Materials Included In Kit

Additional Materials Required

Prelab Preparation

Teacher Demonstration—Diffusion/Osmosis

1. Add 300 mL of distilled or deionized water and 12 mL of iodine solution to one 600-mL beaker.
2. To a second 600-mL beaker, add 100 mL of starch solution and 200 mL of distilled or deionized water.
3. Cut a piece of string and tie it around one end of each soaked piece of dialysis tubing. Add 5 mL of iodine solution and 25 mL of distilled water to one tube. Tie off or knot the open end of the tube, rinse with distilled water and blot dry with a paper towel.
4. Follow step 3 procedure for the second tube but add 15 mL of starch solution and 15 mL of distilled water.
5. Determine the mass of each of the dialysis tubing bags by massing an empty 150-mL beaker. Place the dialysis tubing bags inside the beaker and mass again. Subtract the ending mass from the beginning to find the bag’s mass. Students should record all masses in the Diffusion Demonstration Data Table on the Student Worksheet.
6. Place the dialysis tubing containing the starch and water mixture into the 600-mL beaker containing the iodine solution.
7. Place the dialysis tubing containing the iodine solution inside the 600-mL beaker containing the starch solution.
8. Allow both bags to sit in the solutions for 25 minutes.
9. Remove the bags from each beaker and determine their masses as before (step 5). Students should record additional observations, like color changes and where the changes occurred.

Student Experiment—Starch Digestion

1. Cut 6" lengths of dialysis tubing and place in distilled or deionized water to soften 15 to 30 minutes prior to beginning the lab.
2. Prepare a 2% starch solution for both the demonstration and student experiments by boiling 1 L of distilled or deionized water. Remove from heat source and while stirring, sprinkle in 20 g of soluble starch.
3. Prepare amylase solution by dissolving 1.6 g of amylase powder in 80 mL of distilled/deionized water.

Safety Precautions

Benedict’s qualitative solution is a skin and eye irritant. Iodine solutions are irritating to eyes, irritating and mildly corrosive to skin and toxic by ingestion. Use insulated gloves, clamps or tongs when handling heated glassware. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The mixtures and solutions produced in this lab can all be rinsed down the drain according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period if the teacher demonstration is ready to go when class starts. Recording the data and doing the calculations should be performed during the lab.
• To assist students in understanding this activity, the Prelab Questions should be answered and discussed prior to beginning the student experiment.
• Except for the beaker needed for the hot water bath, the other containers used in the starch digestion experiment may be plastic or Styrofoam cups.
• For best results, only freshly-made solutions of amylase and starch should be used.

Teacher Tips

• This activity also may be used as an introduction to a discussion of respiration, which is the production of energy from food and/or used to discover the basic principles of osmosis and diffusion.
• It is estimated that fully one-half of the carbohydrates ingested by the average person are in the form of starch. Perhaps this fact will help put the importance of an efficient starch digestion mechanism in perspective.
• The disaccharide maltose cannot be metabolized directly and must be further broken down into the monosaccharide glucose. Glucose can be directly metabolized and is readily absorbed by the small intestine. Each maltose molecule yields two glucose molecules.

Answers to Prelab Questions

1. From what you may already know or learned from the reading the Background information, what is the primary purpose of carbohydrates (starches and sugars) in living organisms?

   To provide energy for all the activities of the organism.

2. Look carefully at the reaction sequence diagram above. What do you believe are the “building blocks” of all carbohydrates? (In other words, if all carbohydrates were broken down to their smallest component parts, what would those parts be called, in general.)

   The building blocks of carbohydrates are monosaccharides of which glucose is an example.

   1. What do you understand the prefix di to mean, as in disaccharides?

      di means two

   2. What are disaccharides made of?

      Two monosaccharides linked together.

   3. Why are many large carbohydrates molecules, like starch, referred to as polysaccharides?

      Polysaccharides are made from many monosaccharides linked together.

3. Which of the two mixtures that will be tested in the starch digestion experiment will give the strongest positive reaction with Benedict’s solution? Explain the reason for your answer.

   Student answers will vary. The starch and enzyme mixture will give the strongest positive because the enzyme will break the starch down to maltose and maltose will diffuse through the dialysis tubing membrane.

Sample Data

See Teacher PDF.

Answers to Questions

1. 1. In the diffusion demonstration, did any molecules move? If yes, which one(s)?

      Both the iodine and the water moved into the tubing and out of the tubing (see Figures 1 and 2 in the Sample Data).

   2. How do you know? (Use data from the lab to support your answer.)

      The starch changed color both inside and outside the tubing when it combined with the iodine. The mass of the dialysis tubing increased from the water moving in when starch was inside and decreased in mass when the starch was outside.

2. Based on the results from testing the starch–water mixture, what do you conclude about this mixture’s ability to break down polysaccharides (starch)?

   Water cannot break down starch by itself.

3. Based on the results from testing the starch–enzyme mixture, what do you conclude about this mixture’s ability to break down polysaccharides (starch)?

   Enzymes are necessary to break down starch.

4. 1. Were there molecules that “moved” during the starch digestion experiment? If yes, which one(s)?

      Water moved into the tubing, maltose must have moved out of the tubing (see Figures 3 and 4 in the Sample Data).

   2. How do you know? (Use data from the lab to support your answer.)

      The mass of the tubing increased slightly as water moved in. The outside water had a positive Benedict’s solution test.

5. Propose a logical explanation as to why only the starch–enzyme mixture had a strongly positive Benedict’s solution test?

   Starch was broken down (digested) by the enzyme (amylase) and the product maltose was small enough to diffuse out of the tubing.

6. 1. Why is the starch–enzyme mixture a good model to explain how “carbohydrate loading” can benefit athletes?

      It is a good model because it shows that large molecules can be utilized by organisms if the proper enzymes are present to metabolize them.

   2. Why is “carbohydrate loading” generally not a good idea for the so-called “couch potatoes” of the world? (Hint: Refer to the Background information to help answer these two questions.)

      People who are not active and who eat a lot of carbohydrates will gain weight because the small molecules produced from the digestion of carbohydrates, if not used for energy, are converted into fat.

Teacher Handouts

14118_Teacher1.pdf

References

Morholt, E.; Brandwein, P. F. A Sourcebook for Biological Sciences, 3rd. ed.; Harcourt Brace Jovanovich: Orlando, FL 1986, pp 241–249.

Introduction

The membranes that surround all cells only allow certain substances to go in or out. Small molecules, like iodine, pass readily through cell membranes while large, bulky molecules, such as starch, do not. However, we don’t “eat” huge quantities of iodine but we do eat starch. If we eat starch but it cannot pass through cell membranes, what good is it as a food? If starch is not really used by the human body, because it can’t get inside our cells, then why do many athletes participate in “starch/carbohydrate loading” prior to a competition? Why do people become obese if they eat a lot of starchy food? Experiments will be conducted to discover the answers to such questions.

Concepts

• Diffusion
• Osmosis
• Digestion

Background

The absorption, or uptake, of nutrients derived from the foods we eat requires passage of those nutrients through the membranes of the cells lining the intestines. Those nutrients also pass into the capillaries surrounding the intestinal lining cells which then move the nutrients into the blood vessels and around the body where they are needed. The membranes of cells act as gates regulating the movement of many types of molecules and ions by both active and passive transport mechanisms. Active transport requires the expenditure of energy by the individual cells while passive transport mechanisms rely only on the motion of the molecules and ions themselves. The primary type of passive transport is diffusion—defined as the net movement of a molecule or ion from a region where it is highly concentrated to a region where it is less concentrated.

Cell membranes are said to be selectively permeable in that some types of molecules and ions can diffuse freely through while others cannot. If we envision the membrane as being porous like a sieve, then it is easy to imagine that some molecules are small enough to fit through the pores while others are too large. Water molecules, dissolved gases (e.g., O₂, CO₂) and salt (which dissociates into sodium and chloride ions) are examples of substances that will diffuse freely through membranes.

Dialysis tubing models the cell membrane in this exercise. It is made of cellulose perforated with microscopic pores. The pores are small enough that the tubing can be used to model the behavior of a cell membrane with respect to the sizes of molecules that will (or will not) diffuse through them.

Starch is a complex carbohydrate and a long-chain polysaccharide. It is the most common way that plants store energy. The most familiar sources of dietary starch are potatoes, beans (legumes) and cereal grains (corn, wheat, barley). When starch and iodine are both present in solution they form a complex that has a characteristic blue/black color. Since neither is chemically altered, each can be used to indicate the presence of the other.

The primary source of energy for many organisms is carbohydrates. Starch and other large carbohydrates must be broken down into smaller molecules before they can diffuse through cell membranes. These smaller molecules are then converted into energy molecules. If energy is not needed, due to inactivity, these same small molecules are converted into fat molecules.

There are two parts to this activity, a teacher demonstration and a student experiment. The demonstration involves placing a starch solution inside a dialysis bag and an iodine solution outside as well as the reverse situation with the iodine inside and the starch outside. In the starch digestion experiment, groups will test two mixtures to determine which is the most effective at digesting (breaking down) starch. A substance called Benedict’solution will be used to determine if the starch was broken down into smaller molecules. Benedict’s solution reacts positively with simple reducing sugars to form a brick-red (cuprous oxide) precipitate. All monosaccharides (i.e., glucose and several disaccharides like maltose) are reducing sugars—that is they possess a free or potentially free carbonyl group (C═O) that combines with other elements in a solution to form new compounds.

One mixture to be tested—a solution containing soluble starch and water—will determine if starch solutions spontaneously break down into smaller sugar molecules that will pass through a cell membrane. The second mixture will investigate the role of enzymes, specifically amylase, in the process of digestion. Amylase is an enzyme found in human saliva and maltase is present in the small intestine. Both enzymes are needed for the complete digestion of starches as shown in the reaction sequence below. In this activity, Benedict’s solution will be used to test for the presence of maltose.

Experiment Overview

This demonstration and hands-on lab has two primary objectives:

1. To show that cell membranes are “selectively permeable” and do not allow all molecules to pass through them.
2. To discover how large molecules, containing essential nutrients, are able to pass through cell membranes and become usable to an organism.

Materials

Prelab Questions

1. From what you may already know or have learned from reading the Background information, what is the primary purpose of carbohydrates (starches and sugars) in living organisms?
2. Look carefully at the reaction sequence diagram above. What do you believe are the “building blocks” of all carbohydrates? (In other words, if all carbohydrates were broken down to their smallest component parts, what would those parts be called, in general.)
   1. What do you understand the prefix di to mean, as in disaccharides?
   2. What are disaccharides made of?
   3. Why are large carbohydrate molecules, like starch, referred to as polysaccharides?
3. Which of the two mixtures that will be tested in the starch digestion experiment will give the strongest positive reaction with Benedict’s solution? Explain the reason for your answer.

Safety Precautions

Benedict’s qualitative solution is a skin and eye irritant. Iodine solutions are irritating to eyes, irritating and mildly corrosive to skin and toxic by ingestion. Use insulated gloves, clamps or tongs when handling heated glassware. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Day 1

1. Label two beakers or cups—1 (starch/water) and 2 (starch/enzymes).
2. Obtain two 6" lengths of presoaked, softened, dialysis tubing. Keep tubing moist while tightly knotting each piece at one end with string—creating open “bags.”
3. Open the unknotted end of a dialysis bag by rubbing it between a thumb and forefinger. Fill it with 20 mL of starch solution and 10 mL of distilled water. Tie off the open end without trapping a lot of air. Rinse the bag with tap water and blot dry with a paper towel. Determine the mass of the dialysis tubing bag by massing the empty container first. Then add the bag to the container and mass again. Subtract the ending mass from the beginning mass to find the bag’s mass. Record the masses in the Starch Digestion Data Table on the Student Worksheet.
4. Open the other dialysis bag and fill it with 20 mL of starch, 5 mL of fresh amylase solution and 5 mL of distilled water. Tie off the open end, without trapping a lot of air. Rinse the bag with tap water and blot dry with a paper towel. Determine the mass of the dialysis tubing bag by massing the empty container. Then add the bag to the container and mass again. Subtract the ending mass from the beginning mass to find the bag’s mass. Record the masses in the Starch Digestion Data Table on the Student Worksheet.
5. After determining and recording the mass of each dialysis bag, place the dialysis bags inside their respective containers and fill each with 125 mL of distilled or deionized water. If the bags do not fully submerge, place a weight (i.e., smaller beaker) on top of the bags to keep them under water.
6. Cover the beakers with foil, plastic wrap or paper towels to minimize evaporation and let stand overnight at room temperature.

Day 2

7. Follow your teacher’s instructions in preparing a hot water bath.
8. Remove the dialysis bag from each container, dry them off and mass them in the same manner as Day 1 steps 3 and 4. Record masses in the data table.
9. Remove one full pipet of liquid from each container and place each sample in separate, clean containers. Test each sample for the presence of starch by adding 5 drops of iodine solution with a disposable dropper. Note: Use clean pipets to prevent cross contamination.
10. Carefully open up each dialysis bag and test the bag’s contents for the presence of starch by adding 5 drops of iodine solution. Record the results in the Starch Digestion Experiment Data Table.
11. Obtain two test tubes and fill each with 5 mL of Benedict’s qualitative solution.
12. Label the test tubes to correspond to the two labeled containers.
13. Transfer one full pipet of liquid from each container to the corresponding test tube of Benedict’s solution.
14. Place the test tubes in a beaker of boiling water for 3 to 4 minutes and note any changes in the solution in the Starch Digestion Data Table.
15. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

14118_Student1.pdf