Determine the Functions of Digestive Enzymes in Breading Down Starch (page 2)
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- Are starch and glucose molecules the same size? To determine whether there is a difference in the size of starch and glucose molecules, mix together 1/2 cup (125 ml) of water, 1/2 cup (125 ml) of 1% starch solution, and 1/2 cup (125 ml) of apple juice (contains glucose). Line a funnel with five thicknesses of coffee filters. Pour the mixture of water, starch, and apple juice into the paper-lined funnel. Use the procedure in the experiment to test the filtrate (liquid that passes through the filter) for the presence of starch and glucose. Molecules that are too large to pass through the thick layer of paper will not appear in the filtrate.
- Starch is an example of a carbohydrate. In its edible form, starch molecules are too large to pass through the semipermeable membrane (membrane that selectively allows materials to pass through) of the digestive tract and too insoluble to dissolve in the blood. Glucose is a much smaller molecule and readily passes through the small holes in the membrane lining the digestive organs.
Demonstrate the selectivity of a semipermeable membrane and the ability or lack of ability of starch and glucose molecules to move through the membrane. Sausage casing (the lining of an intestine) behaves as a semipermeable membrane. Use a 1-foot (30 cm) piece of sausage casing (purchased from a meat market). Tie a knot in one end of the casing. Mix together 1/4 cup (63 ml) of water, 1/4 cup (63 ml) of 1% starch solution, and 1/4 cup (63 ml) of apple juice without added sugar. Pour the mixture into the sausage casing and tie the open end tightly. Place the casing in a jar and add enough water to cover it (see Figure 26.2). Use the procedure in the original experiment to test the liquid outside the casing for glucose. Test for glucose every ten minutes for one hour. At the end of the hour, test the liquid with iodine for the presence of starch.
- The digestion of large, insoluble starch molecules begins in the mouth. First, the food is physically ground by the teeth and then is chemically broken into smaller molecules. Saliva contains an enzyme called amylase. It is this chemical that breaks starch into a smaller disaccharide (double sugar) called maltose.
Study the effects of saliva on a starch by testing for the presence of glucose and starch in an unchewed and a chewed cracker. Note: Use only your own saliva to do this experiment. Or, check with your teacher about replacing the saliva with a .1% diatase solution. Wash your hands and then place a clean bite-size piece of a saltine cracker into your mouth. Chew and move the cracker around until it is a liquid mush (the longer you chew, the better the results). Place the chewed cracker into a jar and label it "Chewed." Label a second jar "Unchewed" and place an equal-size piece of cracker into it. Add 1 tablespoon (15 ml) of water and mix to form a mush. Repeat the original experiment to test for starch and glucose in the two samples. Note: Disinfect containers containing saliva by washing them with a 5:1 bleach solution (5 parts water and 1 part bleach). CAUTION: Take care not to get bleach on skin or clothes. It can damage skin and decolorize clothing.