Inenergy metabolism, glucose is the most important source of energy in allorganisms. Glucose for metabolism is stored as apolymer, in plants mainly asamylose andamylopectin, and in animals asglycogen. Glucose circulates in the blood of animals asblood sugar.[5][7] The naturally occurring form isd-glucose, while itsstereoisomerl-glucose is produced synthetically in comparatively small amounts and is less biologically active.[7] Glucose is a monosaccharide containing six carbon atoms and analdehyde group, and is therefore analdohexose. The glucose molecule can exist in an open-chain (acyclic) as well as ring (cyclic) form. Glucose is naturally occurring and is found in its free state in fruits and other parts of plants. In animals, it is released from the breakdown of glycogen in a process known asglycogenolysis.
The name glucose is derived fromAncient Greekγλεῦκος (gleûkos) 'wine, must', fromγλυκύς (glykýs) 'sweet'.[10][11] The suffix-ose is a chemical classifier denoting a sugar.
Glucose was first isolated fromraisins in 1747 by the German chemistAndreas Marggraf.[12][13] Glucose was discovered in grapes by another German chemist,Johann Tobias Lowitz, in 1792, and distinguished as being different from cane sugar (sucrose). Glucose is the term coined byJean-Baptiste Dumas in 1838, which has prevailed in the chemical literature.Friedrich August Kekulé proposed the termdextrose (from theLatindexter, meaning "right"), because in aqueous solutions of glucose, the plane of linearly polarized light is turned to the right. In contrast,l-fructose (usually referred to asd-fructose) (a ketohexose) and l-glucose (l-glucose) turn linearlypolarized light to the left. The earlier notation according to the rotation of the plane of linearly polarized light (d andl-nomenclature) was later abandoned in favor of thed- andl-notation, which refers to the absolute configuration of the asymmetric center furthest from the carbonyl group, and in concordance with the configuration ofd- orl-glyceraldehyde.[14][15]
Since glucose is a basic necessity of many organisms, a correct understanding of itschemical makeup and structure contributed greatly to a general advancement inorganic chemistry. This understanding occurred largely as a result of the investigations ofHermann Emil Fischer, a German chemist who received the 1902Nobel Prize in Chemistry for his findings.[16] The synthesis of glucose established the structure of organic material and consequently formed the first definitive validation ofJacobus Henricus van 't Hoff's theories of chemical kinetics and the arrangements of chemical bonds in carbon-bearing molecules.[17] Between 1891 and 1894, Fischer established thestereochemical configuration of all the known sugars and correctly predicted the possibleisomers, applyingVan 't Hoff equation of asymmetrical carbon atoms. The names initially referred to the natural substances. Theirenantiomers were given the same name with the introduction of systematic nomenclatures, taking into account absolute stereochemistry (e.g.Fischer nomenclature,d/l nomenclature).
For the discovery of the metabolism of glucoseOtto Fritz Meyerhof received theNobel Prize in Physiology or Medicine in 1922.[18]Hans von Euler-Chelpin was awarded the Nobel Prize in Chemistry along withArthur Harden in 1929 for their "research on thefermentation of sugar and their share of enzymes in this process".[19][20] In 1947,Bernardo Houssay (for his discovery of the role of thepituitary gland in the metabolism of glucose and the derived carbohydrates) as well asCarl andGerty Cori (for their discovery of the conversion of glycogen from glucose) received the Nobel Prize in Physiology or Medicine.[21][22][23] In 1970,Luis Leloir was awarded the Nobel Prize in Chemistry for the discovery of glucose-derived sugar nucleotides in the biosynthesis of carbohydrates.[24]
Glucose forms white or colorless solids that are highlysoluble in water andacetic acid but poorly soluble inmethanol andethanol. They melt at 146 °C (295 °F) (α) and 150 °C (302 °F) (beta),decompose starting at 188 °C (370 °F) with release of various volatile products, ultimately leaving a residue ofcarbon.[25] Glucose has apKa value of 12.16 at 25 °C (77 °F) in water.[26]
With six carbon atoms, it is classed as ahexose, a subcategory of themonosaccharides.d-Glucose is one of the sixteenaldohexosestereoisomers. Thed-isomer,d-glucose, also known as dextrose, occurs widely in nature, but thel-isomer,l-glucose, does not. Glucose can be obtained byhydrolysis of carbohydrates such as milk sugar (lactose), cane sugar (sucrose),maltose,cellulose,glycogen, etc. Dextrose is commonly commercially manufactured fromstarches, such ascorn starch in the US and Japan, from potato and wheat starch in Europe, and fromtapioca starch in tropical areas.[27] The manufacturing process uses hydrolysis via pressurized steaming at controlledpH in a jet followed by further enzymatic depolymerization.[28] Unbonded glucose is one of the main ingredients ofhoney.[29][30][31][32][33]
The termdextrose is often used in a clinical (related to patient's health status) or nutritional context (related to dietary intake, such as food labels or dietary guidelines), while "glucose" is used in a biological or physiological context (chemical processes and molecular interactions),[34][35][36][37] but both terms refer to the same molecule, specifically D-glucose.[36][38]
Dextrose monohydrate is the hydrated form of D-glucose, meaning that it is a glucose molecule with an additional water molecule attached.[39] Its chemical formula isC6H12O6 · H2O.[39][40] Dextrose monohydrate is also calledhydrated D-glucose, and commonly manufactured from plant starches.[39][41] Dextrose monohydrate is utilized as the predominant type of dextrose in food applications, such as beverage mixes—it is a common form of glucose widely used as a nutrition supplement in production of foodstuffs. Dextrose monohydrate is primarily consumed in North America as acorn syrup orhigh-fructose corn syrup.[36]
Anhydrous dextrose, on the other hand, is glucose that does not have any water molecules attached to it.[41][42] Anhydrous chemical substances are commonly produced by eliminating water from a hydrated substance through methods such as heating or drying up (desiccation).[43][44][45] Dextrose monohydrate can be dehydrated to anhydrous dextrose in industrial setting.[46][47] Dextrose monohydrate is composed of approximately 9.5% water by mass; through the process of dehydration, this water content is eliminated to yield anhydrous (dry) dextrose.[41]
Anhydrous dextrose has the chemical formulaC6H12O6, without any water molecule attached which is the same as glucose.[39] Anhydrous dextrose on open air tends to absorb moisture and transform to the monohydrate, and it is more expensive to produce.[41] Anhydrous dextrose (anhydrous D-glucose) has increased stability and increased shelf life,[44] has medical applications, such as in oralglucose tolerance test.[48]
Whereas molecular weight (molar mass) for D-glucose monohydrate is 198.17 g/mol,[49][50] that for anhydrous D-glucose is 180.16 g/mol[51][52][53] The density of these two forms of glucose is also different.[specify]
In terms of chemical structure, glucose is a monosaccharide, that is, a simple sugar. Glucose contains six carbon atoms and analdehyde group, and is therefore analdohexose. The glucose molecule can exist in anopen-chain (acyclic) as well as ring (cyclic) form—due to the presence ofalcohol andaldehyde orketone functional groups, the form having the straight chain can easily convert into a chair-likehemiacetal ring structure commonly found in carbohydrates.[54]
Glucose is present in solid form as amonohydrate with a closedpyran ring (α-D-glucopyranose monohydrate, sometimes known less precisely by dextrose hydrate). In aqueous solution, on the other hand, a small proportion of glucose can be found in an open-chain configuration while remaining predominantly as α- or β-pyranose, which interconvert. From aqueous solutions, the three known forms can be crystallized: α-glucopyranose, β-glucopyranose and α-glucopyranose monohydrate.[55] Glucose is a building block of the disaccharides lactose and sucrose (cane or beet sugar), ofoligosaccharides such asraffinose and ofpolysaccharides such asstarch,amylopectin,glycogen, andcellulose.[7][56] Theglass transition temperature of glucose is 31 °C (88 °F) and the Gordon–Taylor constant (an experimentally determined constant for the prediction of the glass transition temperature for different mass fractions of a mixture of two substances)[56] is 4.5.[57]
An open-chain form of glucose makes up less than 0.02% of the glucose molecules in an aqueous solution at equilibrium.[58] The rest is one of two cyclic hemiacetal forms. In itsopen-chain form, the glucose molecule has an open (as opposed tocyclic) unbranched backbone of six carbon atoms, where C-1 is part of analdehyde groupH(C=O)−. Therefore, glucose is also classified as analdose, or analdohexose. The aldehyde group makes glucose areducing sugar giving a positive reaction with theFehling test.
In solutions, the open-chain form of glucose (either "D-" or "L-") exists in equilibrium with severalcyclic isomers, each containing a ring of carbons closed by one oxygen atom. In aqueous solution, however, more than 99% of glucose molecules exist aspyranose forms. The open-chain form is limited to about 0.25%, andfuranose forms exist in negligible amounts. The terms "glucose" and "D-glucose" are generally used for these cyclic forms as well. The ring arises from the open-chain form by an intramolecularnucleophilic addition reaction between the aldehyde group (at C-1) and either the C-4 or C-5 hydroxyl group, forming ahemiacetal linkage,−C(OH)H−O−.
The reaction between C-1 and C-5 yields a six-memberedheterocyclic system called a pyranose, which is a monosaccharide sugar (hence "-ose") containing a derivatisedpyran skeleton. The (much rarer) reaction between C-1 and C-4 yields a five-membered furanose ring, named after the cyclic etherfuran. In either case, each carbon in the ring has one hydrogen and one hydroxyl attached, except for the last carbon (C-4 or C-5) where the hydroxyl is replaced by the remainder of the open molecule (which is−(C(CH2OH)HOH)−H or−(CHOH)−H respectively).
The ring-closing reaction can give two products, denoted "α-" and "β-". When a glucopyranose molecule is drawn in theHaworth projection, the designation "α-" means that the hydroxyl group attached to C-1 and the−CH2OH group at C-5 lies on opposite sides of the ring's plane (a trans arrangement), while "β-" means that they are on the same side of the plane (a cis arrangement). Therefore, the open-chain isomerD-glucose gives rise to four distinct cyclic isomers: α-D-glucopyranose, β-D-glucopyranose, α-D-glucofuranose, and β-D-glucofuranose. These five structures exist in equilibrium and interconvert, and the interconversion is much more rapid with acidcatalysis.
Widely proposed arrow-pushing mechanism for acid-catalyzed dynamic equilibrium between the α- and β-anomers of D-glucopyranose
The other open-chain isomerL-glucose similarly gives rise to four distinct cyclic forms ofL-glucose, each the mirror image of the correspondingD-glucose.
The glucopyranose ring (α or β) can assume several non-planar shapes, analogous to the "chair" and "boat" conformations ofcyclohexane. Similarly, the glucofuranose ring may assume several shapes, analogous to the "envelope" conformations ofcyclopentane.
In the solid state, only the glucopyranose forms are observed.
Some derivatives of glucofuranose, such as1,2-O-isopropylidene-D-glucofuranose are stable and can be obtained pure as crystalline solids.[59][60] For example, reaction of α-D-glucose withpara-tolylboronic acidH3C−(C6H4)−B(OH)2 reforms the normal pyranose ring to yield the 4-fold ester α-D-glucofuranose-1,2:3,5-bis(p-tolylboronate).[61]
Mutarotation:d-glucose molecules exist as cyclic hemiacetals that are epimeric (= diastereomeric) to each other. The epimeric ratio α:β is 36:64. In the α-D-glucopyranose (left), the blue-labelled hydroxy group is in the axial position at the anomeric centre, whereas in the β-D-glucopyranose (right) the blue-labelled hydroxy group is in equatorial position at the anomeric centre.
Mutarotation consists of a temporary reversal of the ring-forming reaction, resulting in the open-chain form, followed by a reforming of the ring. The ring closure step may use a different−OH group than the one recreated by the opening step (thus switching between pyranose and furanose forms), or the new hemiacetal group created on C-1 may have the same or opposite handedness as the original one (thus switching between the α and β forms). Thus, though the open-chain form is barely detectable in solution, it is an essential component of the equilibrium.
The open-chain form isthermodynamically unstable, and it spontaneouslyisomerizes to the cyclic forms. (Although the ring closure reaction could in theory create four- or three-atom rings, these would be highly strained, and are not observed in practice.) In solutions atroom temperature, the four cyclic isomers interconvert over a time scale of hours, in a process calledmutarotation.[62] Starting from any proportions, the mixture converges to a stable ratio of α:β 36:64. The ratio would be α:β 11:89 if it were not for the influence of theanomeric effect.[63] Mutarotation is considerably slower at temperatures close to 0 °C (32 °F).
Whether in water or the solid form,d-(+)-glucose isdextrorotatory, meaning it will rotate the direction ofpolarized light clockwise as seen looking toward the light source. The effect is due to thechirality of the molecules, and indeed the mirror-image isomer,l-(−)-glucose, islevorotatory (rotates polarized light counterclockwise) by the same amount. The strength of the effect is different for each of the fivetautomers.
Thed- prefix does not refer directly to the optical properties of the compound. It indicates that the C-5 chiral centre has the same handedness as that ofd-glyceraldehyde (which was so labelled because it is dextrorotatory). The fact thatd-glucose is dextrorotatory is a combined effect of its four chiral centres, not just of C-5; some of the otherd-aldohexoses are levorotatory.
The conversion between the two anomers can be observed in apolarimeter since pure α-d-glucose has a specific rotation angle of +112.2° mL/(dm·g), pure β-d-glucose of +17.5° mL/(dm·g).[64] When equilibrium has been reached after a certain time due to mutarotation, the angle of rotation is +52.7° mL/(dm·g).[64] By adding acid or base, this transformation is much accelerated. The equilibration takes place via the open-chain aldehyde form.
Metabolism of commonmonosaccharides and some biochemical reactions of glucose
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 theamine groups ofproteins.[65] This reaction—glycation—impairs or destroys the function of many proteins,[65] e.g. inglycated hemoglobin. Glucose's low rate of glycation can be attributed to its having a more stablecyclic form compared to other aldohexoses, which means it spends less time than they do in its reactiveopen-chain form.[65] The reason for glucose having the most stable cyclic form of all the aldohexoses is that itshydroxy groups (with the exception of the hydroxy group on the anomeric carbon ofd-glucose) are in theequatorial position. Presumably, glucose is the most abundant natural monosaccharide because it is less glycated with proteins than other monosaccharides.[65][66] Another hypothesis is that glucose, being the onlyd-aldohexose that has all five hydroxy substituents in theequatorial position in the form of β-d-glucose, is more readily accessible to chemical reactions,[67]: 194, 199 for example, foresterification[68]: 363 oracetal formation.[69] For this reason,d-glucose is also a highly preferred building block in natural polysaccharides (glycans). Polysaccharides that are composed solely of glucose are termedglucans.
Glucose is produced by plants through photosynthesis using sunlight,[70][71] 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 andchitin, which are components of the cell wall in plants orfungi andarthropods, 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.Neurons, cells of therenal medulla, anderythrocytes depend on glucose for their energy production.[71] In adult humans, there is about 18 g (0.63 oz) of glucose,[72] of which about 4 g (0.14 oz) is present in the blood.[73] Approximately 180–220 g (6.3–7.8 oz) of glucose is produced in the liver of an adult in 24 hours.[72]
Ingested glucose initially binds to the receptor for sweet taste on the tongue in humans. This complex of the proteinsT1R2 andT1R3 makes it possible to identify glucose-containing food sources.[76][77] Glucose mainly comes from food—about 300 g (11 oz) per day is produced by conversion of food,[77] 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 duringchewing by means ofamylase, which is contained insaliva, as well as bymaltase,lactase, andsucrase on thebrush border of thesmall 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,sphingolipids,mucopolysaccharides, and poly(ADP-ribose). Humans do not produce cellulases, chitinases, or trehalases, but the bacteria in thegut 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 themajor facilitator superfamily. In the small intestine (more precisely, in thejejunum),[78] glucose is taken up into the intestinalepithelium with the help ofglucose transporters[79] via asecondary active transport mechanism called sodium ion-glucosesymport viasodium/glucose cotransporter 1 (SGLT1).[80] Further transfer occurs on thebasolateral side of the intestinal epithelial cells via the glucose transporterGLUT2,[80] as well uptake intoliver cells, kidney cells, cells of theislets of Langerhans,neurons,astrocytes, andtanycytes.[81] Glucose enters the liver via theportal vein and is stored there as a cellular glycogen.[82] In the liver cell, it isphosphorylated byglucokinase at position 6 to formglucose 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.[80] In the other cell types, phosphorylation occurs through ahexokinase, whereupon glucose can no longer diffuse out of the cell.
The glucose transporterGLUT1 is produced by most cell types and is of particular importance for nerve cells and pancreaticβ-cells.[80]GLUT3 is highly expressed in nerve cells.[80] Glucose from the bloodstream is taken up byGLUT4 frommuscle cells (of theskeletal muscle[83] andheart muscle) andfat cells.[84]GLUT14 is expressed exclusively intesticles.[85] Excess glucose is broken down and converted into fatty acids, which are stored astriglycerides. In thekidneys, glucose in the urine is absorbed via SGLT1 andSGLT2 in the apical cell membranes and transmitted via GLUT2 in the basolateral cell membranes.[86] About 90% of kidney glucose reabsorption is via SGLT2 and about 3% via SGLT1.[87]
In plants and someprokaryotes, glucose is a product ofphotosynthesis.[70] Glucose is also formed by the breakdown of polymeric forms of glucose likeglycogen (in animals andmushrooms) or starch (in plants). The cleavage of glycogen is termed glycogenolysis, the cleavage of starch is called starch degradation.[88]
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 allbiomolecules come from the assimilation of carbon dioxide in plants and microbes during photosynthesis.[68]: 359 The free energy of formation of α-d-glucose is 917.2 kilojoules per mole.[68]: 59 In humans, gluconeogenesis occurs in the liver and kidney,[89] but also in other cell types. In the liver about 150 g (5.3 oz) of glycogen are stored, in skeletal muscle about 250 g (8.8 oz).[90] 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 forglucose-6-phosphate. Gluconeogenesis allows the organism to build up glucose from other metabolites, includinglactate or certainamino acids, while consuming energy. The renaltubular cells 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.[91]
Glucose metabolism and various forms of it in the process.Glucose-containing compounds andisomeric forms are digested and taken up by the body in the intestines, includingstarch,glycogen,disaccharides, andmonosaccharides.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[92] and the pentose phosphate pathway.[93] Glycolysis is used by all living organisms,[67]: 551 [94] with small variations, and all organisms generate energy from the breakdown of monosaccharides.[94] In the further course of the metabolism, it can be completely degraded viaoxidative decarboxylation, thecitric acid cycle (synonymKrebs cycle) and therespiratory 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 metaboliteacetyl-CoA from the Krebs cycle can also be used forfatty acid synthesis.[95] Glucose is also used to replenish the body's glycogen stores, which are mainly found in liver and skeletal muscle. These processes arehormonally regulated.
In other living organisms, other forms of fermentation can occur. The bacteriumEscherichia coli can grow on nutrient media containing glucose as the sole carbon source.[68]: 59 In some bacteria and, in modified form, also in archaea, glucose is degraded via theEntner-Doudoroff pathway.[96] With glucose, a mechanism forgene regulation was discovered inE. coli, thecatabolite repression (formerly known asglucose effect).[97]
Use of glucose as an energy source in cells is by either aerobic respiration, anaerobic respiration, or fermentation.[98] The first step of glycolysis is thephosphorylation of glucose by ahexokinase to formglucose 6-phosphate. The main reason for the immediate phosphorylation of glucose is to prevent its diffusion out of the cell as the chargedphosphate group prevents glucose 6-phosphate from easily crossing thecell membrane.[98] Furthermore, addition of the high-energy phosphate groupactivates glucose for subsequent breakdown in later steps of glycolysis.[99]
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).[100] 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.[101]
Click on genes, proteins and metabolites below to link to respective articles.[§ 1]
Tumor cells often grow comparatively quickly and consume an above-average amount of glucose by glycolysis,[102] which leads to the formation of lactate, the end product of fermentation in mammals, even in the presence of oxygen. This is called theWarburg effect. For the increased uptake of glucose in tumors various SGLT and GLUT are overly produced.[103][104]
Inyeast, ethanol is fermented at high glucose concentrations, even in the presence of oxygen (which normally leads to respiration rather than fermentation). This is called theCrabtree 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 °C and a pH of 2.5.[105]
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 inbiology. It is used as an energy source in organisms, from bacteria to humans, through eitheraerobic respiration,anaerobic respiration (in bacteria), orfermentation. Glucose is the human body's key source of energy, through aerobic respiration, providing about 3.75 kilocalories (16 kilojoules) offood energy per gram.[106] Breakdown of carbohydrates (e.g., starch) yields mono- anddisaccharides, most of which is glucose. Throughglycolysis and later in the reactions of thecitric acid cycle andoxidative phosphorylation, glucose isoxidized to eventually formcarbon dioxide and water, yielding energy mostly in the form ofadenosine 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[107] or 15.7 kJ/g (3.74 kcal/g).[108] 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 ofNADPH as a reductant foranabolism that would otherwise have to be generated indirectly.[109]
Glucose and oxygen supply almost all the energy for thebrain,[110] so its availability influencespsychological processes. Whenglucose is low, psychological processes requiring mental effort (e.g.,self-control, effortful decision-making) are impaired.[111][112][113][114] In the brain, which is dependent on glucose and oxygen as the major source of energy, the glucose concentration is usually 4 to 6 mM (5 mM equals 90 mg/dL),[72] but decreases to 2 to 3 mM when fasting.[115]Confusion occurs below 1 mM andcoma at lower levels.[115]
The glucose in the blood is calledblood sugar. Blood sugar levels are regulated by glucose-binding nerve cells in thehypothalamus.[116] In addition, glucose in the brain binds to glucose receptors of thereward system in thenucleus accumbens.[116] 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.[117]Artificial sweeteners do not lower blood sugar levels.[117]
The blood sugar content of a healthy person in the short-time fasting state, e.g. after overnight fasting, is about 70 to 100 mg/dL of blood (4 to 5.5 mM). Inblood plasma, the measured values are about 10–15% higher. In addition, the values in thearterial blood are higher than the concentrations in thevenous blood since glucose is absorbed into the tissue during the passage of thecapillary 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 hormonesinsulin,incretin, andglucagon.[116][118] Insulin lowers the glucose level, glucagon increases it.[72] Furthermore, the hormonesadrenaline,thyroxine,glucocorticoids,somatotropin, andadrenocorticotropin lead to an increase in the glucose level.[72] There is also a hormone-independent regulation, which is referred to asglucose autoregulation.[119] After food intake the blood sugar concentration increases. Values over 180 mg/dL in venous whole blood are pathological and are termedhyperglycemia, values below 40 mg/dL are termedhypoglycaemia.[120] When needed, glucose is released into the bloodstream by glucose-6-phosphatase from glucose-6-phosphate originating from liver and kidney glycogen, thereby regulating thehomeostasis of blood glucose concentration.[89][71] Inruminants, the blood glucose concentration is lower (60 mg/dL incattle and 40 mg/dL insheep), because the carbohydrates are converted more by their gut microbiota intoshort-chain fatty acids.[121]
Some glucose is converted tolactic acid byastrocytes, which is then utilized as an energy source bybrain cells; some glucose is used by intestinal cells andred blood cells, while the rest reaches theliver,adipose tissue, andmuscle cells, where it is absorbed and stored as glycogen (under the influence ofinsulin). 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. Infat cells, glucose is used to power reactions that synthesize somefat 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 inglucose tests that are common medicalblood tests.[122] 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 ofprediabetes ordiabetes mellitus.[123]
Theglycemic index is an indicator of the speed of resorption and conversion to blood glucose levels from ingested carbohydrates, measured as thearea under the curve of blood glucose levels after consumption in comparison to glucose (glucose is defined as 100).[124] The clinical importance of the glycemic index is controversial,[124][125] as foods with high fat contents slow the resorption of carbohydrates and lower the glycemic index, e.g. ice cream.[125] An alternative indicator is theinsulin index,[126] measured as the impact of carbohydrate consumption on the blood insulin levels. Theglycemic 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.
Organisms use glucose as a precursor for the synthesis of several important substances. Starch,cellulose, and glycogen ("animal starch") are common glucosepolymers (polysaccharides). Some of these polymers (starch or glycogen) serve as energy stores, while others (cellulose andchitin, 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 andlipids in a process calledglycosylation. This is often critical for their functioning. The enzymes that join glucose to other molecules usually usephosphorylated 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 asvitamin C (ascorbic acid). In living organisms, glucose is converted to several other chemical compounds that are the starting material for variousmetabolic pathways. Among them, all other monosaccharides[127] such as fructose (via thepolyol pathway),[80] mannose (the epimer of glucose at position 2), galactose (the epimer at position 4), fucose, variousuronic acids, and theamino sugars are produced from glucose.[82] In addition to the phosphorylation to glucose-6-phosphate, which is part of the glycolysis, glucose can be oxidized during its degradation toglucono-1,5-lactone. Glucose is used in some bacteria as a building block in thetrehalose or thedextran biosynthesis and in animals as a building block of glycogen. Glucose can also be converted from bacterialxylose isomerase to fructose. In addition, glucosemetabolites produce all nonessential amino acids,sugar alcohols such asmannitol andsorbitol,fatty acids,cholesterol, andnucleic acids.[127] Finally, glucose is used as a building block in theglycosylation of proteins toglycoproteins,glycolipids,peptidoglycans,glycosides, and other substances (catalyzed byglycosyltransferases) and can be cleaved from them byglycosidases.
In addition to its well-known function as a cellular energy source, glucose has been identified as a master regulator of tissue maturation.[128] 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.[129]
Diabetes is a metabolic disorder where the body is unable to regulatelevels of glucose in the blood either because of a lack of insulin in the body or the failure, by cells in the body, to respond properly to insulin. Each of these situations can be caused by persistently high elevations of blood glucose levels, through pancreatic burnout andinsulin resistance. Thepancreas is the organ responsible for the secretion of the hormones insulin and glucagon.[130] Insulin is a hormone that regulates glucose levels, allowing the body's cells to absorb and use glucose. Without it, glucose cannot enter the cell and therefore cannot be used as fuel for the body's functions.[131] If the pancreas is exposed to persistently high elevations of blood glucose levels, theinsulin-producing cells in the pancreas could be damaged, causing a lack of insulin in the body. Insulin resistance occurs when the pancreas tries to produce more and more insulin in response to persistently elevated blood glucose levels. Eventually, the rest of the body becomes resistant to the insulin that the pancreas is producing, thereby requiring more insulin to achieve the same blood glucose-lowering effect, and forcing the pancreas to produce even more insulin to compete with the resistance. This negative spiral contributes to pancreatic burnout, and the disease progression of diabetes.
To monitor the body's response to blood glucose-lowering therapy, glucose levels can be measured.Blood glucose monitoring can be performed by multiple methods, such as the fasting glucose test which measures the level of glucose in the blood after 8 hours of fasting. Another test is the 2-hour glucose tolerance test (GTT) – for this test, the person has a fasting glucose test done, then drinks a 75-gram glucose drink and is retested. This test measures the ability of the person's body to process glucose. Over time the blood glucose levels should decrease as insulin allows it to be taken up by cells and exit the blood stream.
Individuals with diabetes or other conditions that result inlow blood sugar often carry small amounts of sugar in various forms. One sugar commonly used is glucose, often in the form of glucose tablets (glucose pressed into a tablet shape sometimes with one or more other ingredients as a binder),hard candy, orsugar packet.
Most dietary carbohydrates contain glucose, either as their only building block (as in the polysaccharides starch and glycogen), or together with another monosaccharide (as in the hetero-polysaccharides sucrose and lactose).[132] Unbound glucose is one of the main ingredients of honey. Glucose is extremely abundant and has been isolated from a variety of natural sources across the world, including male cones of the coniferous treeWollemia nobilis in Rome,[133] the roots ofIlex asprella plants in China,[134] and straws from rice in California.[135]
Sugar content of selected common plant foods (in grams per 100 g)[136]
^The carbohydrate value is calculated in the USDA database and does not always correspond to the sum of the sugars, the starch, and the "dietary fiber".
Glucose is produced industrially from starch byenzymatichydrolysis usingglucose amylase or by the use ofacids. Enzymatic hydrolysis has largely displaced acid-catalyzed hydrolysis reactions.[137] The result is glucose syrup (enzymatically with more than 90% glucose in the dry matter)[137] with an annual worldwide production volume of 20 million tonnes (as of 2011).[138] This is the reason for the former common name "starch sugar". The amylases most often come fromBacillus licheniformis[139] orBacillus subtilis (strain MN-385),[139] which are more thermostable than the originally used enzymes.[139][140] Starting in 1982,pullulanases fromAspergillus niger were used in the production of glucose syrup to convert amylopectin to starch (amylose), thereby increasing the yield of glucose.[141] The reaction is carried out at a pH = 4.6–5.2 and a temperature of 55–60 °C.[12]Corn syrup has between 20% and 95% glucose in the dry matter.[142][143] The Japanese form of the glucose syrup,Mizuame, is made fromsweet potato orrice starch.[144]
Many crops can be used as the source of starch.Maize,[137] rice,[137]wheat,[137]cassava,[137]potato,[137]barley,[137] sweet potato,[145]corn husk andsago are all used in various parts of the world. In theUnited States,corn starch (from maize) is used almost exclusively. Some commercial glucose occurs as a component ofinvert sugar, a roughly 1:1 mixture of glucose and fructose that is produced from sucrose. In principle, cellulose could be hydrolyzed to glucose, but this process is not yet commercially practical.[55]
In the US, almost exclusively corn[citation needed] (more precisely, corn syrup) is used as glucose source for the production ofisoglucose, which is a mixture of glucose and fructose, since fructose has a higher sweetening power – with same physiological calorific value of 374 kilocalories per 100 g.[citation needed] The annual world production of isoglucose is 8 million tonnes (as of 2011).[138] When made from corn syrup, the final product ishigh-fructose corn syrup (HFCS).
Relative sweetness of various sugars in comparison with sucrose[146]
Glucose is mainly used for the production of fructose and of glucose-containing foods. In foods, it is used as a sweetener,humectant, to increase thevolume and to create a softermouthfeel.[137] Various sources of glucose, such as grape juice (for wine) or malt (for beer), are used for fermentation to ethanol during the production ofalcoholic beverages. Most soft drinks in the US use HFCS-55 (with a fructose content of 55% in the dry mass), while most other HFCS-sweetened foods in the US use HFCS-42 (with a fructose content of 42% in the dry mass).[147] In Mexico, on the other hand, soft drinks are sweetened by cane sugar, which has a higher sweetening power.[148] In addition, glucose syrup is used, inter alia, in the production ofconfectionery such ascandies,toffee, andfondant.[149] Typical chemical reactions of glucose when heated under water-free conditions arecaramelization and, in presence of amino acids, theMaillard reaction.
TheFehling test is a classic method for the detection of aldoses.[153] Due to mutarotation, glucose is always present to a small extent as an open-chain aldehyde. By adding the Fehling reagents (Fehling (I) solution and Fehling (II) solution), the aldehyde group is oxidized to acarboxylic acid, while the Cu2+ tartrate complex is reduced to Cu+ and forms a brick red precipitate (Cu2O).
InBarfoed's test,[155] a solution of dissolvedcopper acetate,sodium acetate, and acetic acid is added to the solution of the sugar to be tested and subsequently heated in a water bath for a few minutes. Glucose and other monosaccharides rapidly produce a reddish color and reddish browncopper(I) oxide (Cu2O).
Upon heating a dilutepotassium hydroxide solution with glucose to 100 °C, a strong reddish browning and a caramel-like odor develops.[157] Concentratedsulfuric acid dissolves dry glucose without blackening at room temperature forming sugar sulfuric acid.[157][verification needed] In a yeast solution, alcoholic fermentation produces carbon dioxide in the ratio of 2.0454 molecules of glucose to one molecule ofCO2.[157] Glucose forms a black mass withstannous chloride.[157] In an ammoniacal silver solution, glucose (as well as lactose and dextrin) leads to the deposition of silver. In an ammoniacallead acetate solution, whitelead glycoside is formed in the presence of glucose, which becomes less soluble on cooking and turns brown.[157] In an ammoniacal copper solution, yellowcopper oxide hydrate is formed with glucose at room temperature, while red copper oxide is formed during boiling (same with dextrin, except for with an ammoniacal copper acetate solution).[157] WithHager's reagent, glucose formsmercury oxide during boiling.[157] An alkalinebismuth solution is used to precipitate elemental, black-brown bismuth with glucose.[157] Glucose boiled in anammonium molybdate solution turns the solution blue. A solution withindigo carmine andsodium carbonate destains when boiled with glucose.[157]
In concentrated solutions of glucose with a low proportion of other carbohydrates, its concentration can be determined with a polarimeter. For sugar mixtures, the concentration can be determined with arefractometer, for example in theOechsle determination in the course of the production of wine.
The enzyme glucose oxidase (GOx) converts glucose into gluconic acid and hydrogen peroxide while consuming oxygen. Another enzyme, peroxidase, catalyzes a chromogenic reaction (Trinder reaction)[158] ofphenol with4-aminoantipyrine to a purple dye.[159]
The test-strip method employs the above-mentioned enzymatic conversion of glucose to gluconic acid to form hydrogen peroxide. The reagents are immobilised on a polymer matrix, the so-called test strip, which assumes a more or less intense color. This can be measured reflectometrically at 510 nm with the aid of an LED-based handheld photometer. This allows routine blood sugar determination by nonscientists. In addition to the reaction of phenol with 4-aminoantipyrine, new chromogenic reactions have been developed that allow photometry at higher wavelengths (550 nm, 750 nm).[159][160]
The electroanalysis of glucose is also based on the enzymatic reaction mentioned above. The produced hydrogen peroxide can be amperometrically quantified by anodic oxidation at a potential of 600 mV.[161] The GOx is immobilized on the electrode surface or in a membrane placed close to the electrode. Precious metals such as platinum or gold are used in electrodes, as well as carbon nanotube electrodes, which e.g. are doped with boron.[162] Cu–CuO nanowires are also used as enzyme-free amperometric electrodes, reaching a detection limit of 50 μmol/L.[163] A particularly promising method is the so-called "enzyme wiring", where the electron flowing during the oxidation is transferred via a molecular wire directly from the enzyme to the electrode.[164]
There are a variety of other chemical sensors for measuring glucose.[165][166] Given the importance of glucose analysis in the life sciences, numerous optical probes have also been developed for saccharides based on the use of boronic acids,[167] which are particularly useful for intracellular sensory applications where other (optical) methods are not or only conditionally usable. In addition to the organic boronic acid derivatives, which often bind highly specifically to the 1,2-diol groups of sugars, there are also other probe concepts classified by functional mechanisms which use selective glucose-binding proteins (e.g. concanavalin A) as a receptor. Furthermore, methods were developed which indirectly detect the glucose concentration via the concentration of metabolized products, e.g. by the consumption of oxygen using fluorescence-optical sensors.[168] Finally, there are enzyme-based concepts that use the intrinsic absorbance or fluorescence of (fluorescence-labeled) enzymes as reporters.[165]
In particular, for the analysis of complex mixtures containing glucose, e.g. in honey, chromatographic methods such ashigh performance liquid chromatography andgas chromatography[169] are often used in combination withmass spectrometry.[170][171] Taking into account the isotope ratios, it is also possible to reliably detect honey adulteration by added sugars with these methods.[172] Derivatization using silylation reagents is commonly used.[173] Also, the proportions of di- and trisaccharides can be quantified.
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