Starch

Starch, by far the most important dietary polysaccharide, consists only of glucose units and is thus a homopolysaccharide and is designated a glucosan or glucan. It is actually composed of two such homopolymers (Fig 3.2): amylose, which has linear (1-4) linked a-D-glucose, and amylopectin, a highly branched form containing both

(1-4) and (1-6) linkages at the branch points. Plants have both forms as insoluble, semicrystalline granules and differing ratios of amylopectin and amylose, depending on the plant source (Ia.b!e 3,3). The salivary and pancreatic amylases act on the interior (1-4) linkages but cannot break the outer glucose-glucose links. Thus, the final breakdown products formed by the amylases are a(1-4)-linked disaccharides (maltose) and trisaccharides (maltotriose).

Figure 3.2. Starch is composed of amylose (15-20%) and amylopectin (80-85%). Amylose is a nonbranching helical chain structure of glucose residues, while amylopectin (a portion shown above) has branched chains of 24-30 glucose residues (solid black) joined by (1®4) glucosidic linkages with (1-6) linkages creating the branching points.

Figure 3.2. Starch is composed of amylose (15-20%) and amylopectin (80-85%). Amylose is a nonbranching helical chain structure of glucose residues, while amylopectin (a portion shown above) has branched chains of 24-30 glucose residues (solid black) joined by (1®4) glucosidic linkages with (1-6) linkages creating the branching points.

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