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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