|March 26, 2016|
D-glucose or dextrose or grape sugar is the dextro-isomer of glucose (C6H12O6). The molecule is the mirror-image of L-glucose . D-glucose is a simple sugar ( monosaccharide) and an important carbohydrate in biology. Cells use it as a source of energy and a metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration. Starch and cellulose are polymers derived from the dehydration of glucose. The name "glucose" comes from the Greek word glukus (), meaning "sweet." The suffix " -ose" denotes a sugar.
Glucose can adopt several different structures, but all of these structures can be divided into two families of mirror-images ( stereoisomers). Only one set of these isomers exists in nature, those derived from the "right-handed form" of glucose, denoted D-glucose. D-glucose is often referred to as dextrose. The term dextrose is derived from dextrorotatory glucose . Solutions of dextrose rotate polarized light to the right (in Latin: dexter = "right" ).
Although it is called a "simple sugar" (meaning that it is a monosaccharide), glucose is a complicated molecule because it adopts several different structures. These structures are usually discussed in the context of the acyclic isomer, which exists in only minor amounts in solution.
Glucose is derived from hexanal, a chain of six carbon atoms terminating with an aldehyde group. The other five carbon atoms each bear alcohol groups. Glucose is called an aldo hexose . In solution, glucose mainly exists as the six-membered ring containing a hemiacetal group, which arises from the reaction of the hydroxy group at C-5 and the aldehyde at C-1. Containing five carbon atoms and one oxygen atom, this ring is a derivative of pyran. This cyclic form of glucose is called a glucopyranose, of which two isomers exist.
The asymmetric center at C-1, the site of the hemiacetal, is called the anomeric carbon atom. The ring closing process can give rise to two isomers, called anomers, which are labeled ??-glucose and ??-glucose. These anomers differ in terms of the relative positioning of the hydroxyl group linked to C-1. When D-glucose is drawn as a Haworth projection or in the standard chain conformation, the designation ?? means that the hydroxyl group attached to C-1 is positioned trans to the -CH2OH group at C-5, while ?? means that it is cis. An inaccurate but superficially attractive alternative method of distinguishing ?? from ?? is observing whether the C-1 hydroxyl is below or above the plane of the ring; this may fail if the glucose ring is drawn upside down or in an alternative chair conformation. The ?? and ?? forms interconvert over a timescale of hours in aqueous solution, to a final stable ratio of ??:?? 36:64, in a process called mutarotation. The ratio would be ??:?? 11:89 if it were not for the influence of the anomeric effect.
Aldohexoses have four chiral centers in their acyclic forms (i.e. ignoring the anomeric carbon). Four chiral centers give rise to 24 = 16 stereoisomers. These stereoisomers are classified into two classes with eight sugars in each, which are mirror images of each other. One class is labeled L and the other D. Only seven of these isomers are found in nature, of which D-glucose (Glu), D- galactose (Gal) and D- mannose (Man) are the most important. These eight isomers (including glucose itself) are diastereoisomers and belong to the D series .
Image:Glucose chain structure numbered.svg|
All forms of glucose are colorless and soluble in water. Depending on conditions, three major forms can be crystallised: ??-glucose and ??-glucose, and the hydrated ??-glucose.
In plants and some prokaryotes, glucose is a product of photosynthesis. In animals and fungi, glucose results from 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.
In some deep-sea bacteria glucose is produced by chemosynthesis.
Glucose is produced commercially via the enzymatic hydrolysis of starch. Many crops can be used as the source of starch. Maize, rice, wheat, cassava, corn husk and sago are all used in various parts of the world. In the United States, cornstarch (from maize) is used almost exclusively. Most commercial glucose occurs as a component of invert sugar, an approximately 1:1 mixture of glucose and fructose. In principle, cellulose could be hydrolysed to glucose, but this process is not yet commercially practical.
Scientists can speculate on the reasons why glucose, and not another monosaccharide such as (Fru), is so widely used in organisms. One reason might be that glucose has a lower tendency, relative to other hexose sugars, to react non-specifically with the [[amine|amino ] groups of proteins. This reaction ( glycation) reduces or destroys the function of many enzymes. The low rate of glycation is due to glucose's preference for the less reactive cyclic isomer. Nevertheless, many of the long-term complications of diabetes (e.g., blindness, renal failure, and peripheral neuropathy) are probably due to the glycation of proteins or lipids. In contrast, enzyme-regulated addition of glucose to proteins by glycosylation is often essential to their function.
As an energy source
Glucose is a ubiquitous fuel in biology. It is used as an energy source in most organisms, from bacteria to humans. Use of glucose may be by either aerobic respiration, anaerobic respiration, or fermentation . Carbohydrates are the human body's key source of energy, through aerobic respiration, providing approximately 3.75 kilocalories (16 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 sources, mostly in the form of ATP . The insulin reaction, and other mechanisms, regulate the concentration of glucose in the blood. A high fasting blood sugar level is an indication of prediabetic and diabetic conditions.
Glucose is a primary source of energy for the brain, and hence its availability influences psychological processes. When glucose is low , psychological processes requiring mental effort (e.g., self-control, effortful decision-making) are impaired.
Glucose in glycolysis
Use of glucose as an energy source in cells is via aerobic or anaerobic respiration. Both of these start with the early steps of the glycolysis metabolic pathway. The first step of this is the phosphorylation of glucose by hexokinase to prepare it for later breakdown to provide energy.
The major reason for the immediate phosphorylation of glucose by a hexokinase is to prevent diffusion out of the cell. The phosphorylation adds a charged phosphate group so the glucose 6-phosphate cannot easily cross the cell membrane. Irreversible first steps of a metabolic pathway are common for regulatory purposes.
As a precursor
Glucose is critical in the production of proteins and in lipid metabolism. In plants and most animals, it is also a precursor for vitamin C ( ascorbic acid) production. It is modified for use in these processes by the glycolysis pathway.
Glucose is used as a precursor for the synthesis of several important substances. Starch, cellulose, and glycogen ("animal starch") are common glucose polymers ( polysaccharides). Lactose, the predominant sugar in milk, is a glucose-galactose disaccharide. In sucrose, another important disaccharide, glucose is joined to fructose. These synthesis processes also rely on the phosphorylation of glucose through the first step of glycolysis.
In industry, glucose is used as a precursor to make vitamin C in the Reichstein process, to make citric acid, gluconic acid, bio- ethanol, polylactic acid, sorbitol.
Most dietary carbohydrates contain glucose, either as their only building block, as in starch and glycogen, or together with another monosaccharide, as in sucrose and lactose.
In the lumen of the duodenum and small intestine, the glucose oligo- and polysaccharides are broken down to monosaccharides by the pancreatic and intestinal glycosidases. Other polysaccharides cannot be processed by the human intestine and require assistance by intestinal flora if they are to be broken down; the most notable exceptions are sucrose ( fructose-glucose) and lactose ( galactose-glucose). Glucose is then transported across the apical membrane of the enterocytes by SLC5A1, and later across their basal membrane by SLC2A2. Some of the glucose is directly utilized as an energy source by brain cells, intestinal cells and red blood cells , while the rest reaches the liver, adipose tissue and muscle cells, where it is absorbed and stored as glycogen (under the influence of insulin). Liver cell glycogen can be converted to glucose and returned to the blood when insulin is low or absent; muscle cell glycogen is not returned to the blood because of a lack of enzymes. In fat cells, glucose is used to power reactions that synthesize some fat types and have other purposes. Glycogen is the body's 'glucose energy storage' mechanism because it is much more 'space efficient' and less reactive than glucose itself.
Because glucose is a basic necessity of many organisms, a correct understanding of its chemical makeup and structure contributed greatly to a general advancement in organic chemistry. This understanding occurred largely as a result of the investigations of Emil Fischer, a German chemist who received the 1902 Nobel Prize in Chemistry as a result of his findings. The synthesis of glucose established the structure of organic material and consequently formed the first definitive validation of Jacobus Henricus van't Hoff's theories of chemical kinetics and the arrangements of chemical bonds in carbon-bearing molecules. Between 1891 and 1894, Fischer established the stereochemical configuration of all the known sugars and correctly predicted the possible isomers, applying van't Hoff's theory of asymmetrical carbon atoms.
Glucose is a highly complex chemical compound because it can exist in many isomeric forms and each isomer is subject to rotational isomerism.
GNU Free Documentation License. It uses material from the Wikipedia article "glucose".
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