Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Prepare the cellular fluid.
   1. Add 100 g of dextrose to 250 mL of tap water and mix.
   2. Add 15 drops of red food dye to the dextrose solution and stir to dissolve.
2. Prepare the dialysis tubing.
   1. At least 10 minutes prior to doing the lab, use scissors to cut the dialysis tubing into 15-cm pieces.
   2. Place the pieces into a 600-mL beaker containing 500 mL of tap water. Note: Use distilled or deionized water if your tap water is very hard or contains large amounts sulfur or iron.

Safety Precautions

Red food dye will stain hands and clothes. Although the materials in this lab activity are nonhazardous, follow normal laboratory safety guidelines. Remind students to wash their hands thoroughly with soap and water before leaving class. 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. All solutions may be disposed of down the drain with running water 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. The lab activity can reasonably be completed in one 50-minute class period if the Prelab Questions are completed prior to class. Calculations and Post-Lab Questions may be completed after class.
• Since students will be testing the extracellular water for the presence of glucose, it is critical that the outside of the dialysis tubing be carefully washed after adding the cellular solution and that the bag does not leak during the experiment.
• Benedict’s quantitative reagent can be used to determine the amount of reducing sugar (glucose) in the cellular fluid and the extracellular fluid.

Teacher Tips

• During the prelab discussion, demonstrate diffusion by adding one drop of the red food dye into a beaker of tap water. Discuss the observations in terms of the movement of dye molecules.
• Extend the laboratory by providing groups with cellular fluid made with corn syrup, molasses or honey.
• Glucose test strips give a positive test result with most artificial sweeteners and with dextrose. However, table sugar (sucrose, a non-reducing disaccharide) will give a negative result.
• Extend the laboratory by assigning each group a different temperature for the “extracellular fluid” to determine how temperature affects the rate of osmosis and diffusion.

Answers to Prelab Questions

1. When a drop of food coloring is added to a container of water, why does the food coloring eventually become evenly distributed throughout the water?

   The food dye molecules become evenly distributed because the moving molecules bump into each other causing them to spread out from the area of higher concentration until the concentration of food-dye molecules is equal throughout the container of water. The process is called diffusion.

2. When all the water is the same color (Question 1), do the molecules stop moving? Explain your answer.

   No, molecules continue to move but the movement is no longer visible due to the even distribution of color. This is termed a dynamic equilibrium.

Sample Data

Answers to Questions

1. Based upon the test results, did glucose move across the selectively permeable membrane in this experiment? Explain.

   Yes, the glucose moved from the area of high concentration (inside the cell) to the area of lower concentration (outside the cell). The extracellular fluid tested positive for glucose at the end of the experiment.

2. What evidence was obtained in this experiment to show that water molecules moved across the selectively permeable membrane? What was the net “direction” in which water moved across the membrane? Explain.

   Yes, the artificial cell gained mass. The cell would have lost mass if the glucose had moved out of the cell and no water had moved into the cell. Since the cell gained mass, water must have moved into the cell.

3. Based on the data, did the red dye molecules move across the selectively permeable membrane? Explain.

   Yes, the extracellular fluid became red as the red molecules moved from the area of high concentration (inside the cell) to the area of lower concentration (outside the cell).

4. Predict whether a strawberry placed in concentrated saltwater will gain or lose water due to osmosis. Defend your prediction.

   Fruit, such as strawberries, contain a higher concentration of water inside their cells compared to that found in a sugar or saltwater solution. The water inside the cells osmoses from the cells to create equilibrium. Since this happens in every cell, the entire fruit or vegetable will become soft and shrink in size.

Introduction

Osmosis is the movement of water through a membrane. How can osmosis be measured or observed? In this activity the movement of water by osmosis and other molecules by diffusion will be observed.

Concepts

• Diffusion
• Osmosis
• Dynamic equilibrium
• Selectively permeable membranes

Background

Molecules are in constant motion. Collisions between the molecules cause them to move in many directions. Let’s consider the example of a drop of food coloring dye dropped into a glass of water. The dye molecules collide with each other, causing these molecules to change direction and spread out from the area with a higher concentration of dye molecules to an area with a lower concentration of dye molecules (see Figure 1). Once the dye molecules become evenly distributed in the glass of water they are said to be in dynamic equilibrium—the molecules are still in continuous random motion, but concentrations are no longer changing in different areas. The random movement of dye molecules from an area of higher concentration to an area of lower concentration is called diffusion. Even after the molecules are evenly distributed, it is important to remember that they continue to move, collide and redistribute themselves. Diffusion is one of the key processes involved in the movement of materials throughout living systems and especially into and out of cells.

Osmosis is defined as the diffusion of water through a selectively permeable membrane from an area where water is more concentrated to an area where water is less concentrated. In a selectively permeable or semipermeable membrane, some types of molecules and ions can diffuse freely through the pores of the membrane while others cannot. In nature, cell membranes are selectively permeable. If a membrane is envisioned 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 sodium chloride (which dissociates into sodium and chloride ions) are important examples of substances that will diffuse freely through cell membranes.

In this lab, dialysis tubing will be used as a model for the cell membrane. It is made of cellulose that is perforated with microscopic pores. The tubing pores are small enough to simulate the behavior of a cell membrane with respect to the sizes of molecules that will (or will not) diffuse through the membrane.

Experiment Overview

The purpose of this activity is to determine whether diffusion and osmosis occur across a selectively permeable membrane. In this activity, a dialysis bag filled with a colored glucose solution will simulate a cell. The cell will be immersed in water. The ability of water, glucose and dye molecules to move across the dialysis membrane will be investigated.

Materials

Prelab Questions

1. When a drop of food coloring is added to a container of water, why does the food coloring eventually become evenly distributed throughout the water?
2. When all the water is the same color (Question 1), do the molecules stop moving? Explain your answer.

Safety Precautions

The cellular fluid will stain hands and clothes. Although the materials in this lab activity are nonhazardous, follow normal safety precautions. Wear safety glasses or goggles whenever working with chemicals, heat or glassware. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Using a marker or wax pencil, write the group members’ initials on the bottom of a plastic cup and a weighing dish.
2. Use a balance to determine the mass of the weighing dish to the nearest 0.01 g. Record the mass in the data table on the Diffusion/Osmosis Worksheet.
3. Use a clean, graduated pipet to remove one drop of the cellular fluid. Drop the cellular fluid onto a glucose test strip. Wait 20 seconds before “reading” the glucose test result. In the data table, record whether the test result is positive or negative for the presence of glucose in the cellular fluid.
4. Prepare the artificial cell.
   1. Obtain a 15-cm piece of dialysis tubing by removing the tubing from the water. Note: Hold the tubing within 3 cm of the end.
   2. Place the tubing under a slow stream of tap water while rubbing the end between two fingers to open the tubing.
   3. Once the tubing is open, twist one end of the tubing two times about 3 cm from the end and fold the twist (see Figure 2).
      {10827_Procedure_Figure_2}
   4. Place one of the dialysis tubing clamps over the fold (see Figure 2).
   5. Hold the tubing upright (with the clip-side pointing down), carefully open up the opposite end of the tubing. Hold the tubing within 3 cm of the end again. The tubing only needs to be separated enough to fit the tip of a pipet inside (see Figure 2).
   6. Fill the extra-large bulb pipet with cellular fluid and insert the tip of the pipet into the tubing. Squeeze the pipet bulb to empty the solution into the tubing.
   7. Twist the open end of the tubing two times about 3 cm from the end and fold the twist. Place a second clamp over the fold to form an artificial cell (see Figure 2).
   8. Rinse the artificial cell with a gentle stream of tap water. If any leaks are observed, the model will need to be discarded and remade.
   9. Gently pat the artificial cell dry with a clean paper towel.
5. Place the artificial cell inside the previously massed weighing dish and measure the mass to the nearest 0.01 g. Record in the data table.
6. Calculate the mass of the artificial cell by subtracting the mass of the empty weighing dish from the mass of the weighing dish plus the artificial cell. Record in the data table.
7. Use a graduated cylinder to fill the cup with 150 mL of warm tap water. Record the volume and initial color of the extracellular fluid in the data table. Note: The water represents the extracellular fluid that surrounds cells.
8. Dip a glucose test strip into the water. Wait 20 seconds before “reading” the glucose test strip. Record the test result (positive or negative for the presence of glucose) in the data table.
9. Place the artificial cell into the cup containing the extracellular fluid. Leave the artificial cell undisturbed for 15 minutes.
10. After 15 minutes, carefully remove the artificial cell from the cup. Without squeezing the artificial cell, use a clean paper towel to gently pat the outside of the artificial cell dry.
11. Place the artificial cell inside the same weighing dish used in step 5. Measure the total mass of the cell, plus the weighing dish, to the nearest 0.01 g and record the result in the data table.
12. Calculate the final mass of the artificial cell by subtracting the mass of the weighing dish from the mass of the weighing dish plus the artificial cell. Record the final mass of the artificial cell in the data table.
13. Hold a sheet of white paper behind the cup of water used in the experiment. Observe the color and appearance of the “extracellular fluid” and record the observations in the data table.
14. Use a graduated cylinder to measure the amount of “extracellular fluid” remaining in the cup. Record the final volume of extracellular fluid in the data table.
15. Calculate the change in volume of the extracellular fluid and record the difference in the data table.
16. Dip a glucose test strip into the extracellular fluid. Wait 20 seconds and record whether the test strip indicates the presence of glucose in the extracellular fluid in the data table.

Student Worksheet PDF

10827_Student1.pdf