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 tubescontaining Benedict’s reagent, 2%maltose solution, 2% sucrose solution, and 2% lactose solution.

Both maltose and lactose arereducing sugars. The sucrose hasremained 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 1000-2000 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