Carbohydrates are molecules that contain oxygen, hydrogen, and carbon atoms. They may also contain other elements such as sulfur or nitrogen, but these are usually minor components. They consist of monosaccharide sugars, of varying chain lengths, that have the general chemical formula C_n(H₂O)_n or are derivatives of such.The smallest value for n is 3. A 3-carbon sugar is referred to as a triose, whereas a 6-carbon sugar is called a hexose (see monosaccharides below).Certain carbohydrates are important for storing and transporting energy in most organisms, including plants and animals, and are major structural elements in many organisms (eg cellulose in plants). In addition they play major roles in cell to cell communication, the immune system, fertilization, pathogenesis, blood clotting, and development. Carbohydrates can be classified by the number of constituent sugar units: monosaccharides (such as glucose and fructose), disaccharides (such as sucrose and lactose), oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose).


Pure carbohydrates contain carbon, hydrogen, and oxygen atoms, in a 1:2:1 molar ratio, giving the general formula C_n(H₂O)_n. (This applies only to monosaccharides, see below, although all carbohydrates have the more general formula C_n(H₂O)_m.) However, many important carbohydrates deviate from this, such as deoxyribose and glycerol. Sometimes compounds containing other elements are also counted as carbohydrates (e.g. glucosamine and chitin, which contain nitrogen).

The simplest carbohydrates are monosaccharides, which are small straight-chain aldehydes and ketones with many hydroxyl groups added, usually one on each carbon except the functional group. Other carbohydrates are composed of monosaccharide units and break down under hydrolysis. These may be classified as disaccharides, oligosaccharides, or polysaccharides, depending on whether they have two, several, or many monosaccharide units.

Monosaccharides may be divided into aldoses, which have an aldehyde group on the first carbon atom, and ketoses, which typically have a ketone group on the second. They may also be divided into trioses, tetroses, pentoses, hexoses,and many more. This all depends on how many carbon atoms they contain. For instance, glucose is an aldohexose, fructose a ketohexose, and ribose an aldopentose.

Further, each carbon atom that supports a hydroxyl group (except for the first and last) is a stereogenic centre, allowing a number of different enantiomers and stereoisomers for carbohydrates with the same basic structure. For instance, galactose is an aldohexose but has different properties from glucose because the atoms are arranged differently.



The straight-chain structure described here is only one of the forms a monosaccharide may take. The aldehyde or ketone group may react with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, in which case there is an oxygen bridge between the two carbon atoms, forming a heterocyclic ring. Rings with five and six atoms are called furanose and pyranose forms and exist in equilibrium with the straight-chain form.

It should be noted that the ring form has one more stereogenic centre than the straight-chain form, and so has both an alpha and a beta form, which interconvert in equilibrium. However, the carbohydrate may further react with an alcohol to form an acetal or ketal, in which case the two forms become distinct. This is the basic type of link between the monosaccharide units of larger carbohydrates. Excessive consumption may cause obesity.

Carbohydrates require less water to digest than proteins or fats and are the most common source of energy. Proteins and fat are vital building components for body tissue and cells, and thus it could be considered advisable not to deplete such resources by necessitating their use in energy production. Carbohydrates, like proteins, contain 4 kilocalories per gram while fats contain 9 kilocalories and alcohol contains 7 kilocalories per gram.

Based on evidence for risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 40-65% of dietary energy from carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates.