both hydrolysis and condensation reactions (most common ones being phosphate ester hydrolysis and phosphoryl transfer) that would otherwise require pH values greatly removed from physiological values.

=== Essential role in the biological activity of ATP ===
[[Adenosine triphosphate|ATP]] (adenosine triphosphate), the main source of energy in cells, must be bound to a magnesium ion in order to be biologically active. What is called ATP is often actually Mg-ATP.<ref name="cancerweb.ncl.ac.uk"/>

===Nucleic acids===
[[Nucleic acid]]s have an important range of interactions with Mg<sup>2+</sup>. The binding of Mg<sup>2+</sup> to [[DNA]] and [[RNA]] stabilises structure; this can be observed in the increased melting temperature (''T''<sub>m</sub>) of double-stranded DNA in the presence of Mg<sup>2+</sup>.<ref name="Cowan 1995"/> In addition, [[ribosome]]s contain large amounts of Mg<sup>2+</sup> and the stabilisation provided is essential to the complexation of this ribo-protein.<ref name="Sperazza 1983">{{cite journal| last=Sperazza| first= J. M.|author2=Spremulli LL | year=1983| title= Quantitation of cation binding to wheat germ ribosomes: influences on subunit association equilibria and ribosome activity| journal= Nucleic Acids Research| volume= 11| pages= 2665–2679| doi=10.1093/nar/11.9.2665| pmid=6856472| issue=9| pmc=325916}}</ref> A large number of enzymes involved in the biochemistry of nucleic acids bind Mg<sup>2+</sup> for activity, using the ion for both activation and catalysis. Finally, the [[autocatalysis]] of many [[ribozymes]] (enzymes containing only RNA) is Mg<sup>2+</sup> dependent (e.g. the yeast mitochondrial group II self splicing introns<ref name="Smith 1995">{{cite journal| last=Smith| first= R. L.|author2=Thompson LJ |author3=Maguire ME | year=1995| title= Cloning and characterization of MgtE, a putative new class of Mg2+ transporter from Bacillus firmus OF4| journal= Journal of Bacteriology| volume= 177| pages= 1233–1238| pmid=7868596| issue=5| pmc=176728| doi= 10.1128/jb.177.5.1233-1238.1995}}</ref>).

Magnesium ions can be critical in maintaining the positional integrity of closely clustered phosphate groups. These clusters appear in numerous and distinct parts of the [[cell nucleus]] and [[cytoplasm]]. For instance, hexahydrated Mg<sup>2+</sup> ions bind in the deep [[DNA#Grooves|major groove]] and at the outer mouth of A-form nucleic acid [[Nucleic acid double helix|duplexes]].<ref name=ref1>{{cite journal|title=Hexahydrated magnesium ions bind in the deep major groove and at the outer mouth of A-form nucleic acid duplexes|first1=Howard|last1=Robinson|first2=Yi-Gui|last2=Gao|first3=Ruslan|last3=Sanishvili|first4=Andrzej|last4=Joachimiak|first5=Andrew H.-J.|last5=Wang|date=15 April 2000|journal=Nucleic Acids Research|volume=28|issue=8|pages=1760–1766|doi=10.1093/nar/28.8.1760|pmid=10734195|pmc=102818}}</ref>

===Cell membranes and walls===
Biological [[cell membrane]]s and [[cell wall]]s are polyanionic surfaces. This has important implications for the transport of ions, in particular because it has been shown that different membranes preferentially bind different ions.<ref name="Cowan 1995"/> Both Mg<sup>2+</sup> and Ca<sup>2+</sup> regularly stabilize membranes by the cross-linking of [[Carboxylation|carboxylated]] and [[Phosphorylation|phosphorylated]] head groups of lipids. However, the envelope membrane of ''[[E. coli]]'' has also been shown to bind Na<sup>+</sup>, K<sup>+</sup>, Mn<sup>2+</sup> and Fe<sup>3+</sup>. The transport of ions is dependent on both the concentration gradient of the ion and the electric potential (ΔΨ) across the membrane, which will be affected by the charge on the membrane surface. For example, the specific binding of Mg<sup>2+</sup> to the [[chloroplast]] envelope has been implicated in a loss of photosynthetic efficiency by the blockage of K<sup>+</sup> uptake and the subsequent acidification of the chloroplast stroma.<ref name="Berkowitz 1993"/>

===Proteins===
The Mg<sup>2+</sup> ion tends to bind only weakly to [[protein]]s ([[Acid dissociation constant|''K''<sub>a</sub>]] ≤ 10<sup>5</sup><ref name="Cowan 1995"/>) and this can be exploited by the cell to switch [[enzymatic]] activity on and off by changes in the local concentration of Mg<sup>2+</sup>. Although the concentration of free cytoplasmic Mg<sup>2+</sup> is on the order of 1&nbsp;mmol/L, the total Mg<sup>2+</sup> content of animal cells is 30&nbsp;mmol/L<ref name="Ebel 1980">{{cite journal| last=Ebel| first= H.|author2=Gunther T | year= 1980| title= Magnesium metabolism: a review| journal= Journal of Clinical Chemistry and Clinical Biochemistry| volume= 18| issue= 5| pages= 257–270| doi=10.1515/cclm.1980.18.5.257| pmid= 7000968| s2cid= 37427719| url= http://edoc.hu-berlin.de/18452/13563}}</ref> and in plants the content of leaf endodermal cells has been measured at values as high as 100&nbsp;mmol/L (Stelzer ''et al.'', 1990), much of which buffered in storage compartments. The cytoplasmic concentration of free Mg<sup>2+</sup> is buffered by binding to [[Chelation|chelators]] (e.g., ATP), but also, what is more important, it is buffered by storage of Mg<sup>2+</sup> in intracellular compartments.{{cn|date=November 2024}} The transport of Mg<sup>2+</sup> between intracellular compartments may be a major part of regulating enzyme activity. The interaction of Mg<sup>2+</sup> with proteins must also be considered for the transport of the ion across biological membranes.{{cn|date=November 2024}}

===Manganese===
In biological systems, only [[manganese]] (Mn<sup>2+</sup>) is readily capable of replacing Mg<sup>2+</sup>, but only in a limited set of circumstances. Mn<sup>2+</sup> is very similar to Mg<sup>2+</sup> in terms of its chemical properties, including inner and outer shell complexation. Mn<sup>2+</sup> effectively binds ATP and allows hydrolysis of the energy molecule by most ATPases. Mn<sup>2+</sup> can also replace Mg<sup>2+</sup> as the activating ion for a number of Mg<sup>2+</sup>-dependent enzymes, although some enzyme activity is usually lost.<ref name="Cowan 1995"/> Sometimes such enzyme metal preferences vary among closely related species: For example, the [[reverse transcriptase]] enzyme of [[lentivirus]]es like [[HIV]], [[Simian immunodeficiency virus|SIV]] and [[FIV]] is typically dependent on Mg<sup>2+</sup>, whereas the analogous enzyme for other [[retrovirus]]es prefers Mn<sup>2+</sup>.

==Measuring magnesium in biological samples==
===By radioactive isotopes===
The use of radioactive tracer elements in ion uptake assays allows the calculation of km, Ki and Vmax and determines the initial change in the ion content of the cells. <sup>28</sup>Mg decays by the emission of a high-energy beta or gamma particle, which can be measured using a scintillation counter. However, the radioactive half-life of <sup>28</sup>Mg, the most stable of the radioactive magnesium isotopes, is only 21 hours. This severely restricts the experiments involving the nuclide. Also, since 1990, no facility has routinely produced <sup>28</sup>Mg, and the price per mCi is now predicted to be approximately US$30,000.<ref name="Maguire 2002">{{cite journal| last=Maguire| first= M.E.|author2=Cowan JA | year=2002| title= Magnesium chemistry and biochemistry| journal=BioMetals| volume= 15| pages= 203–210| doi=10.1023/A:1016058229972| pmid=12206387| issue=3| s2cid= 31622669}}</ref> The chemical nature of Mg<sup>2+</sup> is such that it is closely approximated by few other cations.<ref name="Tevelev 1995">{{cite book| last=Tevelev| first= A.|author2=Cowan JA | year=1995| title= Metal substitution as a probe of the biological chemistry of magnesium ion | editor=J.A. Cowan | location=New York| publisher=VCH}}</ref> However, Co<sup>2+</sup>, Mn<sup>2+</sup> and Ni<sup>2+</sup> have been used successfully to mimic the properties of Mg<sup>2+</sup> in some enzyme reactions, and radioactive forms of these elements have been employed successfully in cation transport studies. The difficulty of using metal ion replacement in the study of enzyme function is that the relationship between the enzyme activities with the replacement ion compared to the original is very difficult to ascertain.<ref name="Tevelev 1995"/>

===By fluorescent indicators===
A number of chelators of divalent cations have different fluorescence spectra in the bound and unbound states.<ref name="Drakenberg 1995">{{cite book| last=Drakenberg| first= T. | year= 1995| title= Physical methods for studying the biological chemistry of magnesium| editor=J. A. Cowan| location=New York| publisher=VCH}}</ref> Chelators for Ca<sup>2+</sup> are well established, have high affinity for the cation, and low interference from other ions. Mg<sup>2+</sup> chelators lag behind and the major fluorescence dye for Mg<sup>2+</sup> (mag-fura 2<ref name="Raju 1989">{{cite journal| last=Raju| first= B.|author2=Murphy E |author3=Levy LA |author4=Hall RD |author5=London RE | year=1989| title= A fluorescent indicator for measuring cytosolic free magnesium| journal= Am J Physiol Cell Physiol| volume= 256| issue= 3 Pt 1| pages= C540–548| doi= 10.1152/ajpcell.1989.256.3.C540| pmid= 2923192}}</ref>) actually has a higher affinity for Ca<sup>2+</sup>.<ref name="Grubbs 2002">{{cite journal| last=Grubbs| first= R. D. | year= 2002| title= Intracellular magnesium and magnesium buffering| journal=BioMetals| volume= 15| pages= 251–259| doi=10.1023/A:1016026831789| pmid=12206391| issue=3| s2cid= 20873166 }}</ref> This limits the application of this dye to cell types where the resting level of Ca<sup>2+</sup> is < 1 μM and does not vary with the experimental conditions under which Mg<sup>2+</sup> is to be measured. Recently, Otten ''et al.'' (2001) have described work into a new class of compounds that may prove more useful, having significantly better binding affinities for Mg<sup>2+</sup>.<ref name="Otten 2001">{{cite journal| last=Otten| first= P.A.|author2=London RE |author3=Levy LA | year=2001| title= 4-Oxo-4H-quinolizine-3-carboxylic acids as Mg<sup>2+</sup> selective, fluorescent indicators| journal= Bioconjugate Chemistry| volume= 12| pages= 203–212| doi=10.1021/bc000087d| pmid=11312681| issue=2}}</ref> The use of the fluorescent dyes is limited to measuring the free Mg<sup>2+</sup>. If the ion concentration is buffered by the cell by chelation or removal to subcellular compartments, the measured rate of uptake will give only minimum values of km and Vmax.{{cn|date=November 2024}}

===By electrophysiology===
First, ion-specific microelectrodes can be used to measure the internal free ion concentration of cells and organelles. The major advantages are that readings can be made from cells over relatively long periods of time, and that unlike dyes very little extra ion buffering capacity is added to the cells.<ref name="Gunzel 2002">{{cite journal| last=Gunzel| first= D.|author2=Schlue, W.-R. | year= 2002| title= Determination of [Mg<sup>2+</sup>]i - an update on the use of Mg<sup>2+</sup>-selective electrodes| journal=BioMetals| volume= 15| pages= 237–249| doi=10.1023/A:1016074714951| pmid=12206390| issue=3| s2cid= 27877817}}</ref>

Second, the technique of two-electrode voltage-clamp allows the direct measurement of the ion flux across the membrane of a cell.<ref name="Hille 1992 ch2">{{cite book| last=Hille| first= B. | year=1992| title= Ionic channels of excitable membranes| chapter-url=https://archive.org/details/ionicchannelsofe00hill| chapter-url-access=registration| location=Sunderland| publisher= Sinauer Associates Inc.| chapter=2| isbn=978-0-87893-322-8}}</ref> The membrane is held at an electric potential and the responding current is measured. All ions passing across the membrane contribute to the measured current.{{cn|date=November 2024}}

Third, the technique of patch-clamp uses isolated sections of natural or artificial membrane in much the same manner as voltage-clamp but without the secondary effects of a cellular system. Under ideal conditions the conductance of individual channels can be quantified. This methodology gives the most direct measurement of the action of ion channels.<ref name="Hille 1992 ch2"/>

===By absorption spectroscopy===
Flame [[atomic absorption spectroscopy]] (AAS) determines the total magnesium content of a biological sample.<ref name="Drakenberg 1995"/> This method is destructive; biological samples must be broken down in concentrated acids to avoid clogging the fine nebulising apparatus.{{cn|date=November 2024}} Beyond this, the only limitation is that samples must be in a volume of approximately 2 mL and at a concentration range of 0.1 – 0.4 μmol/L for optimum accuracy.{{cn|date=November 2024}} As this technique cannot distinguish between Mg<sup>2+</sup> already present in the cell and that taken up during the experiment, only content not uptaken can be quantified.{{cn|date=November 2024}}

[[Inductively coupled plasma]] (ICP) using either the [[mass spectrometry]] (MS) or [[atomic emission spectroscopy]] (AES) modifications also allows the determination of the total ion content of biological samples.<ref name="Dean 1997">See Chapters 5 and 6 in {{cite book| last=Dean| first= J.R. | year= 1997| title= Atomic Absorption and Plasma Spectroscopy| location=Chichester| publisher= John Wiley & Sons| isbn=978-0-471-97255-6}} for descriptions of the methodology as applied to analytical chemistry.</ref>

==Magnesium transport==
{{main|Magnesium transport}}

The chemical and biochemical properties of Mg<sup>2+</sup> present the cellular system with a significant challenge when transporting the ion across biological membranes. The dogma of ion transport states that the transporter recognises the ion then progressively removes the water of hydration, removing most or all of the water at a selective pore before releasing the ion on the far side of the membrane.<ref name="Hille 1992 ch11">Hille, 1992. Chapter 11</ref> Due to the properties of Mg<sup>2+</sup>, large volume change from hydrated to bare ion, high energy of hydration and very low rate of ligand exchange in the inner [[coordination sphere]], these steps are probably more difficult than for most other ions. To date, only the ZntA protein of [[Paramecium]] has been shown to be a Mg<sup>2+</sup> channel.<ref name="Haynes 2002">{{cite journal| last=Haynes| first= W. J.|author2=Kung C |author3=Saimi Y |author4=Preston RR | year=2002| title= An exchanger-like protein underlies the large Mg2+ current in Paramecium| journal= PNAS| volume= 99| pages= 15717–15722| doi=10.1073/pnas.242603999| pmid=12422021| issue=24| pmc=137782| bibcode= 2002PNAS...9915717H| doi-access= free}}</ref> The mechanisms of Mg<sup>2+</sup> transport by the remaining proteins are beginning to be uncovered with the first three-dimensional structure of a Mg<sup>2+</sup> transport complex being solved in 2004.<ref name="Warren 2004">{{cite journal| last=Warren| first= M. A. |author2=Kucharski LM |author3=Veenstra A |author4=Shi L |author5=Grulich PF |author6=Maguire ME |year=2004| title= The CorA Mg2+ Transporter Is a Homotetramer| journal=Journal of Bacteriology | volume=186| pages=4605–4612| doi=10.1128/JB.186.14.4605-4612.2004| pmid=15231793| issue=14| pmc=438605}}</ref>

The [[hydration shell]] of the Mg<sup>2+</sup> ion has a very tightly bound inner shell of six water molecules and a relatively tightly bound second shell containing 12–14 water molecules (Markham ''et al.'', 2002). Thus, it is presumed that recognition of the Mg<sup>2+</sup> ion requires some mechanism to interact initially with the hydration shell of Mg<sup>2+</sup>, followed by a direct recognition/binding of the ion to the protein.<ref name="Maguire 2002"/>

In spite of the mechanistic difficulty, Mg<sup>2+</sup> must be transported across membranes, and a large number of Mg<sup>2+</sup> fluxes across membranes from a variety of systems have been described.<ref name="Gardner 2003">{{cite journal| last=Gardner| first= R. C. | year= 2003| title= Genes for magnesium transport| journal= Current Opinion in Plant Biology| volume= 6| pages= 263–267| doi=10.1016/S1369-5266(03)00032-3| pmid=12753976| issue=3| bibcode= 2003COPB....6..263G }}</ref> However, only a small selection of Mg<sup>2+</sup> transporters have been characterised at the molecular level.

=== Ligand ion channel blockade ===
{{unreferenced section|date=November 2024}}

[[Magnesium]] [[ion]]s (Mg<sup>2+</sup>) in [[cellular biology]] are usually in almost all senses opposite to [[calcium in biology|Ca<sup>2+</sup>]] ions, because they are [[Bivalent (chemistry)|bivalent]] too, but have greater [[electronegativity]] and thus exert greater pull on water molecules, preventing passage through the channel (even though the magnesium itself is smaller). Thus, Mg<sup>2+</sup> ions block Ca<sup>2+</sup> channels such as ([[NMDA channel]]s) and have been shown to affect gap junction channels forming [[electrical synapse]]s.

==Plant physiology of magnesium==
{{unreferenced section|date=November 2024}}

The previous sections have dealt in detail with the chemical and biochemical aspects of Mg<sup>2+</sup> and its transport across cellular membranes. This section will apply this knowledge to aspects of whole plant physiology, in an attempt to show how these processes interact with the larger and more complex environment of the multicellular organism.

===Nutritional requirements and interactions===
Mg<sup>2+</sup> is essential for plant growth and is present in higher plants in amounts on the order of 80 μmol g<sup>−1</sup> dry weight.<ref name="Marschner 1995"/> The amounts of Mg<sup>2+</sup> vary in different parts of the plant and are dependent upon nutritional status. In times of plenty, excess Mg<sup>2+</sup> may be stored in vascular cells (Stelzer ''et al.'', 1990;<ref name="Shaul 1999"/> and in times of starvation Mg<sup>2+</sup> is redistributed, in many plants, from older to newer leaves.<ref name="Marschner 1995"/><ref name="Laing 2000">{{cite journal| last=Laing| first= W.|author2=Greer D |author3=Sun O |author4=Beets P |author5=Lowe A |author6=Payn T | year=2000| title= Physiological impacts of Mg deficiency in Pinus radiata: growth and photosynthesis| journal= New Phytol| volume= 146| issue= 1| pages= 47–57| doi=10.1046/j.1469-8137.2000.00616.x| doi-access=free| bibcode= 2000NewPh.146...47L}}</ref>

Mg<sup>2+</sup> is taken up into plants via the roots. Interactions with other cations in the [[Rhizosphere (ecology)|rhizosphere]] can have a significant effect on the uptake of the ion.(Kurvits and Kirkby, 1980;<ref name="Heenan 1981">{{cite journal| last=Heenan| first= D.P.|author2=Campbell LC | year=1981| title= Influence of potassium and manganese on growth and uptake of magnesium by soybeans (Glycine max (L.) Merr. cv Bragg| journal=Plant Soil| volume= 61| pages= 447–456| doi=10.1007/BF02182025| issue=3| bibcode= 1981PlSoi..61..447H| s2cid= 12271923}}</ref> The structure of root cell walls is highly permeable to water and ions, and hence ion uptake into root cells can occur anywhere from the root hairs to cells located almost in the centre of the root (limited only by the [[Casparian strip]]). Plant cell walls and membranes carry a great number of negative charges, and the interactions of cations with these charges is key to the uptake of cations by root cells allowing a local concentrating effect.<ref name="Hope 1952">{{cite journal| last=Hope| first= A. B.|author2=Stevens PG | year=1952| title= Electrical potential differences in bean roots on their relation to salt uptake| journal= Australian Journal of Scientific Research, Series B| volume= 5| pages= 335–343}}</ref> Mg<sup>2+</sup> binds relatively weakly to these charges, and can be displaced by other cations, impeding uptake and causing deficiency in the plant.

Within individual plant cells, the Mg<sup>2+</sup> requirements are largely the same as for all cellular life; Mg<sup>2+</sup> is used to stabilise membranes, is vital to the utilisation of ATP, is extensively involved in the nucleic acid biochemistry, and is a cofactor for many enzymes (including the ribosome). Also, Mg<sup>2+</sup> is the coordinating ion in the chlorophyll molecule. It is the intracellular compartmentalisation of Mg<sup>2+</sup> in plant cells that leads to additional complexity. Four compartments within the plant cell have reported interactions with Mg<sup>2+</sup>. Initially, Mg<sup>2+</sup> will enter the cell into the cytoplasm (by an as yet unidentified system), but free Mg<sup>2+</sup> concentrations in this compartment are tightly regulated at relatively low levels (≈2&nbsp;mmol/L) and so any excess Mg<sup>2+</sup> is either quickly exported or stored in the second intracellular compartment, the vacuole.<ref>Section 8.5.2 in Marschner, 1995</ref> The requirement for Mg<sup>2+</sup> in mitochondria has been demonstrated in yeast<ref name="Bui 1999">{{cite journal| last=Bui| first= D.M.|author2=Gregan J |author3=Jarosch E |author4=Ragnini A |author5=Schweyen RJ | year=1999| title= The bacterial magnesium transporter CorA can functionally substitute for its putative homologue Mrs2p in the yeast inner mitochondrial membrane| journal= Journal of Biological Chemistry| volume= 274| pages= 20438–20443| doi=10.1074/jbc.274.29.20438| pmid=10400670| issue=29| doi-access=free}}</ref> and it seems highly likely that the same will apply in plants. The chloroplasts also require significant amounts of internal Mg<sup>2+</sup>, and low concentrations of cytoplasmic Mg<sup>2+</sup>.<ref name="Demmig 1979">{{cite journal| last=Demmig| first= B.|author2=Gimmler H | year= 1979| title= Effect of divalent cations on cation fluxes across the chloroplast envelope and on photosynthesis of intact chloroplasts| journal=Zeitschrift für Naturforschung| volume= 24C| issue= 3–4| pages= 233–241| doi= 10.1515/znc-1979-3-413| s2cid= 42750442| doi-access= free}}</ref><ref name="Huber 1980">{{cite journal| last=Huber| first= S.C.|author2=Maury WJ | year=1980| title= Effects of Magnesium on Intact Chloroplasts: I. EVIDENCE FOR ACTIVATION OF (SODIUM) POTASSIUM/PROTON EXCHANGE ACROSS THE CHLOROPLAST ENVELOPE| journal= Plant Physiology| volume= 65| pages= 350–354| pmid=16661188| issue=2| pmc=440325| doi=10.1104/pp.65.2.350}}</ref> In addition, it seems likely that the other subcellular organelles (e.g., Golgi, endoplasmic reticulum, etc.) also require Mg<sup>2+</sup>.

===Distributing magnesium ions within the plant===
Once in the cytoplasmic space of root cells Mg<sup>2+</sup>, along with the other cations, is probably transported radially into the stele and the vascular tissue.<ref>Section 2.7 in Marschner, 1995</ref> From the cells surrounding the xylem the ions are released or pumped into the xylem and carried up through the plant. In the case of Mg<sup>2+</sup>, which is highly mobile in both the xylem and phloem,<ref>Section 3.3 in Marschner, 1995</ref> the ions will be transported to the top of the plant and back down again in a continuous cycle of replenishment. Hence, uptake and release from vascular cells is probably a key part of whole plant Mg<sup>2+</sup> homeostasis. Figure 1 shows how few processes have been connected to their molecular mechanisms (only vacuolar uptake has been associated with a transport protein, AtMHX).

The diagram shows a schematic of a plant and the putative processes of Mg<sup>2+</sup> transport at the root and leaf where Mg<sup>2+</sup> is loaded and unloaded from the vascular tissues.<ref name="Marschner 1995"/> Mg<sup>2+</sup> is taken up into the root cell wall space (1) and interacts with the negative charges associated with the cell walls and membranes. Mg<sup>2+</sup> may be taken up into cells immediately (symplastic pathway) or may travel as far as the Casparian band (4) before being absorbed into cells (apoplastic pathway; 2). The concentration of Mg<sup>2+</sup> in the root cells is probably buffered by storage in root cell vacuoles (3). Note that cells in the root tip do not contain vacuoles. Once in the root cell cytoplasm, Mg<sup>2+</sup> travels toward the centre of the root by [[plasmodesmata]], where it is loaded into the xylem (5) for transport to the upper parts of the plant. When the Mg<sup>2+</sup> reaches the leaves it is unloaded from the xylem into cells (6) and again is buffered in vacuoles (7). Whether cycling of Mg<sup>2+</sup> into the phloem occurs via general cells in the leaf (8) or directly from xylem to phloem via [[transfer cells]] (9) is unknown. Mg<sup>2+</sup> may return to the roots in the phloem sap.

[[Image:whole plant mg transport.png|frame|center|Figure 1: Magnesium in the whole plant]]
When a Mg<sup>2+</sup> ion has been absorbed by a cell requiring it for metabolic processes, it is generally assumed that the ion stays in that cell for as long as the cell is active.<ref name="Marschner 1995"/> In vascular cells, this is not always the case; in times of plenty, Mg<sup>2+</sup> is stored in the vacuole, takes no part in the day-to-day metabolic processes of the cell (Stelzer ''et al.'', 1990), and is released at need. But for most cells it is death by senescence or injury that releases Mg<sup>2+</sup> and many of the other ionic constituents, recycling them into healthy parts of the plant. In addition, when Mg<sup>2+</sup> in the environment is limiting, some species are able to mobilise Mg<sup>2+</sup> from older tissues.<ref name="Laing 2000"/> These processes involve the release of Mg<sup>2+</sup> from its bound and stored states and its transport back into the vascular tissue, where it can be distributed to the rest of the plant. In times of growth and development, Mg<sup>2+</sup> is also remobilised within the plant as source and sink relationships change.<ref name="Marschner 1995"/>

The homeostasis of Mg<sup>2+</sup> within single plant cells is maintained by processes occurring at the plasma membrane and at the vacuole membrane (see Figure 2). The major driving force for the translocation of ions in plant cells is ΔpH.<ref>Section 2.4 in Marschner, 1995</ref> [[Proton ATPase|H<sup>+</sup>-ATPases]] pump H<sup>+</sup> ions against their concentration gradient to maintain the pH differential that can be used for the transport of other ions and molecules. H<sup>+</sup> ions are pumped out of the cytoplasm into the extracellular space or into the vacuole. The entry of Mg<sup>2+</sup> into cells may occur through one of two pathways, via channels using the ΔΨ (negative inside) across this membrane or by [[Symporter|symport]] with H<sup>+</sup> ions. To transport the Mg<sup>2+</sup> ion into the vacuole requires a Mg<sup>2+</sup>/H<sup>+</sup> [[Antiporter|antiport]] transporter (such as AtMHX). The H<sup>+</sup>-ATPases are dependent on Mg<sup>2+</sup> (bound to ATP) for activity, so that Mg<sup>2+</sup> is required to maintain its own homeostasis.

A schematic of a plant cell is shown including the four major compartments currently recognised as interacting with Mg<sup>2+</sup>. [[Proton ATPase|H<sup>+</sup>-ATPases]] maintain a constant ΔpH across the plasma membrane and the vacuole membrane. Mg<sup>2+</sup> is transported into the vacuole using the energy of ΔpH (in ''A. thaliana'' by AtMHX). Transport of Mg<sup>2+</sup> into cells may use either the negative ΔΨ or the ΔpH. The transport of Mg<sup>2+</sup> into mitochondria probably uses ΔΨ as in the mitochondria of yeast, and it is likely that [[chloroplast]]s take Mg<sup>2+</sup> by a similar system. The mechanism and the molecular basis for the release of Mg<sup>2+</sup> from vacuoles and from the cell is not known. Likewise, the light-regulated Mg<sup>2+</sup> concentration changes in chloroplasts are not fully understood, but do require the transport of H<sup>+</sup> ions across the [[thylakoid]] membrane.

[[File:Magnesium in plant cell.svg|thumb|500px|center|Figure 2: Magnesium in the plant cell]]

===Magnesium, chloroplasts and photosynthesis===
Mg<sup>2+</sup> is the coordinating metal ion in the chlorophyll molecule, and in plants where the ion is in high supply about 6% of the total Mg<sup>2+</sup> is bound to chlorophyll.<ref name="Marschner 1995"/><ref name="Scott 1990a">{{cite journal| last=Scott| first= B. J.|author2=Robson AD | year=1990| title= Distribution of magnesium in subterranean clover (''Trifolium subterranean'' L.) in relation to supply| journal= Australian Journal of Agricultural Research| volume= 41| pages= 499–510| doi=10.1071/AR9900499| issue=3}}</ref><ref name="Scott 1990b">{{cite journal| last=Scott| first= B. J.|author2=Robson AD | year=1990b| title= Changes in the content and form of magnesium in the first trifoliate leaf of subterranean clover under altered or constant root supply| journal= Australian Journal of Agricultural Research| volume= 41| pages= 511–519| doi=10.1071/AR9900511| issue=3}}</ref> Thylakoid stacking is stabilised by Mg<sup>2+</sup> and is important for the efficiency of photosynthesis, allowing phase transitions to occur.<ref name="Fork 1986">{{cite journal| last=Fork| first= D.C. | year= 1986| title= The control by state transitions of the distribution of excitation energy in photosynthesis| journal= Annual Review of Plant Physiology and Plant Molecular Biology| volume= 37| pages= 335–361| doi=10.1146/annurev.arplant.37.1.335}}</ref>

Mg<sup>2+</sup> is probably taken up into chloroplasts to the greatest extent during the light-induced development from proplastid to chloroplast or etioplast to chloroplast. At these times, the synthesis of chlorophyll and the biogenesis of the thylakoid membrane stacks absolutely require the divalent cation.<ref name="Gregory 1989">{{cite book| last=Gregory| first= R. P. F. | year= 1989| title= Structure and function of the photosynthesising cell| location=New York| publisher= John Wiley and Sons}}</ref><ref name="Lu 1995">{{cite journal |vauthors=Lu YK, Chen YR, Yang CM, Ifuku K | year=1995| title= Influence of Fe- and Mg-deficiency on the thylakoid membranes of a chlorophyll-deficient ch5 mutant of Arabidopsis thaliana| journal= Botanical Bulletin of Academia Sinica| volume= 36}}</ref>

Whether Mg<sup>2+</sup> is able to move into and out of chloroplasts after this initial developmental phase has been the subject of several conflicting reports. Deshaies ''et al.'' (1984) found that Mg<sup>2+</sup> did move in and out of isolated chloroplasts from young pea plants,<ref name="Deshaies 1984">{{cite journal| last=Deshaies| first= R. J.|author2=Fish LE |author3=Jagendorf AT | year= 1984| title= Permeability of Chloroplast Envelopes to Mg2+: Effects on Protein Synthesis| journal= Plant Physiology| volume=74| pages=956–961| pmid=16663541| issue=4| pmc=1066800| doi=10.1104/pp.74.4.956}}</ref> but Gupta and Berkowitz (1989) were unable to reproduce the result using older spinach chloroplasts.<ref name="Gupta 1989">{{cite journal| last=Gupta| first= A. S.|author2=Berkowitz GA | year=1989| title= Development and Use of Chlorotetracycline Fluorescence as a Measurement Assay of Chloroplast Envelope-Bound Mg2+| journal= Plant Physiology| volume= 89| pages= 753–761| doi=10.1104/pp.89.3.753| pmid=16666617| issue=3| pmc=1055918}}</ref> Deshaies ''et al.'' had stated in their paper that older pea chloroplasts showed less significant changes in Mg<sup>2+</sup> content than those used to form their conclusions. The relative proportion of immature chloroplasts present in the preparations may explain these observations.

The metabolic state of the chloroplast changes considerably between night and day. During the day, the chloroplast is actively harvesting the energy of light and converting it into chemical energy. The activation of the metabolic pathways involved comes from the changes in the chemical nature of the stroma on the addition of light. H<sup>+</sup> is pumped out of the stroma (into both the cytoplasm and the lumen) leading to an alkaline pH.<ref name="Heldt 1973">{{cite journal| last=Heldt| first= H.W.|author2=Werdan K |author3=Milovancev M |author4=Geller G | year=1973| title= Alkalization of the chloroplast stroma caused by light-dependent proton flux into the thylakoid space| journal= Biochimica et Biophysica Acta (BBA) - Bioenergetics| volume= 314| pages= 224–241| doi=10.1016/0005-2728(73)90137-0| pmid=4747067| issue=2}}</ref><ref name="Hind 1974">{{cite journal| last=Hind| first= G.|author2=Nakatani HY |author3=Izawa S | year=1974| title= Light-Dependent Redistribution of Ions in Suspensions of Chloroplast Thylakoid Membranes| journal= Proceedings of the National Academy of Sciences of the United States of America| volume= 71| pages= 1484–1488| doi=10.1073/pnas.71.4.1484| pmid=4524652| issue=4| pmc=388254| bibcode= 1974PNAS...71.1484H| doi-access= free}}</ref> Mg<sup>2+</sup> (along with K<sup>+</sup>) is released from the lumen into the stroma, in an electroneutralisation process to balance the flow of H<sup>+</sup>.<ref name="Bulychev 1976">{{cite journal| last=Bulychev| first= A. A.|author2=Vredenberg WJ | title=Effect of ionophores A-23187 and nigericin on the light induced redistribution of magnesium potassium and hydrogen ions across the thylakoid membrane| date= 1980| journal= American Journal of Neuroradiology| volume=449| pages= 48–58| doi=10.1016/0005-2728(76)90006-2| pmid=10009| issue=1| pmc= 8333438}}</ref><ref name="Krause 1977">{{cite journal| last=Krause| first= G. H. | year=1977| title= Light-induced movement of magnesium ions in intact chloroplasts. Spectroscopic determination with Eriochrome Blue SE| journal= Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume= 460| pages= 500–510| doi=10.1016/0005-2728(77)90088-3| pmid=880298| issue=3}}</ref><ref name="Portis 1981">{{cite journal| last=Portis| first= A. R. | year=1981| title= Evidence of a Low Stromal Mg2+ Concentration in Intact Chloroplasts in the Dark: I. STUDIES WITH THE IONOPHORE A23187| journal= Plant Physiology| volume= 67| pages= 985–989| doi=10.1104/pp.67.5.985| pmid=16661806| issue=5| pmc=425814}}</ref><ref name="Ishijima 2003">{{cite journal| last=Ishijima| first= S.|author2=Uchibori A |author3=Takagi H |author4=Maki R |author5=Ohnishi M | year=2003| title= Light-induced increase in free Mg<sup>2+</sup> concentration in spinach chloroplasts: Measurement of free Mg<sup>2+</sup> by using a fluorescent probe and intensity of stromal alkalinization| journal= Archives of Biochemistry and Biophysics| volume= 412| pages= 126–132| doi=10.1016/S0003-9861(03)00038-9| pmid=12646275| issue=1}}</ref> Finally, thiol groups on enzymes are reduced by a change in the redox state of the stroma.<ref name="Sharkey 1998">{{cite journal| last=Sharkey| first= T. D. | year=1998| title= Photosynthetic carbon reduction| journal= Photosynthesis: A Comprehensive Treatise| editor= A. Raghavendra| location=Cambridge| publisher=Cambridge University Press| pages=111–122}}</ref> Examples of enzymes activated in response to these changes are fructose 1,6-bisphosphatase, sedoheptulose bisphosphatase and [[RuBisCO|ribulose-1,5-bisphosphate carboxylase]].<ref name="Marschner 1995"/><ref name="Black 1995b"/><ref name="Sharkey 1998"/> During the dark period, if these enzymes were active a wasteful cycling of products and substrates would occur.

Two major classes of the enzymes that interact with Mg<sup>2+</sup> in the stroma during the light phase can be identified.<ref name="Black 1995b"/> Firstly, enzymes in the glycolytic pathway most often interact with two atoms of Mg<sup>2+</sup>. The first atom is as an allosteric modulator of the enzymes' activity, while the second forms part of the active site and is directly involved in the catalytic reaction. The second class of enzymes includes those where the Mg<sup>2+</sup> is complexed to nucleotide di- and tri-phosphates (ADP and ATP), and the chemical change involves phosphoryl transfer. Mg<sup>2+</sup> may also serve in a structural maintenance role in these enzymes (e.g., enolase).

===Magnesium stress===
{{Main|magnesium deficiency (agriculture)}}
Plant stress responses can be observed in plants that are under- or over-supplied with Mg<sup>2+</sup>. The first observable signs of Mg<sup>2+</sup> stress in plants for both starvation and toxicity is a depression of the rate of photosynthesis, it is presumed because of the strong relationships between Mg<sup>2+</sup> and chloroplasts/chlorophyll. In pine trees, even before the visible appearance of yellowing and necrotic spots, the photosynthetic efficiency of the needles drops markedly.<ref name="Laing 2000"/> In Mg<sup>2+</sup> deficiency, reported secondary effects include carbohydrate immobility, loss of RNA transcription and loss of protein synthesis.<ref>Section 8.5.6 of Marschner, 1995</ref> However, due to the mobility of Mg<sup>2+</sup> within the plant, the deficiency phenotype may be present only in the older parts of the plant. For example, in Pinus radiata starved of Mg<sup>2+</sup>, one of the earliest identifying signs is the [[chlorosis]] in the needles on the lower branches of the tree. This is because Mg<sup>2+</sup> has been recovered from these tissues and moved to growing (green) needles higher in the tree.<ref name="Laing 2000"/>

A Mg<sup>2+</sup> deficit can be caused by the lack of the ion in the media (soil), but more commonly comes from inhibition of its uptake.<ref name="Marschner 1995"/> Mg<sup>2+</sup> binds quite weakly to the negatively charged groups in the root cell walls, so that excesses of other cations such as K<sup>+</sup>, NH<sub>4</sub><sup>+</sup>, Ca<sup>2+</sup>, and Mn<sup>2+</sup> can all impede uptake.(Kurvits and Kirkby, 1980;<ref name="Heenan 1981"/> In acid soils Al<sup>3+</sup> is a particularly strong inhibitor of Mg<sup>2+</sup> uptake.<ref name="Rengel 1989">{{cite journal| last=Rengel| first= Z.|author2=Robinson DL | year=1989| title= Competitive Al3+ Inhibition of Net Mg2+ Uptake by Intact Lolium multiflorum Roots : I. Kinetics| journal= Plant Physiology| volume= 91| pages= 1407–1413| doi=10.1104/pp.91.4.1407| pmid=16667193| issue=4| pmc=1062198}}</ref><ref name="Marschner 1991">{{cite book| last=Marschner| first= H. | year=1991| title= Root-induced changes in the availability of micronutrients in the rhizosphere | editor=Y. Waisel |editor2=A. Eshel |editor3=U. Kafikfai| location=New York| publisher=Marcel Dekker}}</ref> The inhibition by Al<sup>3+</sup> and Mn<sup>2+</sup> is more severe than can be explained by simple displacement, hence it is possible that these ions bind to the Mg<sup>2+</sup> uptake system directly.<ref name="Marschner 1995"/> In bacteria and yeast, such binding by Mn<sup>2+</sup> has already been observed. Stress responses in the plant develop as cellular processes halt due to a lack of Mg<sup>2+</sup> (e.g. maintenance of ΔpH across the plasma and vacuole membranes). In Mg<sup>2+</sup>-starved plants under low light conditions, the percentage of Mg<sup>2+</sup> bound to chlorophyll has been recorded at 50%.<ref name="Dorenstouter 1985">{{cite journal| last=Dorenstouter| first= H.|author2=Pieters GA |author3=Findenegg GR | year= 1985| title= Distribution of magnesium between chloroplhyll and other photosynthetic functions in magnesium deficient 'sun' and 'shade' leaves of poplar| journal= Journal of Plant Nutrition| volume= 8| pages= 1088–1101| doi=10.1080/01904168509363409| issue=12}}</ref> Presumably, this imbalance has detrimental effects on other cellular processes.

Mg<sup>2+</sup> toxicity stress is more difficult to develop. When Mg<sup>2+</sup> is plentiful, in general the plants take up the ion and store it (Stelzer ''et al.'', 1990). However, if this is followed by drought then ionic concentrations within the cell can increase dramatically. High cytoplasmic Mg<sup>2+</sup> concentrations block a K<sup>+</sup> channel in the inner envelope membrane of the chloroplast, in turn inhibiting the removal of H<sup>+</sup> ions from the chloroplast stroma. This leads to an acidification of the stroma that inactivates key enzymes in [[carbon fixation]], which all leads to the production of oxygen [[free radicals]] in the chloroplast that then cause oxidative damage.<ref name="Wu 1991">{{cite journal| last=Wu| first= W.|author2=Peters J |author3=Berkowitz GA | year=1991| title= Surface Charge-Mediated Effects of Mg2+ on K+ Flux across the Chloroplast Envelope Are Associated with Regulation of Stromal pH and Photosynthesis| journal= Plant Physiology| volume= 97| pages= 580–587| doi=10.1104/pp.97.2.580| pmid=16668438| issue=2| pmc=1081046}}</ref>

==See also==

* [[:Category:Biology and pharmacology of chemical elements|Biology and pharmacology of chemical elements]]
* {{annotated link|Calcium in biology}}
* {{annotated link|Iodine in biology}}
* {{annotated link|Ion channels}}
* [[Chlorosis|Magnesium deficiency (agriculture)]]
* {{annotated link|Magnesium deficiency (medicine)}}
* {{annotated link|Myers' cocktail}}
* {{annotated link|Osteoporosis}}
* {{annotated link|Potassium in biology}}
* {{annotated link|Selenium in biology}}
* {{annotated link|Sodium in biology}}

==Notes==
{{reflist|2}}

==References==
*{{cite book|last1=Romani |first1=Andrea M.P.|chapter=Magnesium Homeostasis in Mammalian Cells |editor1-first=Lucia|editor1-last=Banci |series=Metal Ions in Life Sciences |volume=12 |title=Metallomics and the Cell |year=2013 |pages=69–118|publisher=Springer |isbn=978-94-007-5560-4|doi=10.1007/978-94-007-5561-1_4|pmid=23595671|issn=1559-0836}} electronic-book {{ISBN|978-94-007-5561-1}} electronic-{{issn|1868-0402}}
* {{cite journal | doi = 10.1023/A:1025861522935 | last1 = Findling | first1 = R. L. | year = 1997 | last2 = Maxwell | first2 = K | last3 = Scotese-Wojtila | first3 = L | last4 = Huang | first4 = J | last5 = Yamashita | first5 = T | last6 = Wiznitzer | first6 = M | title = High-dose pyridoxine and magnesium administration in children with autistic disorder: an absence of salutary effects in a double-blind, placebo-controlled study | journal = J Autism Dev Disord | volume = 27 | issue = 4| pages = 467–478 | pmid = 9261669 | s2cid = 39143708 }}
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== External links ==
{{External links|date=January 2018}}
*[http://www.ctds.info/5_13_magnesium.html Magnesium Deficiency]
*[https://foodstruct.com/list/foods-high-in-magnesium-mg List of foods rich in Magnesium]
*[http://www.mgwater.com/index.shtml The Magnesium Website]- Includes full text papers and textbook chapters by leading magnesium authorities Mildred Seelig, Jean Durlach, Burton M. Altura and Bella T. Altura. Links to over 300 articles discussing magnesium and magnesium deficiency.
*[http://iom.edu/Activities/Nutrition/SummaryDRIs/~/media/Files/Activity%20Files/Nutrition/DRIs/DRI_Elements.ashx Dietary Reference Intake]

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[[Category:Physiology]]
[[Category:Plant physiology]]
[[Category:Magnesium]]
[[Category:Biology and pharmacology of chemical elements]]
[[Category:Biological systems]]