Editing Glucose (section)

==Biochemical properties==
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! Metabolism of common [[monosaccharide]]s and some biochemical reactions of glucose
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Glucose is the most abundant monosaccharide. Glucose is also the most widely used aldohexose in most living organisms. One possible explanation for this is that glucose has a lower tendency than other aldohexoses to react nonspecifically with the [[amine]] groups of [[protein]]s.<ref name=Higgins>{{cite journal|last1=Bunn|first1=H. F.|last2=Higgins|first2=P. J.|title=Reaction of monosaccharides with proteins: possible evolutionary significance |journal=Science|date=1981|volume=213 |issue=4504|pages=222–24|doi=10.1126/science.12192669|pmid=12192669| bibcode=1981Sci...213..222B}}<!--|access-date=13 May 2015--></ref> This reaction—[[glycation]]—impairs or destroys the function of many proteins,<ref name=Higgins/> e.g. in [[glycated hemoglobin]]. Glucose's low rate of glycation can be attributed to its having a more stable [[#Cyclic forms|cyclic form]] compared to other aldohexoses, which means it spends less time than they do in its reactive [[#Open-chain form|open-chain form]].<ref name=Higgins/> The reason for glucose having the most stable cyclic form of all the aldohexoses is that its [[hydroxy group]]s (with the exception of the hydroxy group on the anomeric carbon of {{sm|d}}-glucose) are in the [[Cyclohexane conformation|equatorial position]]. Presumably, glucose is the most abundant natural monosaccharide because it is less glycated with proteins than other monosaccharides.<ref name=Higgins /><ref name="Stryer 531">Jeremy M. Berg: ''Stryer Biochemie'' {{In lang|de}}. Springer-Verlag, 2017, {{ISBN|978-3-662-54620-8}}, p.&nbsp;531.</ref> Another hypothesis is that glucose, being the only {{sm|d}}-aldohexose that has all five hydroxy substituents in the [[Equatorial bond|equatorial]] position in the form of β-{{sm|d}}-glucose, is more readily accessible to chemical reactions,<ref name="Garrett">{{cite book |last1=Garrett |first1=Reginald H. |title=Biochemistry |date=2013 |publisher=Brooks/Cole, Cengage Learning |location=Belmont, CA |isbn=978-1-133-10629-6 |edition=5th}}</ref>{{rp|194, 199}} for example, for [[esterification]]<ref name=Voet>{{cite book |last1=Voet |first1=Donald |last2=Voet |first2=Judith G. |title=Biochemistry |date=2011 |publisher=John Wiley & Sons |location=Hoboken, NJ |isbn=978-0-470-57095-1 |edition=4th}}</ref>{{rp|363}} or [[acetal]] formation.<ref>Albert L. Lehninger, ''Biochemistry, 6th printing'', Worth Publishers Inc. 1972, {{ISBN|0-87901-009-6}} p. 228.</ref> For this reason, {{sm|d}}-glucose is also a highly preferred building block in natural polysaccharides (glycans). Polysaccharides that are composed solely of glucose are termed [[glucan]]s.

Glucose is produced by plants through photosynthesis using sunlight,<ref name="photo"/><ref name="Löffler/Petrides 195"/> water and carbon dioxide and can be used by all living organisms as an energy and carbon source. However, most glucose does not occur in its free form, but in the form of its polymers, i.e. lactose, sucrose, starch and others which are energy reserve substances, and cellulose and [[chitin]], which are components of the cell wall in plants or [[fungi]] and [[arthropod]]s, respectively. These polymers, when consumed by animals, fungi and bacteria, are degraded to glucose using enzymes. All animals are also able to produce glucose themselves from certain precursors as the need arises. [[Neuron]]s, cells of the [[renal medulla]] and [[erythrocytes]] depend on glucose for their energy production.<ref name="Löffler/Petrides 195">Peter C. Heinrich: ''Löffler/Petrides Biochemie und Pathobiochemie'' {{In lang|de}}.. Springer-Verlag, 2014, {{ISBN|978-3-642-17972-3}}, p.&nbsp;195.</ref> In adult humans, there is about {{cvt|18|g}} of glucose,<ref name="Satyanarayana">U. Satyanarayana: ''Biochemistry''. Elsevier Health Sciences, 2014, {{ISBN|978-8-131-23713-7}}. p. 674.</ref> of which about {{cvt|4|g}} is present in the blood.<ref>{{Cite journal |doi=10.1152/ajpendo.90563.2008 |pmc=2636990 |pmid=18840763|year=2009 |last1=Wasserman |first1=D. H. |title=Four grams of glucose |journal=American Journal of Physiology. Endocrinology and Metabolism |volume=296 |issue=1 |pages=E11–21 }}</ref> Approximately {{cvt|180-220|g}} of glucose is produced in the liver of an adult in 24 hours.<ref name="Satyanarayana" />

Many of the long-term complications of [[diabetes]] (e.g., [[Visual impairment|blindness]], [[kidney failure]], and [[peripheral neuropathy]]) are probably due to the glycation of proteins or [[lipid]]s.<ref>{{citation | title = High Blood Glucose and Diabetes Complications: The buildup of molecules known as AGEs may be the key link | journal = Diabetes Forecast | url = http://forecast.diabetes.org/magazine/features/high-blood-glucose-and-diabetes-complications | year = 2010 | publisher = American Diabetes Association | issn = 0095-8301 | access-date = 20 May 2010 | archive-url = https://web.archive.org/web/20131014173838/http://forecast.diabetes.org/magazine/features/high-blood-glucose-and-diabetes-complications | archive-date = 14 October 2013 | url-status = dead }}</ref> In contrast, [[enzyme]]-regulated addition of sugars to protein is called [[glycosylation]] and is essential for the function of many proteins.<ref name="varki">{{Cite book|edition = 2nd|publisher = Cold Spring Harbor Laboratories Press|isbn = 978-0-87969-770-9| editor-first= Ajit |editor-last=Varki |title = Essentials of Glycobiology|url = https://www.ncbi.nlm.nih.gov/books/NBK1908/|url-status = live|archive-url = https://web.archive.org/web/20161206081633/https://www.ncbi.nlm.nih.gov/books/NBK1908/|archive-date = 6 December 2016|year = 2009| pmid=20301239 | last1=Varki | first1=A. | last2=Cummings | first2=R. D. | last3=Esko | first3=J. D. | last4=Freeze | first4=H. H. | last5=Stanley | first5=P. | last6=Bertozzi | first6=C. R. | last7=Hart | first7=G. W. | last8=Etzler | first8=M. E. }}</ref>

===Uptake===
Ingested glucose initially binds to the receptor for sweet taste on the tongue in humans. This complex of the proteins [[T1R2]] and [[T1R3]] makes it possible to identify glucose-containing food sources.<ref name="fca">{{cite web | url=https://foodb.ca/compounds/FDB012530 | title=Showing Compound D-Glucose (FDB012530) - FooDB | access-date=18 March 2024 | archive-date=6 December 2022 | archive-url=https://web.archive.org/web/20221206084746/https://www.foodb.ca/compounds/FDB012530 | url-status=live }}</ref><ref name="Löffler/Petrides 404"/> Glucose mainly comes from food—about {{cvt|300|g}} per day is produced by conversion of food,<ref name="Löffler/Petrides 404">Peter C. Heinrich: ''Löffler/Petrides Biochemie und Pathobiochemie''. Springer-Verlag, 2014, {{ISBN|978-3-642-17972-3}}, p.&nbsp;404.</ref> but it is also synthesized from other metabolites in the body's cells. In humans, the breakdown of glucose-containing polysaccharides happens in part already during [[chewing]] by means of [[amylase]], which is contained in [[saliva]], as well as by [[maltase]], [[lactase]], and [[sucrase]] on the [[brush border]] of the [[small intestine]]. Glucose is a building block of many carbohydrates and can be split off from them using certain enzymes. [[Glucosidases]], a subgroup of the glycosidases, first catalyze the hydrolysis of long-chain glucose-containing polysaccharides, removing terminal glucose. In turn, disaccharides are mostly degraded by specific glycosidases to glucose. The names of the degrading enzymes are often derived from the particular poly- and disaccharide; inter alia, for the degradation of polysaccharide chains there are amylases (named after amylose, a component of starch), cellulases (named after cellulose), chitinases (named after chitin), and more. Furthermore, for the cleavage of disaccharides, there are maltase, lactase, sucrase, [[trehalase]], and others. In humans, about 70 genes are known that code for glycosidases. They have functions in the digestion and degradation of glycogen, [[sphingolipid]]s, [[mucopolysaccharides]], and poly([[Adenosine diphosphate ribose|ADP-ribose]]). Humans do not produce cellulases, chitinases, or trehalases, but the bacteria in the [[gut microbiota]] do.

In order to get into or out of cell membranes of cells and membranes of cell compartments, glucose requires special transport proteins from the [[major facilitator superfamily]]. In the small intestine (more precisely, in the [[jejunum]]),<ref name="Harper 641">Harold A. Harper: ''Medizinische Biochemie'' {{In lang|de}}. Springer-Verlag, 2013, {{ISBN|978-3-662-22150-1}}, p.&nbsp;641.</ref> glucose is taken up into the intestinal [[epithelium]] with the help of [[glucose transporter]]s<ref>{{Cite journal |doi=10.1007/s12551-015-0186-2 |pmc=5425736 |pmid=28510148|year=2016 |last1=Navale |first1=A. M. |title=Glucose transporters: Physiological and pathological roles |journal=Biophysical Reviews |volume=8 |issue=1 |pages=5–9 |last2=Paranjape |first2=A. N. }}</ref> via a [[secondary active transport]] mechanism called sodium ion-glucose [[symport]] via [[sodium/glucose cotransporter 1]] (SGLT1).<ref name="Löffler/Petrides 199" /> Further transfer occurs on the [[basolateral]] side of the intestinal epithelial cells via the glucose transporter [[GLUT2]],<ref name="Löffler/Petrides 199" /> as well uptake into [[hepatocyte|liver cells]], kidney cells, cells of the [[Pancreatic islets|islets of Langerhans]], [[neuron]]s, [[astrocyte]]s, and [[tanycyte]]s.<ref>{{Cite journal|doi=10.1007/s00125-014-3451-1|pmid=25421524|year=2015|last1=Thorens|first1=B.|title=GLUT2, glucose sensing and glucose homeostasis|journal=Diabetologia|volume=58|issue=2|pages=221–32|doi-access=free|url=http://doc.rero.ch/record/331705/files/125_2014_Article_3451.pdf|access-date=18 March 2024|archive-date=2 December 2023|archive-url=https://web.archive.org/web/20231202190651/http://doc.rero.ch/record/331705/files/125_2014_Article_3451.pdf|url-status=live}}</ref> Glucose enters the liver via the [[portal vein]] and is stored there as a cellular glycogen.<ref name="Löffler/Petrides 214" /> In the liver cell, it is [[Phosphorylation|phosphorylated]] by [[glucokinase]] at position 6 to form [[glucose 6-phosphate]], which cannot leave the cell. [[Glucose 6-phosphatase]] can convert glucose 6-phosphate back into glucose exclusively in the liver, so the body can maintain a sufficient blood glucose concentration. In other cells, uptake happens by passive transport through one of the 14 GLUT proteins.<ref name="Löffler/Petrides 199" /> In the other cell types, phosphorylation occurs through a [[hexokinase]], whereupon glucose can no longer diffuse out of the cell.

The glucose transporter [[GLUT1]] is produced by most cell types and is of particular importance for nerve cells and pancreatic [[Beta cell|β-cell]]s.<ref name="Löffler/Petrides 199" /> [[GLUT3]] is highly expressed in nerve cells.<ref name="Löffler/Petrides 199" /> Glucose from the bloodstream is taken up by [[GLUT4]] from [[muscle cell]]s (of the [[skeletal muscle]]<ref>{{Cite journal |doi=10.1016/j.cmet.2007.03.006 |pmid=17403369|year=2007|last1=Huang| first1=S.|title=The GLUT4 glucose transporter|journal=Cell Metabolism|volume=5|issue=4|pages=237–52|last2=Czech|first2=M. P.|doi-access=free}}</ref> and [[heart muscle]]) and [[fat cell]]s.<ref>{{Cite book |pmid=25344989|date=2014| last1=Govers|first1=R.|title=Cellular regulation of glucose uptake by glucose transporter GLUT4|series=Advances in Clinical Chemistry|volume=66|pages=173–240|publisher=Elsevier |doi=10.1016/B978-0-12-801401-1.00006-2|isbn=978-0-12-801401-1}}</ref> [[GLUT14]] is expressed exclusively in [[testicle]]s.<ref>{{cite journal |last1=Wu |first1=Xiaohua |last2=Freeze |first2=Hudson H. |title=GLUT14, a Duplicon of GLUT3, is Specifically Expressed in Testis as Alternative Splice Forms |journal=Genomics |date=December 2002 |volume=80 |issue=6 |pages=553–7 |doi=10.1006/geno.2002.7010 |pmid=12504846}}</ref> Excess glucose is broken down and converted into fatty acids, which are stored as [[triglyceride]]s. In the [[kidney]]s, glucose in the urine is absorbed via SGLT1 and [[SGLT2]] in the apical cell membranes and transmitted via GLUT2 in the basolateral cell membranes.<ref>{{Cite journal |doi=10.1007/s00125-018-4656-5 |pmid=30132032|pmc=6133168|year=2018|last1=Ghezzi|first1=C.|title=Physiology of renal glucose handling via SGLT1, SGLT2, and GLUT2|journal=Diabetologia|volume=61|issue=10|pages=2087–2097|author2=Loo DDF|last3=Wright|first3=E. M.}}</ref> About 90% of kidney glucose reabsorption is via SGLT2 and about 3% via SGLT1.<ref>{{Cite journal |doi=10.1097/MNH.0000000000000152 |pmc=5364028 |pmid=26125647|year=2015 |last1=Poulsen |first1=S. B. |title=Sodium-glucose cotransport |journal=Current Opinion in Nephrology and Hypertension |volume=24 |issue=5 |pages=463–9 |last2=Fenton |first2=R. A. |last3=Rieg |first3=T. }}</ref>

===Biosynthesis===
{{main|Gluconeogenesis|Glycogenolysis}}
In plants and some [[prokaryote]]s, glucose is a product of [[photosynthesis]].<ref name="photo">{{Cite web|url=http://www.rsc.org/Education/Teachers/Resources/cfb/Photosynthesis.htm|title=Chemistry for Biologists: Photosynthesis|website=www.rsc.org|access-date=5 February 2018|url-status=live|archive-url=https://web.archive.org/web/20160804060610/http://www.rsc.org/Education/Teachers/Resources/cfb/Photosynthesis.htm|archive-date=4 August 2016}}</ref> Glucose is also formed by the breakdown of polymeric forms of glucose like [[glycogen]] (in animals and [[mushroom]]s) or starch (in plants). The cleavage of glycogen is termed glycogenolysis, the cleavage of starch is called starch degradation.<ref>{{cite journal |last1= Smith, Alison M. |last2=Zeeman, Samuel C. |last3= Smith, Steven M. |year= 2005 |title= Starch Degradation |journal= Annu. Rev. Plant Biol. |volume= 56 |issue=1 |pages= 73–98 |doi= 10.1146/annurev.arplant.56.032604.144257 |pmid=15862090 |bibcode=2005AnRPB..56...73S }}</ref>

The metabolic pathway that begins with molecules containing two to four carbon atoms (C) and ends in the glucose molecule containing six carbon atoms is called gluconeogenesis and occurs in all living organisms. The smaller starting materials are the result of other metabolic pathways. Ultimately almost all [[biomolecule]]s come from the assimilation of carbon dioxide in plants and microbes during photosynthesis.<ref name=Voet/>{{rp|359}} The free energy of formation of α-{{sm|d}}-glucose is 917.2 kilojoules per mole.<ref name=Voet/>{{rp|59}} In humans, gluconeogenesis occurs in the liver and kidney,<ref name="Szablewski">Leszek Szablewski: ''Glucose Homeostasis and Insulin Resistance''. Bentham Science Publishers, 2011, {{ISBN|978-1-608-05189-2}}, p.&nbsp;46.</ref> but also in other cell types. In the liver about {{cvt|150|g}} of glycogen are stored, in skeletal muscle about {{cvt|250|g}}.<ref name="Löffler/Petrides 389">Peter C. Heinrich: ''Löffler/Petrides Biochemie und Pathobiochemie'' {{In lang|de}}. Springer-Verlag, 2014, {{ISBN|978-3-642-17972-3}}, p.&nbsp;389.</ref> However, the glucose released in muscle cells upon cleavage of the glycogen can not be delivered to the circulation because glucose is phosphorylated by the hexokinase, and a glucose-6-phosphatase is not expressed to remove the phosphate group. Unlike for glucose, there is no transport protein for [[Glucose-6-phosphate dehydrogenase (coenzyme-F420)|glucose-6-phosphate]]. Gluconeogenesis allows the organism to build up glucose from other metabolites, including [[lactic acid|lactate]] or certain [[amino acid]]s, while consuming energy. The renal [[tubular cell]]s can also produce glucose.

Glucose also can be found outside of living organisms in the ambient environment. Glucose concentrations in the atmosphere are detected via collection of samples by aircraft and are known to vary from location to location. For example, glucose concentrations in atmospheric air from inland China range from 0.8 to 20.1 pg/L, whereas east coastal China glucose concentrations range from 10.3 to 142 pg/L.<ref>{{cite journal |last1=Wang |first1=Gehui |last2=Kawamura |first2=Kimitaka |last3=Hatakeyama |first3=Shiro |last4=Takami |first4=Akinori |last5=Li |first5=Hong |last6=Wang |first6=Wei |title=Aircraft Measurement of Organic Aerosols over China |journal=Environmental Science & Technology |date=May 2007 |volume=41 |issue=9 |pages=3115–3120 |doi=10.1021/es062601h |pmid=17539513 |bibcode=2007EnST...41.3115W }}</ref>

===Glucose degradation===
{{Main|Glycolysis|Pentose phosphate pathway}}
[[File:Glucose metabolism.svg|thumb|Glucose metabolism and various forms of it in the process.{{pb}}Glucose-containing compounds and [[isomer]]ic forms are digested and taken up by the body in the intestines, including [[starch]], [[glycogen]], [[disaccharide]]s and [[monosaccharide]]s.{{pb}}Glucose is stored in mainly the liver and muscles as glycogen. It is distributed and used in tissues as free glucose.]]

In humans, glucose is metabolized by glycolysis<ref>{{Cite journal |doi=10.1042/BSR20160385 |pmc=5293555 |pmid=27707936|year=2016 |last1=Adeva-Andany |first1=M. M. |title=Liver glucose metabolism in humans |journal=Bioscience Reports |volume=36 |issue=6 |pages=e00416 |last2=Pérez-Felpete |first2=N. |last3=Fernández-Fernández |first3=C. |last4=Donapetry-García |first4=C. |last5=Pazos-García |first5=C. }}</ref> and the pentose phosphate pathway.<ref name="Horton">H. Robert Horton, [[Laurence A. Moran]], K. Gray Scrimgeour, Marc D. Perry, J. David Rawn: ''Biochemie'' {{In lang|de}}. Pearson Studium; 4. aktualisierte Auflage 2008; {{ISBN|978-3-8273-7312-0}}; pp. 490–496.</ref> Glycolysis is used by all living organisms,<ref name="Garrett"/>{{rp|551}}<ref name="Hall">Brian K. Hall: ''Strickberger's Evolution''. Jones & Bartlett Publishers, 2013, {{ISBN|978-1-449-61484-3}}, p.&nbsp;164.</ref> with small variations, and all organisms generate energy from the breakdown of monosaccharides.<ref name="Hall" /> In the further course of the metabolism, it can be completely degraded via [[oxidative decarboxylation]], the [[citric acid cycle]] (synonym ''Krebs cycle'') and the [[respiratory chain]] to water and carbon dioxide. If there is not enough oxygen available for this, the glucose degradation in animals occurs anaerobic to lactate via lactic acid fermentation and releases much less energy. Muscular lactate enters the liver through the bloodstream in mammals, where gluconeogenesis occurs ([[Cori cycle]]). With a high supply of glucose, the metabolite [[acetyl-CoA]] from the Krebs cycle can also be used for [[fatty acid synthesis]].<ref>{{Cite journal |doi=10.1007/s00125-016-3940-5|pmid=27048250|year=2016|last1=Jones|first1=J. G.|title=Hepatic glucose and lipid metabolism|journal=Diabetologia|volume=59|issue=6|pages=1098–103|doi-access=free}}</ref> Glucose is also used to replenish the body's glycogen stores, which are mainly found in liver and skeletal muscle. These processes are [[Hormone|hormonally]] regulated.

In other living organisms, other forms of fermentation can occur. The bacterium ''[[Escherichia coli]]'' can grow on nutrient media containing glucose as the sole carbon source.<ref name=Voet/>{{rp|59}} In some bacteria and, in modified form, also in archaea, glucose is degraded via the [[Entner-Doudoroff pathway]].<ref>{{cite journal | last1 = Entner | first1 = N. | last2 = Doudoroff | first2 = M. | year = 1952 | title = Glucose and gluconic acid oxidation of Pseudomonas saccharophila | journal = [[J Biol Chem]] | volume = 196 | issue = 2| pages = 853–862 | doi = 10.1016/S0021-9258(19)52415-2 | pmid = 12981024 | doi-access = free }}</ref> With glucose, a mechanism for [[gene regulation]] was discovered in ''E. coli'', the [[catabolite repression]] (formerly known as ''glucose effect'').<ref name="PMID29330542">{{cite journal | vauthors = Ammar EM, Wang X, Rao CV | title = Regulation of metabolism in Escherichia coli during growth on mixtures of the non-glucose sugars: arabinose, lactose, and xylose | journal = Scientific Reports | volume = 8 | issue = 1 | pages = 609 | date = January 2018 | pmid = 29330542 | pmc = 5766520 | doi = 10.1038/s41598-017-18704-0 | bibcode = 2018NatSR...8..609A | url = | issn = }}</ref>

Use of glucose as an energy source in cells is by either aerobic respiration, anaerobic respiration, or fermentation.<ref name="sil"/> The first step of glycolysis is the [[phosphorylation]] of glucose by a [[hexokinase]] to form [[glucose 6-phosphate]]. The main reason for the immediate phosphorylation of glucose is to prevent its diffusion out of the cell as the charged [[phosphate]] group prevents glucose 6-phosphate from easily crossing the [[cell membrane]].<ref name="sil">{{cite journal|last1=Bonadonna|first1=Riccardo C|last2=Bonora|first2=Enzo|last3=Del Prato|first3=Stefano|last4=Saccomani|first4=Maria|last5=Cobelli|first5=Claudio|last6=Natali|first6=Andrea|last7=Frascerra|first7=Silvia|last8=Pecori|first8=Neda|last9=Ferrannini|first9=Eleuterio|last10=Bier|first10=Dennis|last11=DeFronzo|first11=Ralph A|last12=Gulli|first12=Giovanni|title=Roles of glucose transport and glucose phosphorylation in muscle insulin resistance of NIDDM|journal=Diabetes|date=July 1996|volume=45|issue=7|pages=915–25|doi=10.2337/diab.45.7.915|pmid=8666143|s2cid=219249555|url=http://diabetes.diabetesjournals.org/content/45/7/915.full-text.pdf |archive-url=https://web.archive.org/web/20170306131309/http://diabetes.diabetesjournals.org/content/45/7/915.full-text.pdf |archive-date=6 March 2017 |url-status=live|access-date=5 March 2017}}</ref> Furthermore, addition of the high-energy phosphate group [[Activation#Biochemistry|activates]] glucose for subsequent breakdown in later steps of glycolysis.<ref>{{cite web | url=https://go.drugbank.com/drugs/DB09341 | title=Glucose | access-date=18 March 2024 | archive-date=5 December 2023 | archive-url=https://web.archive.org/web/20231205120936/https://go.drugbank.com/drugs/DB09341 | url-status=live }}</ref>

In anaerobic respiration, one glucose molecule produces a net gain of two ATP molecules (four ATP molecules are produced during glycolysis through substrate-level phosphorylation, but two are required by enzymes used during the process).<ref>{{citation | title = Medical Biochemistry at a Glance @Google books | url = https://books.google.com/books?id=9BtxCWxrWRoC&pg=PA52 | year = 2006 | page = 52 | publisher = Blackwell Publishing | isbn = 978-1-4051-1322-9 | url-status = live | archive-url = https://web.archive.org/web/20180223145046/https://books.google.com/books?id=9BtxCWxrWRoC&pg=PA52 | archive-date = 23 February 2018 }}</ref> In aerobic respiration, a molecule of glucose is much more profitable in that a maximum net production of 30 or 32 ATP molecules (depending on the organism) is generated.<ref>{{citation|title=Medical Biochemistry at a Glance @Google books|url=https://books.google.com/books?id=9BtxCWxrWRoC&pg=PA50|page=50|year=2006|archive-url=https://web.archive.org/web/20180223145046/https://books.google.com/books?id=9BtxCWxrWRoC&pg=PA50|publisher=Blackwell Publishing|isbn=978-1-4051-1322-9|archive-date=23 February 2018|url-status=live}}</ref>

{{GlycolysisGluconeogenesis_WP534|highlight=Glucose}}

[[Tumor]] cells often grow comparatively quickly and consume an above-average amount of glucose by glycolysis,<ref>{{Cite journal |doi=10.1097/MCO.0b013e32833a5577 |pmid=20473153|year=2010|last1=Annibaldi|first1=A.|title=Glucose metabolism in cancer cells|journal=Current Opinion in Clinical Nutrition and Metabolic Care|volume=13|issue=4|pages=466–70|last2=Widmann|first2=C.|s2cid=205782021}}</ref> which leads to the formation of lactate, the end product of fermentation in mammals, even in the presence of oxygen. This is called the [[Warburg effect (oncology)|Warburg effect]]. For the increased uptake of glucose in tumors various SGLT and GLUT are overly produced.<ref>{{Cite journal |doi=10.1016/j.bbcan.2012.12.004|pmid=23266512|year=2013|last1=Szablewski|first1=L.|title=Expression of glucose transporters in cancers|journal=Biochimica et Biophysica Acta (BBA) - Reviews on Cancer|volume=1835|issue=2|pages=164–9}}</ref><ref>{{Cite journal |doi=10.1097/CCO.0b013e328356da72 |pmid=22913968|year=2012|last1=Adekola|first1=K.|title=Glucose transporters in cancer metabolism|journal=Current Opinion in Oncology|volume=24|issue=6|pages=650–4|last2=Rosen|first2=S. T.|last3=Shanmugam|first3=M.|pmc=6392426}}</ref>

In [[yeast]], ethanol is fermented at high glucose concentrations, even in the presence of oxygen (which normally leads to respiration rather than fermentation). This is called the [[Crabtree effect]].

Glucose can also degrade to form carbon dioxide through abiotic means. This has been demonstrated to occur experimentally via oxidation and hydrolysis at 22&nbsp;°C and a pH of 2.5.<ref>{{cite journal |last1=Schümann |first1=U. |last2=Gründler |first2=P. |title=Electrochemical degradation of organic substances at PbO2 anodes: Monitoring by continuous CO2 measurements |journal=Water Research |date=September 1998 |volume=32 |issue=9 |pages=2835–2842 |doi=10.1016/s0043-1354(98)00046-3 }}</ref>

===Energy source===
[[File:Glucose catabolism intermediates de.png|thumb|upright=1.2|class=skin-invert-image|Diagram showing the possible intermediates in glucose degradation; Metabolic pathways orange: glycolysis, green: Entner-Doudoroff pathway, phosphorylating, yellow: Entner-Doudoroff pathway, non-phosphorylating]]
Glucose is a ubiquitous fuel in [[biology]]. It is used as an energy source in organisms, from bacteria to humans, through either [[aerobic respiration]], [[anaerobic respiration]] (in bacteria), or [[Fermentation (biochemistry)|fermentation]]. Glucose is the human body's key source of energy, through aerobic respiration, providing about 3.75&nbsp;[[kilocalorie]]s (16&nbsp;[[kilojoule]]s) of [[food energy]] per gram.<ref>{{citation | title = Food energy&nbsp;– methods of analysis and conversion factors | chapter-url = http://www.fao.org/docrep/006/Y5022E/y5022e04.htm | chapter = Chapter 3: Calculation of the Energy Content of Foods – Energy Conversion Factors | series = FAO Food and Nutrition Paper 77 | publisher = Food and Agriculture Organization | location = Rome | year = 2003 | isbn = 978-92-5-105014-9 | url-status = live | archive-url = https://web.archive.org/web/20100524003622/http://www.fao.org/DOCREP/006/Y5022E/y5022e04.htm | archive-date = 24 May 2010 }}</ref> Breakdown of carbohydrates (e.g., starch) yields mono- and [[disaccharide]]s, most of which is glucose. Through [[glycolysis]] and later in the reactions of the [[citric acid cycle]] and [[oxidative phosphorylation]], glucose is [[oxidize]]d to eventually form [[carbon dioxide]] and water, yielding energy mostly in the form of [[adenosine triphosphate]] (ATP). The insulin reaction, and other mechanisms, regulate the concentration of glucose in the blood. The physiological caloric value of glucose, depending on the source, is 16.2 kilojoules per gram<ref name="Schwedt">Georg Schwedt: ''Zuckersüße Chemie'' {{In lang|de}}. John Wiley & Sons, 2012, {{ISBN|978-3-527-66001-8}}, p.&nbsp;100.</ref> or 15.7 kJ/g (3.74 kcal/g).<ref>Schmidt, Lang: ''Physiologie des Menschen'', 30. Auflage. Springer Verlag, 2007, p.&nbsp;907 {{in lang|de}}.</ref> The high availability of carbohydrates from plant biomass has led to a variety of methods during evolution, especially in microorganisms, to utilize glucose for energy and carbon storage. Differences exist in which end product can no longer be used for energy production. The presence of individual genes, and their gene products, the enzymes, determine which reactions are possible. The metabolic pathway of glycolysis is used by almost all living beings. An essential difference in the use of glycolysis is the recovery of [[NADPH]] as a reductant for [[anabolism]] that would otherwise have to be generated indirectly.<ref>{{Cite journal |pmc=1220531 |pmid=10493919 |year=1999 |last1=Dandekar |first1=T. |title=Pathway alignment: Application to the comparative analysis of glycolytic enzymes |journal=The Biochemical Journal |volume=343 |pages=115–124 |last2=Schuster |first2=S. |last3=Snel |first3=B. |last4=Huynen |first4=M. |last5=Bork |first5=P. |doi=10.1042/bj3430115 |issue=1}}</ref>

Glucose and oxygen supply almost all the energy for the [[brain]],<ref>{{cite web |first = Pramod|last = Dash |url =http://neuroscience.uth.tmc.edu/s4/chapter11.html |title=Blood Brain Barrier and Cerebral Metabolism (Section 4, Chapter 11) |work =Neuroscience Online: An Electronic Textbook for the Neurosciences |publisher =Department of Neurobiology and Anatomy – The University of Texas Medical School at Houston  |url-status=dead |archive-url=https://web.archive.org/web/20161117104126/http://neuroscience.uth.tmc.edu/s4/chapter11.html |archive-date=17 November 2016 }}</ref> so its availability influences [[psychological]] processes. When [[Hypoglycaemia|glucose is low]], psychological processes requiring mental effort (e.g., [[self-control]], effortful decision-making) are impaired.<ref>{{citation | last1 = Fairclough | first1 = Stephen H. | last2 = Houston | first2 = Kim | s2cid = 44500072 | title = A metabolic measure of mental effort | journal = Biol. Psychol.  | year = 2004 | volume = 66 | issue = 2 | pages = 177–190 | pmid = 15041139  | doi = 10.1016/j.biopsycho.2003.10.001}}</ref><ref>{{citation | last1 = Gailliot | first1 = Matthew T. | last2 = Baumeister | first2 = Roy F. | last3 = DeWall | first3 = C. Nathan | last4 = Plant | first4 = E. Ashby | last5 = Brewer | first5 = Lauren E. | last6 = Schmeichel | first6 = Brandon J. | last7 = Tice | first7 = Dianne M. | last8 = Maner | first8 = Jon K. | title = Self-Control Relies on Glucose as a Limited Energy Source: Willpower is More than a Metaphor | journal = J. Pers. Soc. Psychol. | year = 2007 | volume = 92 | issue = 2 | pages = 325–336 | pmid = 17279852 | doi = 10.1037/0022-3514.92.2.325 | url = http://www.uky.edu/~njdewa2/gailliotetal07JPSP.pdf | url-status = live | archive-url = https://web.archive.org/web/20170818111037/http://www.uky.edu/~njdewa2/gailliotetal07JPSP.pdf | archive-date = 18 August 2017 | citeseerx = 10.1.1.337.3766 | s2cid = 7496171 }}</ref><ref>{{citation | last1 = Gailliot | first1 = Matthew T. | last2 = Baumeister | first2 = Roy F.  | title = The Physiology of Willpower: Linking Blood Glucose to Self-Control | journal = Personal. Soc. Psychol. Rev. | year = 2007 | volume = 11 | issue = 4 | pages = 303–327 | doi = 10.1177/1088868307303030 | pmid = 18453466 | citeseerx = 10.1.1.475.9484 | s2cid = 14380313 }}</ref><ref>{{citation | last1 = Masicampo | first1 = E. J. | last2 = Baumeister | first2 = Roy F. | title = Toward a Physiology of Dual-Process Reasoning and Judgment: Lemonade, Willpower, and Expensive Rule-Based Analysis | journal = Psychol. Sci. | year = 2008 | volume = 19 | issue = 3 | pages = 255–60 | doi = 10.1111/j.1467-9280.2008.02077.x | pmid = 18315798| s2cid = 38596025 }}</ref> In the brain, which is dependent on glucose and oxygen as the major source of energy, the glucose concentration is usually 4 to 6&nbsp;mM (5&nbsp;mM equals 90&nbsp;mg/dL),<ref name="Satyanarayana" /> but decreases to 2 to 3&nbsp;mM when fasting.<ref name="Dwyer" /> [[Confusion]] occurs below 1&nbsp;mM and [[coma]] at lower levels.<ref name="Dwyer" />

The glucose in the blood is called [[blood sugar]]. Blood sugar levels are regulated by glucose-binding nerve cells in the [[hypothalamus]].<ref name="Koekkoek">{{Cite journal |doi=10.3389/fnins.2017.00716 |pmc=5742113 |pmid=29311793|year=2017 |last1=Koekkoek |first1=L. L. |title=Glucose-Sensing in the Reward System |journal=Frontiers in Neuroscience |volume=11 |pages=716 |last2=Mul |first2=J. D. |last3=La Fleur |first3=S. E. |doi-access=free }}</ref> In addition, glucose in the brain binds to glucose receptors of the [[reward system]] in the [[nucleus accumbens]].<ref name="Koekkoek" /> The binding of glucose to the sweet receptor on the tongue induces a release of various hormones of energy metabolism, either through glucose or through other sugars, leading to an increased cellular uptake and lower blood sugar levels.<ref name="Tucker">{{Cite journal |doi=10.1016/j.physbeh.2017.09.016 |pmid=28939430 |year=2017 |last1=Tucker |first1=R. M. |title=Do non-nutritive sweeteners influence acute glucose homeostasis in humans? A systematic review |journal=Physiology & Behavior |volume=182 |pages=17–26 |last2=Tan |first2=S. Y. |s2cid=38764657 |url=https://ap01.alma.exlibrisgroup.com/view/delivery/61USOUTHAUS_INST/12149779800001831 |access-date=7 June 2020 |archive-date=29 July 2020 |archive-url=https://web.archive.org/web/20200729195754/https://ap01.alma.exlibrisgroup.com/view/delivery/61USOUTHAUS_INST/12149779800001831 |url-status=dead |url-access=subscription }}</ref> [[Artificial sweetener]]s do not lower blood sugar levels.<ref name="Tucker" />

The blood sugar content of a healthy person in the short-time fasting state, e.g. after overnight fasting, is about 70 to 100&nbsp;mg/dL of blood (4 to 5.5&nbsp;mM). In [[blood plasma]], the measured values are about 10–15% higher. In addition, the values in the [[artery|arterial]] blood are higher than the concentrations in the [[vein|venous]] blood since glucose is absorbed into the tissue during the passage of the [[capillary bed]]. Also in the capillary blood, which is often used for blood sugar determination, the values are sometimes higher than in the venous blood. The glucose content of the blood is regulated by the hormones [[insulin]], [[incretin]] and [[glucagon]].<ref name="Koekkoek" /><ref>{{Cite book |doi=10.1016/B978-0-444-53480-4.00026-6 |pmid=25410233 |year=2014 |last1=La Fleur |first1=S. E. |series=Handbook of Clinical Neurology |volume=126 |pages=341–351 |last2=Fliers |first2=E. |last3=Kalsbeek |first3=A. |title=Diabetes and the Nervous System |chapter=Neuroscience of glucose homeostasis |isbn=978-0-444-53480-4}}.</ref> Insulin lowers the glucose level, glucagon increases it.<ref name="Satyanarayana" /> Furthermore, the hormones [[adrenaline]], [[thyroxine]], [[glucocorticoid]]s, [[somatotropin]] and [[adrenocorticotropin]] lead to an increase in the glucose level.<ref name="Satyanarayana" /> There is also a hormone-independent regulation, which is referred to as [[glucose autoregulation]].<ref>{{Cite journal |doi=10.1002/cphy.c140009 |pmid=25589267 |year=2015 |last1=Bisschop |first1=P. H. |title=Autonomic regulation of hepatic glucose production |journal=Comprehensive Physiology |volume=5 |issue=1 |pages=147–165 |last2=Fliers |first2=E. |last3=Kalsbeek |first3=A.}}</ref> After food intake the blood sugar concentration increases. Values over 180&nbsp;mg/dL in venous whole blood are pathological and are termed [[hyperglycemia]], values below 40&nbsp;mg/dL are termed [[hypoglycaemia]].<ref>W. A. Scherbaum, B. M. Lobnig<!--: ''Abschnittstitel''.-->, In: Hans-Peter Wolff, Thomas R. Weihrauch: ''Internistische Therapie 2006, 2007''. 16th Edition. Elsevier, 2006, {{ISBN|3-437-23182-0}}, p.&nbsp;927, 985 {{in lang|de}}.</ref> When needed, glucose is released into the bloodstream by glucose-6-phosphatase from glucose-6-phosphate originating from liver and kidney glycogen, thereby regulating the [[homeostasis]] of blood glucose concentration.<ref name="Szablewski" /><ref name="Löffler/Petrides 195" /> In [[ruminant]]s, the blood glucose concentration is lower (60&nbsp;mg/dL in [[cattle]] and 40&nbsp;mg/dL in [[sheep]]), because the carbohydrates are converted more by their gut microbiota into [[short-chain fatty acid]]s.<ref name="Harper">Harold A. Harper: ''Medizinische Biochemie''. Springer-Verlag, 2013, {{ISBN|978-3-662-22150-1}}, p.&nbsp;294.</ref>

Some glucose is converted to [[lactic acid]] by [[astrocyte]]s, which is then utilized as an energy source by [[brain cells]]; some glucose is used by intestinal cells and [[red blood cell]]s, while the rest reaches the [[liver]], [[adipose tissue]] and [[muscle]] cells, where it is absorbed and stored as glycogen (under the influence of [[insulin]]). Liver cell glycogen can be converted to glucose and returned to the blood when insulin is low or absent; muscle cell glycogen is not returned to the blood because of a lack of enzymes. In [[Adipocyte|fat cells]], glucose is used to power reactions that synthesize some [[fat]] types and have other purposes. Glycogen is the body's "glucose energy storage" mechanism, because it is much more "space efficient" and less reactive than glucose itself.

As a result of its importance in human health, glucose is an analyte in [[glucose test]]s that are common medical [[blood test]]s.<ref>{{Cite journal |pmid=22872934 |year=2012 |last1=Clarke |first1=S. F. |title=A history of blood glucose meters and their role in self-monitoring of diabetes mellitus |journal=British Journal of Biomedical Science |volume=69 |issue=2 |pages=83–93 |last2=Foster |first2=J. R. |citeseerx=10.1.1.468.2196 |doi=10.1080/09674845.2012.12002443|s2cid=34263228 }}</ref> Eating or fasting prior to taking a blood sample has an effect on analyses for glucose in the blood; a high fasting glucose blood sugar level may be a sign of [[prediabetes]] or [[diabetes mellitus]].<ref>{{Cite web |url=http://www.diabetes.org/diabetes-basics/diagnosis/ |title=Diagnosing Diabetes and Learning About Prediabetes |website=American Diabetes Association |language=en |access-date=20 February 2018 |url-status=live |archive-url=https://web.archive.org/web/20170728020224/http://www.diabetes.org/diabetes-basics/diagnosis/ |archive-date=28 July 2017}}</ref>

The [[glycemic index]] is an indicator of the speed of resorption and conversion to blood glucose levels from ingested carbohydrates, measured as the [[area under a curve|area under the curve]] of blood glucose levels after consumption in comparison to glucose (glucose is defined as 100).<ref name="Harvey 366">Richard A. Harvey, Denise R. Ferrier: ''Biochemistry''. 5th Edition, Lippincott Williams & Wilkins, 2011, {{ISBN|978-1-608-31412-6}}, p.&nbsp;366.</ref> The clinical importance of the glycemic index is controversial,<ref name="Harvey 366" /><ref name="Satyarayana 508">U Satyanarayana: ''Biochemistry''. Elsevier Health Sciences, 2014, {{ISBN|978-8-131-23713-7}}, p.&nbsp;508.</ref> as foods with high fat contents slow the resorption of carbohydrates and lower the glycemic index, e.g. ice cream.<ref name="Satyarayana 508" /> An alternative indicator is the [[insulin index]],<ref>{{Cite journal |doi=10.1093/ajcn/66.5.1264 |pmid=9356547 |year=1997 |last1=Holt |first1=S. H. |title=An insulin index of foods: The insulin demand generated by 1000-kJ portions of common foods |journal=The American Journal of Clinical Nutrition |volume=66 |issue=5 |pages=1264–1276 |last2=Miller |first2=J. C. |last3=Petocz |first3=P. |doi-access=free}}</ref> measured as the impact of carbohydrate consumption on the blood insulin levels. The [[glycemic load]] is an indicator for the amount of glucose added to blood glucose levels after consumption, based on the glycemic index and the amount of consumed food.

===Precursor===
Organisms use glucose as a precursor for the synthesis of several important substances. Starch, [[cellulose]], and glycogen ("animal starch") are common glucose [[polymer]]s (polysaccharides). Some of these polymers (starch or glycogen) serve as energy stores, while others (cellulose and [[chitin]], which is made from a derivative of glucose) have structural roles. Oligosaccharides of glucose combined with other sugars serve as important energy stores. These include lactose, the predominant sugar in milk, which is a glucose-galactose disaccharide, and sucrose, another disaccharide which is composed of glucose and fructose. Glucose is also added onto certain proteins and [[lipid]]s in a process called [[glycosylation]]. This is often critical for their functioning. The enzymes that join glucose to other molecules usually use [[phosphorylation|phosphorylated]] glucose to power the formation of the new bond by coupling it with the breaking of the glucose-phosphate bond.

Other than its direct use as a monomer, glucose can be broken down to synthesize a wide variety of other biomolecules. This is important, as glucose serves both as a primary store of energy and as a source of organic carbon. Glucose can be broken down and converted into lipids. It is also a precursor for the synthesis of other important molecules such as [[vitamin C]] (ascorbic acid). In living organisms, glucose is converted to several other chemical compounds that are the starting material for various [[metabolic pathway]]s. Among them, all other monosaccharides<ref name="Löffler/Petrides 27">Peter C. Heinrich: ''Löffler/Petrides Biochemie und Pathobiochemie'' {{In lang|de}}. Springer-Verlag, 2014, {{ISBN|978-3-642-17972-3}}, p.&nbsp;27.</ref> such as fructose (via the [[polyol pathway]]),<ref name="Löffler/Petrides 199">Peter C. Heinrich: ''Löffler/Petrides Biochemie und Pathobiochemie'' {{In lang|de}}. Springer-Verlag, 2014, {{ISBN|978-3-642-17972-3}}, p.&nbsp;199, 200.</ref> mannose (the epimer of glucose at position 2), galactose (the epimer at position 4), fucose, various [[uronic acids]] and the [[amino sugar]]s are produced from glucose.<ref name="Löffler/Petrides 214">Peter C. Heinrich: ''Löffler/Petrides Biochemie und Pathobiochemie'' {{In lang|de}}. Springer-Verlag, 2014, {{ISBN|978-3-642-17972-3}}, p.&nbsp;214.</ref> In addition to the phosphorylation to glucose-6-phosphate, which is part of the glycolysis, glucose can be oxidized during its degradation to [[glucono-1,5-lactone]]. Glucose is used in some bacteria as a building block in the [[trehalose]] or the [[dextran]] biosynthesis and in animals as a building block of glycogen. Glucose can also be converted from bacterial [[xylose isomerase]] to fructose. In addition, glucose [[metabolite]]s produce all nonessential amino acids, [[sugar alcohol]]s such as [[mannitol]] and [[sorbitol]], [[fatty acid]]s, [[cholesterol]] and [[nucleic acid]]s.<ref name="Löffler/Petrides 27" /> Finally, glucose is used as a building block in the [[glycosylation]] of proteins to [[glycoprotein]]s, [[glycolipid]]s, [[peptidoglycan]]s, [[glycoside]]s and other substances (catalyzed by [[glycosyltransferase]]s) and can be cleaved from them by [[glycosidase]]s.

=== Regulatory role in cell differentiation ===
In addition to its well-known function as a cellular energy source, glucose has been identified as a master regulator of tissue maturation.<ref>{{Cite web |title=Study reveals glucose's surprising role as master manipulator of tissue maturation |url=https://news.stanford.edu/stories/2025/03/glucose-tissue-regeneration-study-diabetes-cancer |access-date=2025-04-01 |website=news.stanford.edu |language=en}}</ref> A 2025 study by Stanford Medicine uncovered that glucose, in its intact (non-metabolized) form, can bind to various regulatory proteins involved in gene expression. One such protein is IRF6, which alters its conformation upon glucose binding, thereby influencing the expression of genes associated with stem cell differentiation. This regulatory role is independent of glucose’s catabolic function and has been observed across multiple tissue types, including skin, bone, fat, and white blood cells. The research demonstrated that even glucose analogs incapable of metabolism could promote differentiation, suggesting a signaling function for glucose. These findings have potential implications in understanding and treating diseases characterized by impaired differentiation, such as diabetes and certain cancers.<ref>{{cite journal |last1=Lopez-Pajares |first1=Vanessa |last2=Khavari |first2=Paul A. |date=2025-03-21 |title=Glucose is a global regulator of tissue maturation independent of its metabolic role |url=https://med.stanford.edu/news/all-news/2025/03/glucose-tissue-regeneration.html |journal=Cell Stem Cell |publisher=Stanford University School of Medicine}}</ref>