Classification of Carbohydrates

Monosaccharides (simple sugars) can be classified according to the number of carbon atoms they contain. Examples of some monosaccharides commonly found in humans are listed in Figure 1.

They can also be classified by the type of carbonyl group they contain. Carbohydrates with an aldehyde as their carbonyl group are called aldoses, whereas those with a keto as their carbonyl group are called ketoses (Fig. 2). For example, glyceraldehyde is an aldose, whereas dihydroxyacetone is a ketose. Carbohydrates that have a free carbonyl
group have the suffix -ose. [Note: Ketoses have an additional “ul” in their suffix such as xylulose. There are exceptions, such as fructose, to this rule.] Monosaccharides can be linked by glycosidic bonds to create larger structures (Fig. 3). Disaccharides contain two monosaccharide units, oligosaccharides contain three to ten monosaccharide units, and polysaccharides contain more than ten monosaccharide units and can be hundreds of sugar units in length.

Figure 1 Examples of monosaccharides found in humans, classified according to the number of carbons they contain.

Figure 2 Examples of an aldose (A) and a ketose (B) sugar.

Figure 3 A glycosidic bond between two hexoses producing a disaccharide.