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.
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.
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. It is also broken down from polysaccharides before use.
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. Whether a carbohydrate is D or L has to do with the isomeric
conformation of the hydroxyl on carbon 5. If it is to the right in the Fischer projection, then the ring form will be the D enantiomer, if it is to the left, it
will be the L enantiomer. This is easy to remember, as the D is for "dextro," which is a Latin root for "right," where as L is for "levo" which comes from the Latin root
for "left." The ring structure itself may form in two additionally 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
- The product of photosynthesis in plants and some prokaryotes.
- Formed in the liver and skeletal muscle by the breakdown of glycogen stores
(glucose polymers).
- 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
(17 kilojoules) of food
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.
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