![]() Cotton and wood, for example, are completely insoluble in water and have considerable mechanical strength. As a result, cellulose exhibits little interaction with water or any other solvent. This extreme linearity allows a great deal of hydrogen bonding between OH groups on adjacent chains, as shown in the figure below. It differs, however, in that the glucose units are joined by β-1,4-glycosidic linkages, producing a more extended structure than amylose. Like amylose, cellulose is a linear polymer of glucose. Although the use of noncellulose synthetic fibers is increasing, rayon (made from cellulose) and cotton still account for over 70% of textile production. ![]() The largest use of cellulose is in the manufacture of paper and paper products. Cotton fibrils and filter paper are almost entirely cellulose (about 95%), wood is about 50% cellulose, and the dry weight of leaves is about 10%–20% cellulose. Because the earth is covered with vegetation, cellulose is the most abundant of all carbohydrates, accounting for over 50% of all the carbon found in the vegetable kingdom. This color test is sensitive enough to detect even minute amounts of starch in solution.Ĭellulose, a fibrous carbohydrate found in all plants, is the structural component of plant cell walls. The characteristic blue-violet color that appears when starch is treated with iodine is due to the formation of the amylose-iodine complex. When coiled in this fashion, amylose has just enough room in its core to accommodate an iodine molecule. Experimental evidence indicates that amylose is not a straight chain of glucose units but instead is coiled like a spring, with six glucose monomers per turn (part (b) of Figure 16.9 “Amylose”). Amylose is a linear polysaccharide composed entirely of D-glucose units joined by the α-1,4-glycosidic linkages we saw in maltose (part (a) of Figure 16.9 “Amylose”). ![]() Natural starches consist of about 10%–30% amylose and 70%–90% amylopectin. Starch is a mixture of two polymers: amylose and amylopectin. We often think of potatoes as a “starchy” food, yet other plants contain a much greater percentage of starch (potatoes 15%, wheat 55%, corn 65%, and rice 75%). The breakdown of starch to glucose nourishes the plant during periods of reduced photosynthetic activity. It occurs in plants in the form of granules, and these are particularly abundant in seeds (especially the cereal grains) and tubers, where they serve as a storage form of carbohydrates. Starch is the most important source of carbohydrates in the human diet and accounts for more than 50% of our carbohydrate intake.
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