Glucose is by far the most common carbohydrate and classified as a monosaccharide, an aldose, a hexose, and is a reducing sugar. It is also known as dextrose, because it is dextrorotatory (meaning that as an optical isomer is rotates plane polarized light to the right and also an origin for the D designation.
Glucose (Glc), a monosaccharide , is one of the most important carbohydrates . The cell uses it as a source of energy and metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration . The natural form ( D -glucose) is also referred to as dextrose , especially in the food industry. This article deals with the D-form of glucose (see Isomers -section below)
| Synonym for D-glucose
| Varieties of D-glucose
|| -D-glucose; ¦Â-D-glucose
| Chemical Formula
| Molecular mass
|| 180.16 g mol -1
|| ¦Á-D-glucose: 146¡ãC
Glucose is also called blood sugar as it circulates in the blood at a concentration of 65-110 mg/mL of blood.
Glucose is initially synthesized by chlorophyll in plants using carbon dioxide from the air and sunlight as an energy source. Glucose is further converted to starch for storage.
Structure as following:
Glucose contains six carbon atoms and an aldehyde group and is therefore referred to as an aldo hexose . The glucose molecule can exist in an open-chain (acyclic) and ring (cyclic) form, the latter being the result of an intramolecular reaction between the aldehyde C atom and the C-5 hydroxyl group to form an intramolecular hemiacetal . In water solution both forms are in equilibrium, and at pH 7 the cyclic one is the predominant. As the ring contains 5 carbon and one oxygen atoms, which resembles the structure of pyran , the cyclic form of glucose is also referred to as glucopyranose. In this ring, each carbon is linked to an hydroxyl side group with the exception of the fifth atom, which links to a sixth carbon atom outside the ring, forming a CH2OH group.
Glucose has 4 optic centers which means that in theory glucose can have 15 optical stereoisomers . Only 7 of these are found in living organisms, and of these galactose (Gal) and mannose (Man) are the most important. These eight isomers (including glucose itself) are all diastereoisomers in relation to each other and all belong to the D -series .
An additional asymmetric center at C-1 (called the anomeric carbon atom ) is created when glucose cyclizes and two ring structures, called anomers , can be formed ¡ª ¦Á-glucose and ¦Â-glucose. They differ structurally in the orientation of the hydroxyl group linked to C-1 in the ring. When D -glucose is drawn as a Haworth projection , the designation ¦Á means that the hydroxyl group attached to C-1 is below the plane of the ring, ¦Â means it is above. The ¦Á and ¦Â forms interconvert over a timescale of hours in aqueous solution, to a final stable ratio of ¦Á:¦Â 36:64, in a process called mutarotation .
- Glucose is one of the products of photosynthesis in plants and some prokaryotes .
- In animals and fungi, glucose is the result of the breakdown of glycogen , a process known as glycogenolysis . In plants - the breakdown substrate is starch .
- In animals, glucose is synthesized in the liver and kidneys from non-carbohydrate intermediates, such as pyruvate and glycerol , by a process known as gluconeogenesis .
Glucose is produced commercially via the enzymatic hydrolysis of starch . Many crops can be used as the source of starch Maize , rice , wheat , potato , cassava , arrowroot , and sago are all used in various parts of the world. In the United States , cornstarch (from maize) is used almost exclusively.
This enzymatic process has two stages. Over the course of 1-2 hours near 100 ¡ãC, these enzymes hydrolyze starch into smaller carbohydrates containing on average 5-10 glucose units each. Some variations on this process briefly heat the starch mixture to 130 ¡ãC or hotter one or more times. This heat treatment improves the solubility of starch in water, but deactivates the enzyme, and fresh enzyme must be added to the mixture after each heating.
In the second step, known as saccharification , the partially hydrolyzed starch is completely hydrolyzed to glucose using the glucoamylase enzyme from the fungus Aspergillus niger . Typical reaction conditions are pH 4.0¨C4.5, 60 ¡ãC, and a carbohydrate concentration of 30¨C35% by weight. Under these conditions, starch can be converted to glucose at 96% yield after 1¨C4 days. Still higher yields can be obtained using more dilute solutions, but this approach requires larger reactors and processing a greater volume of water, and is not generally economical. The resulting glucose solution is then purified by filtration and concentrated in a multiple-effect evaporator . Solid D-glucose is then produced by repeated crystallizations .
We can speculate on the reasons why glucose, and not another monosaccharide such as fructose (Fru) , 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 non-specifically 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.
As an energy source
Glucose is a ubiquitous fuel in biology . Carbohydrates are the human body's key source of energy, providing 4 kilocalories (17 kilojoules ) of food energy per gram . Breakdown of carbohydrates (e.g. starch ) yields mono- and disaccharides, most of which is glucose. Through glycolysis and later in the reactions of the Citric acid cycle (TCAC), glucose is oxidized to eventually form CO2 and water , yielding energy, mostly in the form of ATP .