Struktur Polisakarida (Structural Polysaccharides)

Organisms build strong materials from structural polysaccharides. For example, the polysaccharide called ceUulose is a major component of the tough walls that enclose plant cells. On a global scale, plants produce almost 1014 kg (100 billion tons) of cellulose per year; it is the most abundant organic compound on Earth. Like starch, cellulose is a polymer of glucose, but the glycosidic linkages in these two polymers differ. The difference is based on the fact that there are actually two slightly different ring structures for glucose (Figure S.7a). When glucose forms a ring, the hydroxyl group attached to the number 1 carbon is positioned either below or above the plane of the ring. These two ring forms for glucose are called alpha (0.) and beta (13), respectively. In starch, all the glucose monomers are in the ex configuration (Figure 5.7b), the arrangement we sawin Figures 5.4 and 5.5. In contrast, the glucose monomers of cellulose are all in the 13 configuration, making every other glucose monomer upside down with respect to its neighbors (Figure 5.7c). The differing glycosidic linkages in starch and cellulase give the two molecules distinct three-dimensional shapes. Whereas a starch molecule is mostly helical, a cellulose molecule is straight. Cellulose is never branched, and some hydroxyl groups on its glucase monomers are free to hydrogen-bond with the hydroxyis of other cellulose molecules lying parallel to it. In plant cell walls, parallel cellulose molecules held together in this way are grouped into units called microfibrils (Figure 5.8). These cable-like microfibrils are a strong building material for plants and an important substance for humans because cellulose is the major constituent of paper and the only component of cotton. Enzymes that digest starch by hydrolyzing its a linkages are unable to hydrolyze the f3 linkages of cellulose because of the distinctly different shapes of these two molecules. In fact, few organisms possess enzymes that can digest cellulose. Humans do not; the cellulose in our food passes through the digestive tract and is eliminated with the feces. Along the way, the cellulose abrades the wall ofthe digestive tract and stimulates the lining to secrete mucus, which aids in the smooth passage of food through the tract. Thus, although cellulose is not a nutrient for humans, it is an important part ofa healthful diet. Most fresh fruits, vegetables, and whole grains are rich in cellulose. On food packages, uinsoluble fiber~ refers mainly to cellulose. Some prokaryotes can digest cellulose, breaking it down into glucose monomers. A cow harbors cellulose·digesting prokaryotes in its rumen, the first compartment in its stomach (figure 5.9). The prokaryotes hydrolyze the cellulose of hay and grass and convert the glucose to other nutrients that nourish the cow. Similarly, a termite, which is unable to digest cellulose by itself, has prokaryotes living in its gut that can make a meal of wood. Some fungi can also digest cellulose, there by helping recycle chemical elements within Earth's ecosystems. Another important structural polysaccharide is chitin, the carbohydrate used by arthropods (insects, spiders, crustaceans, and related animals) to build their exoskeletons (figure 5.10). An exoskeleton is a hard case that surrounds the soft parts of an animal. Pure chitin is leathery and flexible, but it becomes hardened when encrusted with calcium carbonate, a salt. Chitin is also found in many fungi, which use this polysaccharide rather than cellulose as the building material for their cell walls. Chitin is similar to cellulose, except that the glucose monomer of chitin has a nitrogen. containing appendage (see Figure 5.l0a).