Carbohydrates or saccharides are the most abundant class of biomolecules. Carbohydrates are used to store energy, though they serve other important functions as well. This is an overview of carbohydrate chemistry, including a look at the types of carbohydrates, their functions, and carbohydrate classification.

What Is a Carbohydrate?

Carbohydrates are a common class of simple organic compouds. A carbohydrate is an aldehyde or a ketone that has additional hydroxyl groups. The simplest carbohydrates are called monosaccharides, which has the basic structure (C·H₂O)_n, where n is three or greater. Monosaccharides link together to form oligosaccharides and polysaccharides. Two monosaccharides link together to form a disaccharide.

Functions of Carbohydrates

Carbohydrates serve several biochemical functions:

• Monosaccharides are a fuel for celular metabolism.
• Monosaccharides are used in several biosynthesis reactions.
• Monosaccharides may be converted into space-saving polysaccharides, such as glyocogen and starch. These molecules provide stored energy for plant and animal cells.
• Carbohydrates are used to form structural elements, such as chitin in animals and cellulose in plants.
• Carbohydrates and modified carbohydrates are important for an organism's fertilization, development, blood clotting and immune system function.

Examples of Carbohydrates

Monosaccharides: glucose, fructose, galactose
Disaccharides: sucrose, lactose
Polysaccharides: chitin, cellulose

Carbohydrate Classification

Three characteristics are used to classify monosaccharides:

• number of carbon atoms in the molecule
• location of the carbonyl group
• the chirality of the carbohydrate

aldose - monosaccharide in which the carbonyl group is an aldehyde
ketone - monosaccharide in which the carbonyl group is a ketone
triose - monosaccharide with 3 carbon atoms
tetrose - monosaccharide with 4 carbon atoms
pentose - monosaccharide with 5 carbon atoms
hexose - monosaccharide with 6 carbon atoms
aldohexose - 6-carbon aldehyde (e.g., glucose)
aldopentose - 5-carbon aldehyde (e.g., ribose)
ketohexose - 6-carbon hexose (e.g., fructose)
A monosaccharide is D or L depending on the orientation of the asymmetric carbon located furthest from the carbonyl group. In a D sugar, the hydroxyl group is on the right the molecule when written as a Fischer projection. If the hydroxyl group is on the left of the molecule, then it is an L sugar.