The general chemical formula of unmodified monosaccharide is (CH2O)n, where ‘n’ is any number of three or greater. There are twenty monosaccharides that occur naturally, of which glucose is the most common monosaccharide.
Monosaccharides (from the Greek monos - single, and sacchar - sugar) are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. Glucose (C6H12O6), the most common monosaccharide, is of central importance in the chemistry of life. The molecule has a carbonyl group (=C=O) and multiple hydroxyl groups ( − OH). Depending on the location of the carbonyl group, a sugar is either an aldose (aldehyde sugar) or a ketose (ketone sugar). Glucose for example, is an aldose; fructose, a structural isomer of glucose is a ketose.
Another criterion for classifying sugars is the size of the carbon skeleton, which ranges from three to seven carbons long. Glucose, fructose, and other sugars that have six carbons are called hexoses. Trioses and pentoses are also common. Another source of diversity for simple sugars is in the spatial arrangement of their parts around asymmetric carbons. Glucose and galactose, for example, differ only in the placement of parts around one asymmetric carbon.
Classification of Monosaccharides
Sugars may be aldoses (aldehyde sugars, top row) or ketoses (ketone sugars, bottom row), depending on the location of the carbonyl group (represented in green). Sugars are also classified according to the length of their carbon skeletons. A third point of variation is the spatial arrangement around asymmetric carbons (compare, for example, the orange portions of glucose and galactose). Monosaccharides, particularly glucose, are major nutrients for cells. Monosaccharides are the major source of fuel for metabolism, being used both as an energy source (glucose being the most important in nature) and in biosynthesis. In the process known as cellular respiration, cells extract the energy stored in glucose molecules. Not only are simple sugar molecules a major fuel for cellular work, but their carbon skeletons serve as raw material for the synthesis of other types of small organic molecules, such as amino acids and fatty acids. When monosaccharides are not needed by cells they are quickly converted into another form, such as disaccharides or polysaccharides.