Diffusion and Osmosis
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. In this activity, Benedict’s solution will be used to test for the presence of maltose.
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Experiment Overview
This demonstration and hands-on lab has two primary objectives:
- To show that cell membranes are “selectively permeable” and do not allow all molecules to pass through them.
- To discover how large molecules, containing essential nutrients, are able to pass through cell membranes and become usable to an organism.
Materials
Student Experiment—Starch Digestion Amylase solution, 5 mL Benedict’s qualitative solution, 10 mL Iodine solution, 3 mL Starch solution, 40 mL Water, distilled or deionized Balance, 0.01-g precision Beakers/cups, 150-mL, 3 Dialysis tubing, two 6" lengths Graduated cylinder, 25 mL Graduated cylinder, 100 mL Hot plates (for boiling water bath) Pipets, Beral-type, 2 Sampling containers, 2 Scissors String Test tubes, 16 x 125 mm or larger, 2 Wax pencil or washable marker Teacher Demonstration—Diffusion/Osmosis Iodine (potassium iodide solution), 17 mL Starch solution, 2%, 115 mL Water, distilled or deionized Beaker, 150-mL Beakers, 600-mL, 2 Dialysis tubing, 6", 2 Scissors String
Prelab Questions
- 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?
- 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.)
a. What do you understand the prefix di to mean, as in disaccharides? b. What are disaccharides made of? c. Why are large carbohydrate molecules, like starch, referred to as polysaccharides?
- 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
Student Experiment—Starch Digestion
Day 1
- Label two beakers or cups—1 (starch/water) and 2 (starch/enzymes).
- Obtain two 6" lengths of pre-soaked, softened, dialysis tubing. Keep tubing moist while tightly knotting each piece at one end with string—creating open “bags.”
- 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 aiRinse 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.
- 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.
- 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).
- Cover the beakers with foil, plastic wrap or paper towels to minimize evaporation and let stand overnight at room temperature.
Day 2
- Follow your teacher’s instructions in preparing a hot water bath.
- 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.
- 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.
- 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.
- Obtain two test tubes and fill each with 5 mL of Benedict’s qualitative solution.
- Label the test tubes to correspond to the two labeled containers.
- Transfer one full pipet of liquid from each container to the corresponding test tube of Benedict’s solution.
- 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.
- Consult your instructor for appropriate disposal procedures.
Teacher Demonstration—Diffusion/Osmosis
- Add 250 mL of distilled or deionized water and 10 mL of iodine solution to one 600-mL beaker.
- To a second 600-mL beaker, add 85 mL of starch solution and 265 mL of distilled or deionized water.
- 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.
- Follow step 3 procedure for the second tube but add 15 mL of starch solution and 15 mL of distilled water.
- 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.
- Place the dialysis tubing containing the starch and water mixture into the 600-mL beaker containing the iodine solution.
- Place the dialysis tubing containing the iodine solution inside the 600-mL beaker containing the starch solution.
- Allow both bags to sit in the solutions for 25 minutes.
- 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
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