The simple carbohydrates are the sugars just mentioned: glucose, fructose, and galactose, either singly or paired with glucose. In contrast, the complex carbohydrates contain many glucose units and a few other monosaccharides strung together as polysaccharides. Three are important in nutrition: glycogen, starches, and fibers.
Glycogen is a storage form of energy in the animal body; starches play that role in plants; and the fibers of plants serve as structural elements in stems, trunks, roots, leaves, and skins. Both glycogen and starch are built of glucose units, but they are linked together differently. The fibers are composed of a variety of mono-saccharides and other carbohydrate derivatives.
Glycogen is found only to a limited extent in meats and not at all in plants.* For this reason, glycogen is not a significant food source of carbohydrate, but it does perform an important role in the body. The human body stores much of its glucose as glycogen—many glucose molecules linked together in highly branched chains (see the left side of Figure 4-8). This arrangement permits rapid hydrolysis.
*Glycogen in animal muscles rapidly hydrolyzes after slaughter.
Glycogen and Starch Molecules Compared (Small Segments)
Notice that the more highly branched the structure, the greater the number of ends from which glucose can be released. (These units would have to be magnified millions of times to appear at the size shown in this figure. For details of the chemical structures, see Appendix C.)
Glycogen Starch (amylopectin) Starch (amylose)
A glycogen molecule contains hundreds A starch molecule contains hundreds of glucose molecules in either of glucose units in long, highly branched occasionally branched chains (amylopectin) or unbranched chains chains. (amylose).
starches: plant polysaccharides composed of glucose.
fibers: in plant foods, the nonstarch polysaccharides that are not digested by human digestive enzymes, although some are digested by GI tract bacteria; fibers include cellulose, hemicelluloses, pectins, gums, and mucilages and the nonpolysaccharides lignins, cutins, and tannins.
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