Carbohydrates are present in every plant or animal cell, and make up the largest portion, in terms of mass, of organic compounds present on Earth. Together with fats and proteins, they are the organic nutrients of humans and animals. Carbohydrates are a large class of naturally occurring polyhdroxycarbonyl compounds which have the general molecular formula (C)_n(H2O)_n. They were originally characterized as hydrated forms of carbon and the name has been retained, although it is chemically inaccurate. Today, other compounds having different elemental compositions are also categorized as carbohydrates, e.g., aldonic acids, uronic acids, deoxysugars, amino sugars and mucopolysaccharides.

The carbohydrates are subdivided on the basis of their molecular size. Monosaccharides, or simple sugars, cannot be further hydrolysed. Glucose, galactose and fructose are three common monosaccharides of physiological importance which share the same molecular formula: C₆H₁₂O_6. Because of their six carbon atoms...