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 (e.g., blindness, kidney failure, and peripheral neuropathy) are probably due to the glycosylation of proteins.
In respiration, through a series of enzyme-catalysed reactions, glucose is oxidized to eventually form carbon dioxide and water, yielding energy, mostly in the form of ATP. It is also broken down from polysaccharides before use.
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.
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 over a timescale of hours in aqueous solution, to a final stable ratio of α:β 36:64, in a process called mutarotation. This process can be speeded up.
D-Glucose has the same configuration at its penultimate carbon as D-glyceraldehyde.
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.