Plane of mirror
The —OH groups on sugars can be replaced by many other groups such as carboxyl, amino, and acetyl groups to form molecules that are important in the structures of the cell. For example, acetyl glucosamine is an important component of the cell wall of bacteria. ■ bacterial cell wall, p. 57
Monosaccharides are classified by the number of carbon atoms they contain. The most common monosaccharides are those with 5- or 6-carbon atoms. The 5-carbon sugars, ribose and deoxyribose, are the sugars in nucleic acids (figure 2.19). Note that these monosaccharides are identical except that deoxyribose has one less molecule of oxygen than does ribose (de means "away from"). Thus, deoxyribose is ribose "away from" oxygen. Common 6-carbon sugars include glucose, galactose, and fructose. It is convenient to number the carbon atoms with carbon atom 1 being closest to the aldehyde or keto group.
Sugars can be drawn in two forms, a linear and a ring form. The linear form of ribose, the ring form, and the relationship between the two representations are shown in figure 2.19. Both forms naturally occur in the cell, but most molecules are in the ring form. The forms are interconvertible.
Sugars can form two different stereoisomers (mirror images). These stereoisomers result as the ring structure is formed from the linear form, because the —OH group on the C atom involved in the formation of the ring can be above or below the plane of the ring (figure 2.20a). These two different positions yield molecules with different properties when this —OH is linked to another monosaccharide to form larger molecules. The two different forms are termed alpha (a) and beta (b). Every sugar can also exist in two different forms, D and L, which also are mirror images of one another (figure 2.20b). Most monosaccharides in living organisms are of the D-configuration, which is opposite to the situation observed with amino acids. The sugars of different isomeric forms still have the same name.
Sugars also form structural isomers, molecules that contain the same elements but in different arrangements that are not mirror images. They are different sugars and have different names. For example, common hexoses of biological impor-
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