DISACCHARIDES Disaccharides are two monosaccharide units linked together by acetal or ketal glycosidic linkages. A glycosidic linkage is identified by: the numbers associated with the carbon atoms joined together by the linkage. the configuration of the linkage for any anomeric carbon atom joined by the linkage.

IMPORTANT DISACCHARIDES Maltose is: malt sugar. two glucose units joined by  (1→4) glycosidic linkage. formed during the digestion of starch to glucose. found in germinating grain. a hemiacetal, which means maltose is a reducing sugar. hydrolyzed to form two molecules of D-glucose.

IMPORTANT DISACCHARIDES (continued) Lactose is: milk sugar (5% cow milk, 7% human milk by weight). composed of galactose and glucose units joined by  (1→4) glycosidic linkage. a hemiacetal, which means lactose is a reducing sugar.

IMPORTANT DISACCHARIDES (continued) Sucrose is: common household sugar. composed of fructose and glucose units joined by  1→  2 glycosidic linkage. found in many plants (especially sugar cane and sugar beets). not a reducing sugar. hydrolyzed to produce invert sugar (a mixture of equal amounts glucose and fructose).

IMPORTANT DISACCHARIDES (continued) From left to right, four test tubes containing Benedict’s reagent, 2% maltose solution, 2% sucrose solution, and 2% lactose solution. Both maltose and lactose are reducing sugars. The sucrose has remained unreacted.

IMPORTANT DISACCHARIDES (continued)

POLYSACCHARIDES Polysaccharides are condensation polymers containing thousands of units. They are not water-soluble, but hydroxy groups become hydrated individually when exposed to water. They can form thick colloidal dispersions when heated in water. Starch is used as a thickener in sauces, gravies, pie fillings, and other food preparations.

IMPORTANT POLYSACCHARIDES Starch is: a polymer consisting of glucose units. a major storage form of D-glucose in plants. one of two forms, which are: unbranched amylose (10-20%), which is composed of glucose units with  (1→4) glycosidic linkages;

IMPORTANT POLYSACCHARIDES (continued) and branched amylopectin (80-90%) with  (1→4) and  (1→6) glycosidic linkages.

IMPORTANT POLYSACCHARIDES (continued) Amylose complexes with iodine to form a dark blue color. The molecular conformation of starch and the starch-iodine complex: (a) the helical conformation of the amylose chain and (b) the starch-iodine complex.

IMPORTANT POLYSACCHARIDES (continued) Glycogen (animal starch) is: a polymer of glucose units. used by animals to store glucose, especially in the liver and muscles. structurally similar to amylopectin with  (1→4) and  (1→6) linkages, but more highly branched.

IMPORTANT POLYSACCHARIDES (continued) Cellulose is: a polymer of glucose units. the most important structural polysaccharide. the most abundant organic compound on earth. found in plant cell walls. a linear polymer like amylose, but has  (1→4) glycosidic linkages. not easily digested and a constituent of dietary fiber.

IMPORTANT POLYSACCHARIDES (continued) Cellulose structure