Monosaccharide Definition and Functions

A monosaccharide or simple sugar is a carbohydrate that cannot be hydrolyzed into smaller carbohydrates. Like all carbohydrates, a monosaccharide consists of three chemical elements: carbon, hydrogen, and oxygen. It is the simplest type of carbohydrate molecule and often serves as the basis for forming more complex molecules.

Monosaccharides include aldoses, ketoses, and their derivatives. The general chemical formula for a monosaccharide is C_nH_2nO_n or (CH₂O)_n. Examples of monosaccharides include the three most common forms: glucose (dextrose), fructose (levulose), and galactose.

Properties

In pure form, monosaccharides are crystalline, water-soluble, colorless solids. Monosaccharides have a sweet flavor because the orientation of the OH group interacts with the taste receptor on the tongue that detects sweetness. Via a dehydration reaction, two monosaccharides can form a disaccharide, three to ten can form an oligosaccharide, and more than ten can form a polysaccharide.

Functions

Monosaccharides serve two main functions within a cell. They are used to store and produce energy. Glucose is a particularly important energy molecule. Energy is released when its chemical bonds are broken. Monosaccharides are also used as building blocks to form more complex sugars, which are important structural elements.

Structure and Nomenclature

The chemical formula (CH₂O)_n indicates a monosaccharide is a carbon hydrate. However, the chemical formula doesn't indicate the placement of the carbon atom within the molecule or the chirality of the sugar. Monosaccharides are classified based on how many carbon atoms they contain, the placement of the carbonyl group, and their stereochemistry.

The n in the chemical formula indicates the number of carbon atoms in a monosaccharide. Each simple sugar contains three or more carbon atoms. They are categorized by the number of carbons: triose (3), tetrose (4), pentose (5), hexose (6), and heptose (7). Note, all of these classes are named with the -ose ending, indicating they are carbohydrates. Glyceraldehyde is a triose sugar. Erythrose and threose are examples of tetrose sugars. Ribose and xylose are examples of pentose sugars. The most abundant simple sugars are hexose sugars. These include glucose, fructose, mannose, and galactose. Sedoheptulose and mannoheptulose are examples of heptose monosaccharides.

Aldoses have more than one hydroxyl group (-OH) and a carbonyl group (C=O) at the terminal carbon, while ketoses have the hydroxyl group and carbonyl group attached to the second carbon atom.

The classification systems may be combined to describe a simple sugar. For example, glucose is an aldohexose, while ribose is a ketohexose.

Linear vs. Cyclic

Monosaccharides may exist as straight-chain (acyclic) molecules or as rings (cyclic). The ketone or aldehyde group of a straight molecule can reversibly react with a hydroxyl group on another carbon to form a heterocyclic ring. In the ring, an oxygen atom bridges two carbon atoms. Rings made of five atoms are called furanose sugars, while those consisting of six atoms are the pyranose form. In nature, the straight-chain, furanose, and pyranose forms exist in equilibrium. Calling a molecule "glucose" could refer to straight-chain glucose, glucofuranose, glucopyranose, or a mixture of the forms.

Stereochemistry

Monosaccharides exhibit stereochemistry. Each simple sugar can be in either D- (dextro) or L- (levo) form. The D- and L- forms are mirror images of each other. Natural monosaccharides are in the D- form, while synthetically produced monosaccharides are usually in the L-form.

Cyclic monosaccharides also display stereochemistry. The -OH group replacing oxygen from the carbonyl group can be in one of two positions (commonly drawn above or below the ring). The isomers are indicated using the prefixes α- and β-.

Sources

• Fearon, W.F. (1949). Introduction to Biochemistry (2nd ed.). London: Heinemann. ISBN 9781483225395.
• IUPAC (1997) Compendium of Chemical Terminology (2nd ed.). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications. Oxford. doi:10.1351/goldbook.M04021 ISBN 0-9678550-9-8.
• McMurry, John. (2008). Organic Chemistry (7th ed.). Belmont, CA: Thomson Brooks/Cole.
• Pigman, W.; Horton, D. (1972). "Chapter 1: Stereochemistry of the Monosaccharides". In Pigman and Horton (ed.). The Carbohydrates: Chemistry and Biochemistry Vol 1A (2nd ed.). San Diego: Academic Press. ISBN 9780323138338.
• Solomon, E.P.; Berg, L.R.; Martin, D.W. (2004). Biology. Cengage Learning. ISBN 978-0534278281.