Polysaccharides:- Some Homopolysaccharides Serve Structural Roles
Cellulose, a fibrous, tough, water-insoluble substance, is found in the cell walls of plants, particularly in stalks, stems, trunks, and all the woody portions of the plant body. Cellulose constitutes much of the mass of wood, and cotton is almost pure cellulose. Like amylose and the main chains of amylopectin and glycogen, the cellulose molecule is a linear, unbranched homopolysac charide, consisting of 10,000 to 15,000 D-glucose units. But there is a very important difference: in cellulose the glucose residues have the β configuration (Fig. 7–16),
FIGURE 7–15 Amylose and amylopectin, the polysaccharides of starch. (a) A short segment of amylose, a linear polymer of D-glucose residues in (α1→4) linkage. A single chain can contain several thou sand glucose residues. Amylopectin has stretches of similarly linked residues between branch points. (b) An (α1→6) branch point of amylopectin. (c) A cluster of amylose and amylopectin like that believed to occur in starch granules. Strands of amylopectin (red) form double helical structures with each other or with amylose strands (blue). Glucose residues at the nonreducing ends of the outer branches are removed enzymatically during the mobilization of starch for energy production. Glycogen has a similar structure but is more highly branched and more compact.
whereas in amylose, amylopectin, and glycogen the glucose is in the α configuration. The glucose residues in cellulose are linked by (β1→4) glycosidic bonds, in contrast to the (α1→4) bonds of amylose, starch, and glycogen. This difference gives cellulose and amylose very different structures and physical properties. Glycogen and starch ingested in the diet are hydrolyzed by -amylases, enzymes in saliva and intestinal secretions that break (α1→4) glycosidic bonds between glucose units. Most animals cannot use cellulose as a fuel source, because they lack an enzyme to hydrolyze the (β1n4) linkages. Termites readily digest cellulose (and therefore wood), but only because their intestinal tract harbors a symbiotic microorganism, Tri Cho nympha, that secretes cellulase, which hydrolyzes the (β1 → 4) linkages. Wood-rot fungi and bacteria also produce cellulase (Fig. 7–17).
FIGURE 7–16 The structure of cellulose. (a) Two units of a cellulose chain; the D-glucose residues are in (β 1 → 4) linkage. The rigid chair structures can rotate relative to one another. (b) Scale drawing of segments of two parallel cellulose chains, showing the conformation of the D-glucose residues and the hydrogen-bond cross-links. In the hexose unit at the lower left, all hydrogen atoms are shown; in the other three hexose units, the hydrogens attached to carbon have been omit ted for clarity as they do not participate in hydrogen bonding.
FIGURE 7–17 Cellulose breakdown by wood fungi. A wood fungus growing on an oak log. All wood fungi have the enzyme cellulase, which breaks the (β 1 → 4) glycosidic bonds in cellulose, such that wood is a source of metabolizable sugar (glucose) for the fungus. The only vertebrates able to use cellulose as food are cattle and other ruminants (sheep, goats, camels, giraffes). The extra stomach compart ment (rumen) of a ruminant teems with bacteria and protists that secrete cellulase.
Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in linkage (Fig. 7–18). The only chemical difference from cellulose is the re placement of the hydroxyl group at C-2 with an acetylated amino group. Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates. Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods—insects, lobsters, and crabs, for example— and is probably the second most abundant polysaccharide, next to cellulose, in nature.
FIGURE 7–18 Chitin. (a) A short segment of chitin, a homopolymer of N-acetyl-D-glucosamine units in (β 1 → 4) linkage. (b) A spotted June beetle (Pellidnota punetatia), showing its surface armor (exoskeleton) of chitin.
