Monosaccharides
Monosaccharides (mono = “one”, sacchar = “sweet”) are simple sugars. In monosaccharides, the number of carbon atoms usually ranges from three to seven. Most monosaccharide names end with the suffix "-ose". If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R'), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons) and/or hexoses (six carbons).
Figure 1: Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. Aldoses have a carbonyl group at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively.
The most common monosaccharide is glucose, the building block of many important carbohydrates. Galactose (part of lactose or milk sugar) and fructose (found in sucrose in fruit) are other common monosaccharides. Although glucose, galactose and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon.