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Carbohydrates are molecules that contain carbon, hydrogen and oxygen (and sometimes sulphur and nitrogen). They are extremely important for life since they are used for storing and transporting energy. To learn how to test for them see tests.


These are the simplest form of carbohydrates, being made of only one sugar; and often have a sweet taste. An example of a monosaccharide you may have heard of is glucose. It has the formula C6H12O6 and the following structure:

the structure of glucose

Glucose consists of a ring of 5 carbons and an oxygen atom around which is a series of H, OH and a CH2OH. The structure can be simplified to a polygon where each point represents a carbon.

Fructose has the same formula as glucose but a different structure, consisting of a 4 carbon and 1 oxygen ring. For this reason it is said to be an isomer of glucose, and has the following structure.



A disaccharide is made up of two monosaccharides and has the general formula Cn(H2O)n-1. Examples of them include maltose and sucrose. But before we look at them, let's take a look at how disaccharides are formed.

a condensation reaction

In the condensation reaction, an OH from one of the monosaccharides and H from the other are removed and produce water. Leaving an oxygen atom that acts as the bond between the two molecules. This is called a glycosidic bond. The breaking up of a disaccharide is a hydrolysis reaction and involves the addition of water. Maltose and sucrose are composed as follows.

Maltose = Glucose + Glucose
Sucrose = Glucose + Fructose


A polysaccharide is a carbohydrate polymer where the monomers (single units) are monosaccharides. The monomers are joined by glycosidic links, they tend to be insoluble (do not dissolve) and have a very dull taste.

Examples of polysaccharides are starch and glycogen, both made of glucose but arranged slightly differently. The picture below is a simple representation to help you understand the structure of a polysaccharide. So you can imagine for example that each green circle is a glucose, all joined together in a great long chain to make starch

diagram to represent a polysaccharide


As mentioned in the introduction, carbohydrates are extremely important in life, the table below outlines just a few examples of the fundamental uses of carbohydrates.

UseType of CarbohydrateExample
Component in RNA and DNAMonosaccharideRibose and Deoxyribose
Building cell wallsPolysaccharideCellulose or Murecin
Energy storagePolysaccharideStarch or glycogen
Sweeting your food (table sugar)DisaccharideSucrose

Updated: 26 December 2015.