In biochemistry, a sugar is the simplest molecule that can be identified as a carbohydrate. These include monosaccharides and disaccharides, trisaccharides and the oligosaccharides; these being sugars composed of 1, 2, 3 or more units. Sugars contain either aldehyde groups (-CHO) or ketone groups (C=O), where there are carbon-oxygen double bonds, making the sugars reactive. Most sugars conform to (CH2O)n where n is between 3 and 7. A notable exception is deoxyribose, which as the name suggests is "missing" an oxygen. As well as being classified by their reactive group, sugars are also classified by the number of carbons they contain. Derivatives of trioses (C3H6O3) are intermediates in glycolysis. Pentoses ( 5 carbon sugars) include ribose and deoxyribose, which are present in nucleic acids. Ribose is also a component of several chemicals that are important to the metabolic process, including NADH and ATP. Hexoses (6 carbon sugars) include glucose which is a universal substrate for the production of energy in the form of ATP. Through photosynthesis plants produce glucose which is then converted for storage as an energy reserve in the form of other carbohydrates such as starch, or as in cane and beet as sucrose.

Fructose, a monosaccharide

Many pentoses and hexoses are capable of forming ring structures. In these closed-chain forms the aldehyde or ketone group is not free, so many of the reactions typical of these groups cannot occur. Glucose in solution exists mostly in the ring form at equilibrium, with less than 0.1% of the molecules in the open-chain form.

Monosaccharides in a closed-chain form can form glycosidic bonds with other monosaccharides, creating disaccharides, such as sucrose, and polysaccharides such as starch. Glycosidic bonds must be hydrolysed or otherwise broken by enzymes before such compounds can be used in metabolism. After digestion and absorption the principal monosaccharides present in the blood and internal tissues are: glucose, fructose, and galactose.

The term "glyco-" indicates the presence of a sugar in an otherwise non-carbohydrate substance: for example, a glycoprotein is a protein to which one or more sugars are connected.

Simple sugars include sucrose, fructose, glucose, galactose, maltose, lactose and mannose. As far as disaccharides are concerned, the most common are sucrose (cane or beet sugar - made from one glucose and one fructose), lactose (milk sugar - made from one glucose and one galactose) and maltose (made of two glucoses). The formula of these disaccharides is C12H22O11.

Sucrose is a disaccharide of glucose (left) and fructose, important molecules in the body

Sucrose can be converted by hydrolysis into a syrup of fructose and glucose, producing what is called invert sugar. This resulting syrup is sweeter than the original sucrose, and is useful for making confections because it does not crystalize as easily and thus produces a smoother finished product.


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