Carbohydrates Polyhydroxyaldehydes monosaccharides

The simplest carbohydrates, sometimes referred to as monosaccharides, or sugars, are either polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses). They can be derived from polyalcohols (polyols) by oxidation of one carbinol group to a carbonyl group. For example, the simple three-carbon triol, glycerol, can be converted either to the aldotriose, glyceraldehyde, or to the ketotriose, dihydroxyacetone, by loss of two hydrogens (fig. 12.1). 

Monosaccharides, or simple sugars, are carbohydrates that cannot be hydrolyzed to simpler compounds. Figure 23-1 shows Fischer projections of the monosaccharides glucose and fructose. Glucose is a polyhydroxyaldehyde, and fructose is a polyhydroxy ketone. Polyhydroxyaldehydes are called aldoses (aid- is for aldehyde and -ose is the suffix for a sugar), and polyhydroxyketones are called ketoses (ket- for ketone, and -ose for sugar). 

All carbohydrates are polyhydroxyaldehydes, polyhydroxyketones, or molecules that yield polyhydroxy aldehydes or ketones on hydrolysis. Monosaccharides are the smallest carbohydrate molecules and include five- and six-carbon sugars, namely, pentoses and hexoses. Polysaccharides, such as starch and cellulose, lead to many monosaccharides upon hydrolysis. 

Aldoses polyhydroxyaldehydes, one of the two main subdivisions of the monosaccharides (the other is the ketoses see Carbohydrates). A. are characterized by a terminal aldehyde group -CHO, which is always given the number 1 in systematic nomenclature. The A. are formally derived from their simplest representative, glyceraldehyde, by chain extension. According to the number of carbon atoms in their chain, they are classified as trioses, tetroses, etc. The pentoses and hexoses are particularly important metaboli-cally. 

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