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Glucose

A space-filling model of glucose

Glucose, a simple monosaccharide sugar, is one of the most important carbohydrates and is used as a source of energy in animals and plants. Glucose is one of the main products of photosynthesis and starts respiration. The natural form (D-glucose) is also referred to as dextrose, especially in the food industry.

A Haworth projection representation of the structure of glucose (α-D-glucopyranose)

Glucose (C₆H₁₂O₆, molecular weight 180.18) is a hexose -- a monosaccharide containing six carbon atoms. Glucose is an aldehyde (contains a -CHO group). Five of the carbons plus an oxygen atom form a loop called a "pyranose ring", the most stable form for six-carbon aldoses. In this ring, each carbon is linked to hydroxyl and hydrogen side groups with the exception of the fifth atom, which links to a 6th carbon atom outside the ring, forming a CH₂OH group. This ring structure exists in equilibrium with a more reactive acyclic form, which makes up 0.0026% at pH 7.

Glucose is a ubiquitous fuel in biology. We can speculate on the reasons why glucose, and not another monosaccharide such as fructose, is so widely used. Glucose can form from formaldehyde under abiotic conditions, so it may well have been available to primitive biochemical systems. Probably more important to advanced life is the low tendency of glucose, by comparison to other hexose sugars, to nonspecifically react with the amino groups of proteins. This reaction (glycosylation) reduces or destroys the function of many enzymes. The low rate of glycosylation is due to glucose's preference for the less reactive cyclic isomer. Nevertheless, many of the long-term complications of diabetes, blindness, kidney failure and peripheral neuropathy for example, are probably due to the glycosylation of proteins.

The chain form of D-Glucose

In respiration, through a series of enzyme-catalysed reactions, glucose is oxidized to eventually to form carbon dioxide and water, yielding energy, mostly in the form of ATP.

Chemically joined together, glucose and fructose form sucrose. Starch, cellulose, and glycogen are common glucose polymers (polysaccharides).

The older name dextrose arose because a solution of D-glucose rotates polarised light towards the right. In the same vein D-fructose was called "levulose" because a solution of levulose rotates polarised light to the left.

Isomerism

There are two enantiomers (mirror-image isomers) of the sugar -- D-glucose and L-glucose, but in living organisms only the D-isomer is found. The ring structure may form in two different ways, yielding α (alpha) glucose and β (beta) glucose. Structurally, they differ in the orientation of the hydroxyl group linked to the first carbon in the ring. The α form has the hydroxyl group "below" the hydrogen (as the molecule is conventionally drawn, as in the figure above), while the β form has the hydroxyl group "above" the hydrogen. These two forms interconvert on a timescale of hours in aqueous solution, to a ratio of α:β 36:64, in a process called mutarotation.

D-Glucose has the same configuration at its penultimate carbon as D-glyceraldehyde.

Synthesis

1. The product of photosynthesis in plants and some prokaryotes.
2. Formed in the liver and skeletal muscle by the breakdown of glycogen stores (glucose polymers).
3. Synthesized in liver and kidneys from intermediates by a process known as gluconeogenesis.

Role in metabolism

Carbohydrates are the human body's key source of energy, providing 4 calories of energy per gram. Breakdown of carbohydrates (e.g. starch) yields mono- and disaccharides, most of which are glucose. Through glycolysis, glucose is immediately involved in the production of ATP, the cell's energy carrier. In addition, it is critical in the production of protein and in lipid metabolism. As the central nervous system does not metabolise lipids, it is more dependent on glucose than other tissues.

Glucose is absorbed into the bloodstream through the intestinal wall. Some of this glucose goes directly to fuel brain cells, while the rest makes its way to the liver and muscles, where it is stored as glycogen ("animal starch"), and to fat cells, where it is stored as fat. Glycogen is the body's auxiliary energy source, tapped and converted back into glucose when it needs more energy. Although stored fat can also serve as a backup source of energy, it is never directly converted into glucose. Fructose and galactose, other sugar products resulting from the breakdown of carbohydrates, go straight to the liver, where they are converted into glucose.

External links

da:Glukose de:Traubenzucker es:Glucosa eo:Glukozo fr:Glucose nl:Glucose ja:グルコース pl:Glukoza sv:Glukos

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This article uses material from Wikipedia and is licensed under the GNU Free Documentation License.