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 CarbohydratesCarbohydrates 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 CarbohydratesMonosaccharides: glucose, fructose, galactose
Disaccharides: sucrose, lactose
Polysaccharides: chitin, cellulose
Carbohydrate ClassificationThree characteristics are used to classify monosaccharides:
- number of carbon atoms in the molecule
- location of the carbonyl group
- the chirality of the carbohydrate
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.