GLUCOSE is a simple sugar with the molecular formula C 6H 12O 6.
With 6 carbon atoms, it is classed as a hexose , a subcategory of the
monosaccharides . D-
* 1 Function in biology
* 1.1 Energy source * 1.2 Glycolysis * 1.3 Precursors
* 2 Medical uses
* 3 Structure and nomenclature
* 3.1 Open-chain form * 3.2 Cyclic forms * 3.3 Rotational isomers
* 4 Physical properties
* 4.1 Solutions * 4.2 Solid state * 4.3 Optical activity
* 5 Production
* 5.1 Biosynthesis * 5.2 Commercial
* 6 Sources and absorption * 7 History * 8 Research * 9 See also * 10 References * 11 Further reading * 12 External links
FUNCTION IN BIOLOGY
As a result of its importance in human health, glucose is an analyte in common medical blood tests . 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 .
Main article: Glycolysis
Use of glucose as an energy source in cells is by either aerobic respiration, anaerobic respiration, or fermentation. All of these processes follow from an earlier metabolic pathway known as glycolysis . 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 . Furthermore, addition of the high-energy phosphate group activates glucose for subsequent breakdown in later steps of glycolysis. At physiological conditions , this initial reaction is irreversible.
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). 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) through oxidative phosphorylation is generated.
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* ^ The interactive pathway map can be edited at WikiPathways: "GlycolysisGluconeogenesis_WP534".
Organisms use glucose as a precursor for the synthesis of several
Starch , cellulose , and glycogen ("animal
starch") are common glucose polymers (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
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
GLUCOSE IN DIABETES
Diabetes is a metabolic disorder where the body is unable to regulate levels 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. Both of these situations can be caused by persistently high elevations of blood glucose levels, through pancreatic burnout and insulin resistance . The pancreas is the organ responsible for the secretion of insulin . 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. If the pancreas is exposed to persistently high elevations of blood glucose levels, the insulin-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
Individuals with diabetes or other conditions where hypoglycemia (low blood sugar) may occur 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 , or sugar packet .
STRUCTURE AND NOMENCLATURE
In its fleeting open-chain form, the glucose molecule has an open (as opposed to cyclic ) and unbranched backbone of six carbon atoms, C-1 through C-6; where C-1 is part of an aldehyde group H(C=O)-, and each of the other five carbons bears one hydroxyl group -OH. The remaining bonds of the backbone carbons are satisfied by hydrogen atoms -H. Therefore, glucose is both a hexose and an aldose , or an aldohexose . The aldehyde group makes glucose a reducing sugar giving a positive reaction with the Fehling test .
Each of the four carbons C-2 through C-5 is a stereocenter , meaning that its four bonds connect to four different substituents. (Carbon C-2, for example, connects to -(C=O)H, -OH, -H, and -(CHOH)4H.) In D-glucose, these four parts must be in a specific three-dimensional arrangement. Namely, when the molecule is drawn in the Fischer projection , the hydroxyls on C-2, C-4, and C-5 must be on the right side, while that on C-3 must be on the left side.
The positions of those four hydroxyls are exactly reversed in the Fischer diagram of L-glucose . D- and L-glucose are two of the 16 possible aldohexoses; the other 14 are allose , altrose , mannose , gulose , idose , galactose , and talose , each with two enantiomers , "D-" and "L-".
The aldehyde form of glucose
It is important to note that the linear form of glucose makes up less than 3% of the glucose molecules in a water solution. The rest is one of two cyclic forms of glucose that are formed when the hydroxyl group on carbon 5 (C5) bonds to the aldehyde carbon 1 (C1).
Cyclic forms of glucose . From left to right: Haworth projections and ball-and-stick structures of the α- and β- anomers of D-glucopyranose (top row) and D-glucofuranose (bottom row)
In solutions, the open-chain form of glucose (either "D-" or "L-") exists in equilibrium with several cyclic isomers , each containing a ring of carbons closed by one oxygen atom. In aqueous solution however, more than 99% of glucose molecules, at any given time, exist as pyranose forms. The open-chain form is limited to about 0.25% and furanose forms exists 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 intramolecular nucleophilic addition reaction between the aldehyde group (at C-1) and either the C-4 or C-5 hydroxyl group, forming a hemiacetal linkage, -C(OH)H-O-.
The reaction between C-1 and C-5 yields a six-membered heterocyclic system called a pyranose, which is a monosaccharide sugar (hence "–ose") containing a derivatised pyran skeleton. The (much rarer) reaction between C-1 and C-4 yields a five-membered furanose ring, named after the cyclic ether furan . 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 makes carbon C-1 chiral , too, since its
four bonds lead to -H, to -OH, to carbon C-2, and to the ring oxygen.
These four parts of the molecule may be arranged around C-1 (the
anomeric carbon ) in two distinct ways, designated by the prefixes
"α-" and "β-". When a glucopyranose molecule is drawn in the Haworth
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
The other open-chain isomer L-glucose similarly gives rise to four distinct cyclic forms of L-glucose, each the mirror image of the corresponding D-glucose. Chair conformations of α- (left) and β- (right) D-glucopyranose
The rings are not planar, but are twisted in three dimensions. The glucopyranose ring (α or β) can assume several non-planar shapes, analogous to the "chair" and "boat" conformations of cyclohexane . Similarly, the glucofuranose ring may assume several shapes, analogous to the "envelope" conformations of cyclopentane .
In the solid state, only the glucopyranose forms are observed, forming colorless crystalline solids that are highly soluble in water and acetic acid but poorly soluble in methanol and ethanol . They melt at 146 °C (295 °F) (α) and 150 °C (302 °F) (β), and decompose at higher temperatures into carbon and water.
Each glucose isomer is subject to rotational isomerism . Within the cyclic form of glucose, rotation may occur around the O6-C6-C5-O5 torsion angle, termed the ω-angle, to form three staggered rotamer conformations called gauche-gauche (gg), gauche-trans (gt) and trans-gauche (tg). There is a tendency for the ω-angle to adopt a gauche conformation, a tendency that is attributed to the gauche effect .
All forms of glucose are colorless and easily soluble in water, acetic acid , and several other solvents. They are only sparingly soluble in methanol and ethanol .
The open-chain form is thermodynamically unstable , and it spontaneously isomerizes 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 at room temperature , the four cyclic isomers interconvert over a time scale of hours, in a process called mutarotation . 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 the anomeric effect . Mutarotation is considerably slower at temperatures close to 0 °C (32 °F).
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.
Depending on conditions, three major solid forms of glucose can be crystallised from water solutions: α-glucopyranose, β-glucopyranose, and β-glucopyranose hydrate.
Whether in water or in the solid form, D- (+) glucose is dextrorotatory , meaning it will rotate the direction of polarized light clockwise. The effect is due to the chirality of the molecules, and indeed the mirror-image isomer, L- (-)glucose, is levorotatory (rotates polarized light counterclockwise) by the same amount. The strength of the effect is different for each of the five tautomers.
Note that the D- prefix does not refer directly to the optical
properties of the compound. It indicates that the C-5 chiral center
has the same handedness as that of D-glyceraldehyde (which was so
labeled because it is dextrorotatory). The fact that
METABOLISM OF COMMON MONOSACCHARIDES AND SOME BIOCHEMICAL REACTIONS OF GLUCOSE
THIS SECTION NEEDS EXPANSION with: a rigorous, sourced description of glucose biosynthesis in the major phylogenetic divisions. You can help by adding to it . (November 2016)
In plants and some prokaryotes , glucose is a product of photosynthesis. In plants, and in animals and fungi, glucose also is produced by the breakdown of polymeric forms of glucose—glycogen (animals, fungi) or starch (plants); the cleavage of glycogen is termed glycogenolysis , of starch, starch degradation. In animals, glucose is synthesized in the liver and kidneys from non-carbohydrate intermediates, such as pyruvate , lactate and glycerol , in the process of gluconeogenesis . In some deep-sea bacteria , glucose is produced by chemosynthesis .
SOURCES AND ABSORPTION
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 ).
In the lumen of the duodenum and small intestine , the glucose oligo-
and polysaccharides are broken down to monosaccharides by the
pancreatic and intestinal glycosidases. Other polysaccharides cannot
be processed by the human intestine and require assistance by
intestinal flora if they are to be broken down; the most notable
exceptions are sucrose (fructose -glucose) and lactose (galactose
Some glucose is converted to lactic acid by astrocytes, which is then
utilized as an energy source by brain cells ; some glucose is used by
intestinal cells and red blood cells , while the rest reaches the
liver , adipose tissue and muscle cells, where it is absorbed and
stored as glycogen (under the influence of insulin ).
THIS SECTION NEEDS EXPANSION with: a proper encyclopedic description of the history establishing the importance of the molecule, balancing the importance in chemistry with its importance in biochemistry, physiology, and medicine. You can help by adding to it . (November 2016)
* ^ A B Boerio-Goates, Juliana (1991), "Heat-capacity measurements
and thermodynamic functions of crystalline α-