Carbohydrates are large macromolecules made up of carbon (C), hydrogen (H) and oxygen (O) and have the general formula Cx(H2O)y.
Carbohydrates can be classified according to the number of sugar units they contain, as follows:
- Monosaccharides consist of a single sugar. Examples include glucose and fructose. Depending on the number of carbon atoms, the carbohydrate may be a triose, tetrose, pentose or hexose.
- Disaccharides contain two sugar units
- Oligosaccharides contain 3 to 10 sugar units
- Polysaccharides contain more than 10 sugar units.
The general formula for a monosaccharide is (CH2O)n. Monosaccharides are the simplest form of carbohydrate, which means they cannot be hydrolyzed or broken down into smaller carbohydrates. Monosaccharides are important molecules that complex carbohydrates are broken down into, in order to generate energy. They are also essential for building nucleic acids.
Monosaccharides can be classified in three main ways, according to:
- The number of carbon atoms – Monosaccharides containing three carbon atoms are referred to as trioses, while those with four carbons are called tetroses and those with five are called pentoses etc.
- The location of the carbonyl group – If the carbonyl group is an aldehyde, then the monosaccharide is an aldose whereas if the carbonyl group is a ketone, the monosaccharide is a ketose.
- The molecule’s stereochemistry or chiral handedness – This refers to the configuration of the molecule, which may exist in different structural forms or isomers.
The first two classification systems above are often combined and a monosaccharide may be called an aldohexose (e.g. glucose), an aldopentose (e.g. ribose) or a ketohexose (e.g. fructose), for example.
Apart from the first and last carbon atom, each carbon atom bearing a hydroxyl group is asymmetric and may have two possible configurations in space (R or S). A number of isomers can therefore exist for any monosaccharide.
Reviewed by Sally Robertson, BSc