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D-Glucose Molecule

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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
A Haworth projection representation of the structure of glucose

Glucose (C6H12O6) 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. 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 ± (alpha) glucose and ² (beta) glucose. Structurally, they differ in the orientation of the hydroxyl group linked to the first carbon in the ring. The ± form has the hydroxyl group "below" the hydrogen (as the molecule is conventionally drawn, as in the figure above), while the ² form has the hydroxyl group "above" the hydrogen. These two forms interconvert on a timescale of hours in aqueous solution, to a ratio of ±:² 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 by the breakdown of glycogen stores (Glucose polymers).
  3. Synthesized in liver and kidneys from intermediates.
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