Monosaccharide

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Monosaccharides (from Greek monos: single, sacchar: sugar) are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose (levulose), galactose, xylose and ribose. Monosaccharides are the building blocks of disaccharides such as sucrose and polysaccharides (such as cellulose and starch). Further, each carbon atom that supports a hydroxyl group (except for the first and last) 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 chemical and physical properties.

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Structure

With few exceptions (e.g., deoxyribose), monosaccharides have the chemical formula Cx(H2O)y with the chemical structure H(CHOH)nC=O(CHOH)mH. If n or m is zero, it is an aldehyde and is termed an aldose; otherwise, it is a ketone and is termed a ketose. Monosaccharides contain either a ketone or aldehyde functional group, and hydroxyl groups on most or all of the non-carbonyl carbon atoms.

Fischer projections of Simple Monosaccharides

Structures of simple monosaccharides are conveniently represented using Fischer projections. The following tables shows all possible isomers of simple (C3-C6) D-aldoses and selected D-ketoses. Some of these monosaccharides are only synthetically prepared in the laboratory and not found in nature.

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