Figure 2.20 Stereoisomers (a)The a and b forms of ribose are interconvertible and only differ in whether the OH group on carbon 1 is above or below the plane of the ring. (b) As shown in these stereoisomers of glucose, the carbon atoms at the intersections of the lines in the ring structure are understood to be present and are not labeled.
tance include glucose, galactose, and mannose. They all contain the same atoms but differ in the arrangements of the —H and —OH groups relative to the carbon atoms. Glucose and galactose are identical except for the arrangement of the —H and —OH groups attached to carbon 4. Mannose and glucose differ in the arrangement of the —H and —OH groups joined to carbon 2. Structural isomers result in three distinct sugars with different properties and different names. For example, glucose has a sweet taste as does mannose, but mannose has a bitter aftertaste.
The two most common disaccharides in nature are the milk sugar, lactose, and the common table sugar, sucrose. Lactose consists of glucose and galactose, while sucrose, which comes from sugar cane or sugar beets, is composed of glucose and fructose. The monosaccharides are joined together by a dehydration reaction between hydroxyl groups of two monosaccha-rides, with the loss of a molecule of water. Note that this reaction is similar to the joining of two amino acids. The reaction is reversible, so that the addition of a water molecule, the process of hydrolysis, yields the two original molecules. Great diversity is possible in molecules formed by joining monosaccharides. The carbon atoms involved in the joining together of the monosaccharides may differ and the position of the —OH groups, a and b, involved in the bonding may also differ.
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