Monosaccharides (/ˌmɒnˈsækərd, ˌmɒnə-/, from Greek monos: single, sacchar: sugar), also called simple sugars, are the most basic units of carbohydrates.[1] They are fundamental units of carbohydrates and cannot be further hydrolyzed to simpler compounds. The general formula is C
. They are the simplest form of sugar and are usually colorless, water-soluble, and crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose (levulose), and galactose. Monosaccharides are the building blocks of disaccharides (such as sucrose and lactose) and polysaccharides (such as cellulose and starch). Further, each carbon atom that supports a hydroxyl group (so, all of the carbons except for the primary and terminal carbon) is chiral, giving rise to a number of isomeric forms, all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but have different physical structures and chemical properties.

With few exceptions (e.g., deoxyribose), monosaccharides have this chemical formula: Cx(H2O)y, where conventionally x ≥ 3. Monosaccharides can be classified by the number x of carbon atoms they contain: triose (3), tetrose (4), pentose (5), hexose (6), heptose (7), and so on.

The most important monosaccharide, glucose, is a hexose. Examples of heptoses include the ketoses , mannoheptulose and sedoheptulose. Monosaccharides with eight or more carbons are rarely observed as they are quite unstable. In aqueous solutions monosaccharides exist as rings if they have more than four carbons.

Simple monosaccharides have a linear and unbranched carbon skeleton with one carbonyl (C=O) functional group, and one hydroxyl (OH) group on each of the remaining carbon atoms. Therefore, the molecular structure of a simple monosaccharide can be written as H(CHOH)n(C=O)(CHOH)mH, where n + 1 + m = x; so that its elemental formula is CxH2xOx.

By convention, the carbon atoms are numbered from 1 to x along the backbone, starting from the end that is closest to the C=O group. Monosaccharides are the simplest units of carbohydrates and the simplest form of sugar.

If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins with a formyl group H(C=O)− and is technically an aldehyde. In that case, the compound is termed an aldose. Otherwise, the molecule has a keto group, a carbonyl −(C=O)− between two carbons; then it is formally a ketone, and is termed a ketose. Ketoses of biological interest usually have the carbonyl at position 2.

The various classifications above can be combined, resulting in names such as "aldohexose" and "ketotriose".

A more general nomenclature for open-chain monosaccharides combines a Greek prefix to indicate the number of carbons (tri-, tetr-, pent-, hex-, etc.) with the suffixes "-ose" for aldoses and "-ulose" for ketoses.[2] In the latter case, if the carbonyl is not at position 2, its position is then indicated by a numeric infix. So, for example, H(C=O)(CHOH)4H is pentose, H(CHOH)(C=O)(CHOH)3H is pentulose, and H(CHOH)2(C=O)(CHOH)2H is pent-3-ulose.

This page was last edited on 25 June 2018, at 22:46 (UTC).
Reference: under CC BY-SA license.

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