D-galactose D-mannose D-fructose
Note that some of these structures are identical apart from the orientation of groups around the central axis; d-glucose and l-glucose for example differ only in the way H atoms and -OH groups are arranged to the right or left. They are said to be stereoisomers or optical isomers, and are mirror images of each other, just like your right and left hands. (d-and l- are short for dextro- and /aevo-rotatory, meaning that the plane of polarised light is turned to the right and left respectively when passed through a solution of these substances). Generally, living cells will only synthesise one or other stereoisomer, and not both.
or more units. These are termed oligosaccharides (oligo, a few), and chains with many units are polysaccharides. The chemical bond joining the monosaccharide units together is called a glycosidic linkage. The bond between the two glucose molecules that make up maltose is called an a-glycosidic linkage; in lactose, formed from one glucose and one galactose, we have a (3-glycosidic linkage. The two bonds are formed in the same way, with the elimination of water, but they have a different orientation in space. Thus disaccharides bound together by a- and (-glycosidic linkages have a different overall shape and as a result the molecules behave differently in cellular metabolism.
Biologically important molecules such as starch, cellulose and glycogen are all polysaccharides. Another is dextran, a sticky substance produced by some bacteria to aid their adhesion. They differ from monosaccharides in being generally insoluble in water, not tasting sweet and not being able to reduce cupric ions. Most polysaccharides
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