Carbohydrate Basics | Richard Crooks's Website

Carbohydrates are molecules containing carbon, hydrogen and oxygen. They are distinct from proteins, as they do not contain nitrogen, and they are distinct from lipids by having a greater content of oxygen. This means that unlike lipids, they are generally water soluble.

Carbohydrates have a general formula of C_nH_2nO_n. There are some deviations from this (namely polymer carbohydrates, and in deoxyribose, a building block of DNA), but in general they have around as many oxygen atoms as carbon atoms.

The most frequently occurring carbohydrate is glucose, which is a six carbon containing monomer carbohydrate. As a six carbon containing carbohydrates it is a hexose. There are other hexoses, as well as pentoses, which contain five carbon atoms.

The individual units of small carbohydrates adopt both linear and ring structures (Figure 1). The structures of these are interchangeable in aqueous (water based) solutions, however the cyclic form is the dominant form, and is the form which carries out most of the subsequently interesting chemical reactions of carbohydrates.

A picture of molecular models of glucose in its cyclic and linear forms and the exchange between the two. — Figure 1: The cyclic and linear forms of glucose. Both are found in aqueous solution and readily change from one form to the other, with the predominent form being the cyclic glucose, which is also the form that forms carbohydrate polymers.

The ring structures of carbohydrates are able to polymerize, and form chains of individual carbohydrate units (Figure 2). The bonds that form between the units are called glycosidic bonds, and are an example of a condensation polymer, that releases a water molecule when the bond forms.

A picture of molecular models of glucose molecules forming glycosidic bonds — Figure 2: The formation of a glycosidic bond between 2 glucose molecules to form a maltose molecule and a water molecule. The -OH groups that are widespread in carbohydrate molecules are able to form covalent bonds with nearby -OH groups, replacing the two -OH groups with a oxygen atom now linking the two molecules, and releasing a water molecule.

To describe properties of carbohydrate molecules, it is normal to refer to the carbon atoms in the carbohydrate unit (the monosaccharide). The carbon atoms are numbered from 1 to 6 (Figure 3). This nomenclature is used to describe different properties of carbohydrate molecules, based on which carbon atoms are involved. This is most notably used for describing DNA (DNA is translated from the 5 prime to the 3 prime end), but can also describe differences at different carbon atoms, such as how galactose is different from glucose due to the orientation of the groups around the 3 carbon.

Picture of a molecular model of a sugar molecule, with its atoms numbered 1 to 6 — Figure 3: The individual subunits of carbohydrates (called monosaccharides) can be described by labelling the carbon atoms by number. Monosaccharides in this way can be described in terms of differences between at particular carbon atoms. This is important for providing a consistent way of describing structural differences between different monosaccharides.

Carbohydrates are a key fuel source in cellular respiration, but they also take part in important chemical reactions, such as when attached to proteins in cell membranes (to form glycoproteins) or as part of the backbone of DNA and RNA. They have a lot of interesting chemistry about them that you can read more about in this section.