Carbohydrates are organic biomolecules composed mainly of carbon, hydrogen and oxygen, although sometimes they can also have nitrogen, sulfur and phosphorus in their composition. The simplest are the osas or monosaccharides, which are made up of three to seven carbon atoms. A curiosity is that to detect a carbohydrate with reducing power, a test can be carried out in the laboratory. For this, Fehling's reagent is used, which is a solution of cupric sulfate in water and is blue in color. First, the reagent is added to the sugar solutions and, when oxidized, the aldehyde and ketone groups will form acid groups, thus giving up electrons that will be captured by the cupric inons. Later, when these cupric ions capture the electrons released earlier, they will form cuprous oxide, which is responsible for the red color of the precipitate. Therefore, we can deduce that this change from blue to red will determine whether there is the presence of carbohydrates or not.

On the other hand, to represent the cyclic structures of monosaccharides in a plane with their radicals at the top and bottom, the Haworth projection is used. The following photo shows how D-Glucose is cycled and is renamed α-D-glucopyranose. It is α because the -OH of the anomeric carbon, which is carbon 1, is below the plane, and ends in 'pyranose' because the carbon and oxygen atoms form a hexagonal structure. On the contrary, if the -OH of the anomeric carbon will remain above the plane it would be called β, and if a pentagonal structure were formed, as is the case of fructose, it would be called furanose. Finally, note that the formation of the cycle is carried out by a hemiacetal bond, which is a covalent bond between the aldehyde group and an alcohol and that occurs, for example, in cyclized glucose, or a hemicetallic bond between the ketone group and an alcohol such as that which takes place in fructose cyclization.