A carbohydrate is an organic compound consisting of a chain or ring of carbon atoms to which hydrogen and oxygen atoms are attached in the ratio of approximately 2:1. The general formula of carbohydrates is Cx(H2O)y. Carbohydrates form a large and important group of naturally occurring carbon compounds, which can be divided into a number of smaller families.


Types of carbohydrate

A distinction is made between the sugars and the polysaccharides. The sugars are sweet to the taste, are soluble in water, and have fairly simple chemical formulae. By definition, the polysaccharides contain more than 18 carbon atoms in each molecule, but with the three principal polysaccharides – starch, glycogen, and cellulose, which play an essential role in the metabolism of all terrestrial organisms and, in the case of cellulose, the structure of plants – the number of carbon atoms is far greater. None of these compounds is truly soluble in water although starch and glycogen give colloidal solutions (the particles are too large to form a true solution). They are not sweet to the taste, though after some starch grains have been chewed for a short time the enzyme ptyalin in the saliva hydrolyses the starch to give maltose, a sugar which is sweet.


The sugars may be further subdivided into two groups – the monosaccharides and the disaccharides. Glucose (grape sugar) and fructose (fruit sugar) are among the most important of the monosaccharides. They both have the formula C6H12O6. The disaccharides are formed by two monosaccharides combining together at the same time losing one molecule of water from each pair of monosaccharide molecules. For instance, sucrose (cane sugar) is made up of fructose and glucose, while maltose (malt sugar) contains glucose units only. The oligosaccharides (uncommon in nature) consist of three to six monosaccharide molecules linked together.


So-called available carbohydrates, which include sugars and starches, can be metabolized by the human body. Unavailable carbohydrates, such as cellulose and hemicellulose, cannot be broken down by human digestive enzymes and make up the bulk of what is known as dietary fiber.


The term refined carbohydrate usually refers to sucrose refined from cane or beet, with no other nutrients. Cornflour (corn starch) is another refined carbohydrate, consisting of pure starch with no other nutrients.


Carbohydrates in space

In 2000, molecules of the simple sugar glycoaldehyde, comprised of 2 carbon, 2 oxygen, and 4 hydrogen atoms, were detected in a star-forming region near the center of our galaxy. This is biologically significant because glycoaldehyde is a component that can react to form more complex sugars such as ribose – a building block of nucleic acids such as RNA and DNA.


Some years earlier, Fred Hoyle and Chandra Wickramasinghe had made the controversial claim that some features of the spectra of interstellar clouds might indicate the presence of carbohydrates in space.