Ch.5 part dos

Polysaccharides

the polymers of sugars, have storage and structural roles.Polysaccharides are polymers of hundreds to thousands of monosaccharides joined by glycosidic linkages. Some polysaccharides serve for storage and are hydrolyzed as sugars are needed. Other polysaccharides serve as building materials for the cell or the whole organism. Starch is a storage polysaccharide composed entirely of glucose monomers. Most of these monomers are joined by 1-4 linkages (number 1 carbon to number 4 carbon) between the glucose molecules. The simplest form of starch, amylose, is unbranched and forms a helix. Branched forms such as amylopectin are more complex.

storage examples

Plants store surplus glucose as starch granules within plastids, including chloroplasts, and withdraw it as needed for energy or carbon. Animals that feed on plants, especially parts rich in starch, have digestive enzymes that can hydrolyze starch to glucose. Animals store glucose in a polysaccharide called glycogen. Glycogen is highly branched like amylopectin. Humans and other vertebrates store a day's supply of glycogen in the liver and muscles.

The differing glycosidic links in starch and cellulose give the two molecules

distinct three-dimensional shapes. While polymers built with alpha glucose form helical structures, polymers built with beta glucose form straight structures. The straight structures built with beta glucose allow H atoms on one strand to form hydrogen bonds with OH groups on other strands. In plant cell walls, parallel cellulose molecules held together in this way are grouped into units called microfibrils, which form strong building materials for plants (and for humans, as lumber).

The enzymes that digest starch by hydrolyzing its alpha linkages cannot

hydrolyze the beta linkages in cellulose. Cellulose in human food passes through the digestive tract and is eliminated in feces as "insoluble fiber." As it travels through the digestive tract, cellulose abrades the intestinal walls and stimulates the secretion of mucus, aiding in the passage of food. Some microbes can digest cellulose to its glucose monomers through the use of cellulase enzymes. Many eukaryotic herbivores, from cows to termites, have symbiotic relationships with cellulolytic microbes, providing the microbe and the host animal access to a rich source of energy. Some fungi can also digest cellulose.

Cellulose

is a major component of the tough wall of plant cells. Plants produce almost one hundred billion tons of cellulose per year. It is the most abundant organic compound on Earth. Like starch, cellulose is a polymer of glucose. However, 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. These two ring forms differ in whether the hydroxyl group attached to the number 1 carbon is fixed above (beta glucose) or below (alpha glucose) the plane of the ring. Starch is a polysaccharide of alpha glucose monomers. Cellulose is a polysaccharide of beta glucose monomers, making every other glucose monomer upside down with respect to its neighbors.

Another important structural polysaccharide

is chitin, used in the exoskeletons of arthropods (including insects, spiders, and crustaceans). Chitin is similar to cellulose, except that it contains a nitrogen-containing appendage on each glucose monomer. Pure chitin is leathery but can be hardened by the addition of calcium carbonate. Chitin also provides structural support for the cell walls of many fungi.