Materials Included In Kit

Additional Materials Required

Prelab Preparation

Glucose–Starch Solution

1. Prepare a 1% starch solution by heating 200 mL of deionized water to boiling. Remove the water from the heat and slowly sprinkle in the 2 g of soluble starch while stirring. Heat for a few more minutes, with continued stirring, until the starch is dissolved. Add 30 g of dextrose to the 200 mL of 1% starch. Stir until dissolved.
2. Prepare the sucrose solutions by dissolving the following quantities of sucrose in 500 mL of deionized water. (FW = 342.3 g/mol)
   1. 0.2 M = 34.2 g of sucrose
   2. 0.4 M = 68.3 g of sucrose
   3. 0.6 M = 102.7 g of sucrose
   4. 0.8 M = 137 g of sucrose
   5. 1.0 M = 171.2g of sucrose
3. Cut the dialysis tubing into 20-cm pieces and place in water to soak for a minimum of 5 minutes before use. Extra dialysis tubing is included to accommodate those students who cannot properly secure the dialysis tubing.

Safety Precautions

Iodine solutions are irritating to eyes and skin, mildly corrosive and toxic by ingestion. Scalpels and knives are sharp instruments, use caution when cutting. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please consult 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. Iodine solutions may be disposed of according to Flinn Suggested Disposal Method #12a. All other solutions may be disposed of according to Flinn Suggested Disposal Method #26b. Scalpels and glass slides may be disposed of according to Flinn Biological Waste Disposal Method V, sharps and broken glass. All other materials may be disposed of according to Flinn Biological Waste Disposal Method VI, common garbage wastes.

Lab Hints

• Enough materials are provided in this kit for eight groups of students. This laboratory activity can reasonably be completed in three 50-minute class periods. Data analysis and calculations can be completed the day after the lab.
• Securing the ends tightly on the dialysis tubing and rinsing the outside of the bag are the key steps in the student procedure. Demonstrate the techniques and emphasize the importance of these steps with students.
• Starch and iodine form a complex that has a characteristic blue/black color. This reaction is reversible and thus each reactant can be used an an indicator to confirm the presence of the other.
• Glucose test strips contain the chemicals necessary to perform a double sequential enzymatic reaction, involving glucose oxidase and peroxidase in the presence of an indicator. The enzymatic reactions give rise to a color change from yellow to green in the presence of glucose.
• The plant tissue in Activity 4 may be Elodea or red onion.

  Cut a red onion into ¾-inch thick wedges. Separate the layers of onion. Break the onion section and pull the thin layer of skin off of the inside curve of the section. Place the epidermis side (inner tissue) facing up toward the microscope lens on the glass slide.

  {10766_Hints_Figure_6}

Teacher Tips

• Activity 1 can be performed by the teacher as an interactive class demonstration.
• Activity 2 may be completed as a cooperative class project, with each group testing one or two different concentrations of sucrose. For best results, ensure that each sucrose concentration is tested by at least three different student groups. Otherwise, it is too easy to get anomalous results that throw off the analysis.
• Activity 3 can be performed as a class demonstration.
• It is important to help students avoid a common misconception. After the solutions on each side of the membrane have reached a uniform distribution or equal concentration, there is still significant movement across the membrane. However, now the rate of movement in bother directions is equal.

Sample Data

Activity 1. Diffusion 

Pre-Analysis Results

1. Color of the glucose–starch solution. Colorless
2. Color of glucose test strip after reacting with the known glucose–starch solution. Green
3. Color of iodine after reacting with the known glucose–starch solution. Purple-black
4. Color of the deionized water–iodine solution. Dark brown
5. Color of glucose test strip after reacting with the known deionized water–iodine solution. Yellow
6. What compounds are inside the dialysis bag at the beginning of the experiment? Glucose, starch, water
7. What compounds are outside the dialysis bag at the beginning of the experiment? Iodine, water

Analysis Results

1. Color of the solution in the dialysis bag. Black
2. Color of the solution in the beaker. Clear
3. Color of the glucose test strip after reacting with the solution in the beaker. Green
4. Color of the glucose test strip after reacting with the solution in the bag. Yellow to green
5. What compounds are inside the dialysis bag after the experiment? Starch, water, iodine, some glucose
6. What compounds are outside the dialysis bag after the experiment? Glucose, water, some iodine.

Activity 2. Osmosis

Analysis Results

Table 1 Table 2
Data will vary according to class results. The trend of data should be similar to that shown in Table 1.

Activity 3. Water Potential

Analysis Results

Table 1 Activity 4. Plasmolysis

Answers to Questions

Activity 1. Diffusion

1. Based on your observations, rank the following by relative size, beginning with the smallest: glucose molecules, water molecules, iodine molecules, membrane pores, starch molecules.

   Molecules of water, iodine molecules, glucose, membrane pores, starch molecules.

2. Explain the results you obtained. Include the concentration differences and membrane pore size in your discussion.

   All of the substances that moved either into or out of the bag did so because the concentration of that substance was greater in one place than in the other. In all instances the molecules that moved were smaller than the pores in the dialysis tubing.

3. Quantitative data uses numbers to measure observed changes. How could this experiment be modified so that quantitative data could be collected to show that water diffused into the dialysis bag?

   The bags could have been massed before and after the waiting time and the difference noted. A second method would be to measure the volume of the contents of the dialysis tubing both before and after the waiting time. The volume can be measured either by pouring the contents of the bag into a graduated cylinder or by a volume displacement technique.

4. What results would you expect if the experiment started with a glucose and iodine solution inside the bag and only starch and water outside? Why?

   There would be a net movement of the iodine solution out of the bag. The starch–iodine solution would turn blue-black color and not enter the bag because the starch molecules are too big to pass through the pores of the bag. If there is an excess of iodine molecules, it might produces a light tan color that persists inside the bag. Initially, the bag might increase in mass as the water diffuses into the bag (assuming that the rate of water diffusion is greater than the rate of glucose diffusion). Eventually, there would be no net movement of water into the bag as both the water and glucose would attain a dynamic equilibrium.

Activity 2. Osmosis

1. On graph paper draw a graph showing the percent change in mass versus sucrose concentration. Plot both your individual data and the class averages. Determine and label the independent variable, the dependent variable, and the title.

   Independent variable: Molarity of the Contents of the Dialysis Tubing
   Dependent variable: Percent Change in Mass
   Title: Percent Change of Mass of Contents of Dialysis Tubing in Solutions of Different Molarity.

   {10766_Answers_Figure_8}
2. Describe the mathematical relationship between the percent change in mass and the concentration of sucrose within the dialysis bag.

   The percent change in mass increases with an increase in the molarity of the sucrose solution. Water moves from a place of lower solute concentration to a place of greater solute concentration. The higher the concentration of solute in the dialysis tubing, the more water moves into the tubing in the time period allowed. Water moves from a place of greater water potential to a place of less water potential. As the concentration of solute increases in a solution, the water potential will decrease accordingly.

3. Predict what would happen to the mass of each bag in this experiment if all the bags were placed in a 0.4 M sucrose solution instead of in deionized water. Explain your response.

   Water would move out of the bags containing deionized water and 0.2 M sucrose more rapidly than it moves in, because the solute concentration in the bags is less than in the solution surrounding the bags. More (net) water will leave the bag containing deionized water than in the bag containing the 0.2 M sucrose because there is less solute in the deionized water than in the 0.2 M sucrose solution. The mass of the dialysis tubing containing the 0.4 M sucrose will not change because the solute concentration on both sides of the membrane is the same and therefore water moves in and out at equal rates. Water will move into the bags containing 0.6 M, 0.8 M and 1.0 M sucrose. As the molarity increases there will be an increase in the net movement of water into the bags.

4. A dialysis bag is filled with deionized water and then placed in a sucrose solution. The bag’s initial mass is 20 g and its final mass is 18 g.
   1. Calculate the percent change in mass, showing the calculations in the space below.
      {10766_Answers_Equation_1}
   2. Is the sucrose solution in the beaker hypotonic, hypertonic or isotonic relative to the deionized water in the bag?

      Hypertonic

5. A laboratory assistant prepared solutions of 0.8 M, 0.6 M, 0.4 M and 0.2 M sucrose, but forgot to label them. After realizing the error, the assistant randomly labeled the flasks containing these four unknown solutions as flask A, flask B, flask C and flask D. Design an experiment which the assistant could use to determine which of the flasks contained each of the four unknown solutions. Include in your answer a description of how to set up and perform the experiment, the expected results, and an explanation of the results based on the principles involved. (Be sure to clearly state the principles addressed in your discussion.)

   Student answers will vary.

Activity 3. Water Potential

1. On graph paper, complete a graph showing the percent change in mass versus the concentration of sucrose for all the sucrose solutions. For the graph, determine and label the independent variable, the dependent variable and the title.

   Independent variable: Sucrose Molarity in the Beaker
   Dependent variable: Percent Change in Mass
   Title: Percent Change of Mass of Contents of Potato in Solutions of Different Molarity.

   {10766_Answers_Figure_9}
2. Determine the molar concentration of the potato core—the concentration of a sucrose solution in which the mass of the potato core does not change. To find this, draw a “best-fit” straight line through the data points on the graph. The point at which this line crosses the x-axis represents the molar concentration of sucrose having a water potential equal to the potato tissue. At this concentration there should be no net gain or loss of water from the potato core.

   Record this equivalent molar concentration here ≈ ___0.32___ M.

3. What is the water potential of the potato cells?

   Using 0.32 M from Question 2 the water potential of the potato cells would equal –7.8 bars. However, student answers will vary based on class data.

4. Water potential values are useful because they predict the direction of the flow of water. Recall from the discussion that water flows from an area of higher water potential to an area of lower water potential. For the sake of discussion, suppose that a student calculated that the water potential of a solution inside a bag is –6.25 bar (Ψ_s = –6.25, Ψ_p = 0) and the water potential of a solution surrounding the bag is –3.25 bar (Ψ_s = –3.25, Ψ_p = 0). In which direction will the water flow?

   The water would flow from the solution surrounding the bag into the bag because the solution has a higher water potential

5. If a potato core is allowed to dehydrate by sitting in the open air, would the water potential of the potato cells decrease or increase? Explain.

   The water potential of the potato cells would decrease because the concentration of solutes within the cells increases as the potato cells dehydrate.

6. If a plant cell has a lower water potential than its surrounding environment and if the pressure potential is equal to zero, is the cell hypertonic (in terms of solute concentration) or hypotonic to its environment? Will the cell gain water or lose water? Explain.

   The plant cell is hypertonic to its environment and will therefore take up water. The higher the solute concentration, the more hypertonic the cell is to its environment and the lower its solute potential and total water potential. When the pressure potentials of the cell and the environment are both zero, the movement of water is dependent entirely on solute concentration.

Activity 4. Plasmolysis

1. Using the concepts of diffusion, osmosis and water potential, describe what occurred when the sodium chloride solution contacted the plant epidermis cells.

   The water potential is greater inside the cell therefore the water molecules move by osmosis through the semipermeable membrane into the surrounding sodium chloride solution.

2. Consider what would happen to a red blood cell (RBC) placed in deionized water.
   1. Which would have the higher concentration of water molecules? Deionized water
   2. Which would have the higher water potential? Deionized water
   3. What would happen to the RBC? Why? The RBC would acquire water by osmosis and rupture (hemolysis).

Introduction

How do the membranes around cells help regulate the internal makeup of a cell? How does a semipermeable membrane work? What is diffusion? The purpose of this lab is to observe, measure and compare the diffusion of water, starch and glucose through both artificial (dialysis tubing) and natural (potato cell) membranes.

Objectives
After completing this laboratory, you should be able to:

• Measure the water potential of a solution in a controlled experiment.
• Determine the osmotic concentration of living tissue or an unknown solution from experimental dat

Concepts

• Concentration gradient
• Osmosis
• Selectively permeable membrane
• Diffusion
• Plasmolysis

Background

Diffusion is the random movement of molecules from an area of higher concentration of those molecules to an area of lower concentration. How does diffusion occur? Molecules in solutions or cells are in constant motion, and the moving molecules continually collide with one another. The higher the concentration of molecules, the greater the number of collisions will occur. These collisions cause the molecules to change direction and to spread out until they eventually become uniformly distributed. Even after the molecules are evenly distributed, it is important to remember that they continue to move, collide and redistribute themselves. The motion of molecules does not cease even when a uniform distribution is reached. Consequently, uniform distribution is called a dynamic equilibrium, because there is no further net movement of the molecules down a concentration gradient. If there is a difference in concentration across a distance, the measure of this difference is called a concentration gradient. Diffusion is one of the key processes involved in the movement of materials throughout living systems and especially into and out of cells (see Figure 1).

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 while others cannot. In nature, cell membranes are said to be selectively permeable. The absorption, or uptake, of nutrients derived from the foods consumed requires passage of those nutrients through the membranes of the cells lining the intestines. The 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 down the concentration gradient. The primary type of passive transport is diffusion.

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 salt (which dissociates into sodium and chloride ions) are examples of substances that will diffuse freely through 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 pores are small enough that the tubing can be used to simulate the behavior of a cell membrane with respect to the sizes of molecules that will (or will not) diffuse through the membrane.

Carbohydrates are the primary source of energy for many organisms. Monosaccharides or simple sugars are the simplest carbohydrates. These simple sugars, such as glucose and fructose, may be absorbed by the body for direct use inside the cells. A second type of carbohydrate is a disaccharide. Sucrose (table sugar) and lactose (milk sugar) are examples of disaccharides. These must first be digested by enzymes in the body into monosaccarides before they are transported from the digestive system for use throughout the body. Starch is a complex carbohydrate and a long-chain polysaccharide. It is the most common form of energy storage for plants. The most familiar sources of dietary starch are potatoes, beans, and grains. 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.

The terms hypotonic, hypertonic and isotonic are used to compare solutions having different solute concentrations. The hypotonic side is the side with the higher concentration of water and a lower solute concentration. The hypertonic side is the side with the lower concentration of water and a higher solute concentration. Hypotonic and hypertonic represent two unequal concentrations of molecules on either side of a permeable membrane. Water will flow, via osmosis, from the hypotonic side to the hypertonic side until the concentrations on both sides are equal. Water will then continue to move across the membrane in equal amounts creating a dynamic equilibrium. Two solutions (or “sides” across a membrane) are isotonic when both sides have equal concentrations of solute and water percentages. Plasmolysis is the shrinking of the cytoplasm of a plant cell due to the diffusion of water out of the cell and into a hypertonic solution that surrounds the cell. During plasmolysis the cell membrane contracts away from the more rigid cell wall as the cell loses water through its selectively permeable cell membrane to the high solute concentration of the adjacent solution. Osmosis is a spontaneous process, so it must be the result of a “downhill” energy flow. This energy system is called water potential. Water potential is a measure of the free energy of water. Water spontaneously moves from an area of higher water potential (higher free energy; more water molecules) to an area of lower water potential (lower free energy; fewer water molecules). Water potential is represented by the symbol Ψ (Greek letter, “psi”). It is measured in units of pressure, usually kilopascals (kPa) or bars. By convention, the water potential of pure water at atmospheric pressure is defined as zero. The water potential can be positive, zero or negative. Remember that water will move across a membrane in the direction of the lower water potential. The total water potential (Ψ) may be determined by adding the water potential due to pressure (Ψ_p) and the water potential due to the solute concentration (Ψ_s).

Ψ = Ψ_p + Ψ_s

The solute concentration (Ψ_s) in the area surrounding a cell influences the properties of the cell because a solute decreases the water potential and tends to cause water to enter the area of high solute concentration—water will leave the cell and enter the solution. The pressure potential (Ψ_p) affects a cell because an increase in pressure causes the water to leave the cell and enter the lower pressure area. Solute potential is measured using an osmometer. Solute potential depends solely on the number (and not the type) of dissolved particles or molar concentration. The solute potential is calculated using the following formula:

Ψ_s = –iCRT

where

i = ionization constant (for sucrose this is 1.0 because it does not ionize in water)
C = molar concentration
R = pressure constant (R = 0.0831 liter bar/mole K)
T = absolute temperature in Kelvin (K273 + °C of solution)

The units of measure will cancel as in the following example:

For a 1.0 M sucrose solution at 22 °C at standard atmospheric pressure
If the pressure potential is known to be zero (Ψ_p = 0) then the water potential equals the solute potential.
The normal turgid (rigid) state of plant cells is the result of water potential. It is this turgid state that makes the green parts of the plant “stand up” toward sunlight. This phenomenon involves the movement of water by osmosis into the cell, from a region of higher water potential outside the cell to the vacuole of the cell, which has a lower water potential. The increasing volume of water in the vacuole causes it to enlarge and press the cell contents against the cell wall. Eventually a point is reached when the cell wall cannot stretch any more. At this point there will be no further net uptake of water by osmosis—the water potential inside the cell equals that outside the cell. A wilted plant is usually the result of a loss of turgidity of the tissues, as a consequence of excessive
water loss.

Animals must also compensate for the effects of water potential on their cells. In very dilute solutions, animal cells swell up and burst. In concentrated solutions, water exits the cell by osmosis and the cell shrivels. Consequently, animal cells must always be bathed in a solution having the same water potential as their cytoplasm, or the animals must have methods to regulate the water potential. The regulation of water and ion concentrations in the body is called osmoregulation. In humans, the kidney regulates the amount of water and mineral salts in the blood under the direction of the hypothalamus. Other animals have methods of conserving water on dry land or in sea water or of ridding their bodies of excess water if they reside in freshwater habitats.

The traditional laboratory used to demonstrate osmosis involves the use of dialysis tubing, glucose, starch, iodine, water and a glucose test strip. Glucose and starch are mixed together and placed inside a piece of dialysis tubing. The dialysis tube is placed into a beaker of iodine water.

After several minutes, a glucose test strip is used to determine if the glucose has diffused out of the dialysis tubing. At the same time, the iodine acts as an indicator of starch. Brown iodine reacts with starch to form a dark blue complex molecule.

Experiment Overview

In Activity 1, a dialysis bag filled with a glucose/starch solution will be immersed in a dilute iodine solution. Water, glucose, starch, and iodine molecules will all be in motion, and the net movement of molecules will be from a region of high concentration to a region of lower concentration unless the molecule is too large to pass through the membrane.

In Activity 2, six dialysis bags will be filled with different molarities of sucrose solution and placed into deionized water. The dialysis bags will demonstrate the relationship between solute concentration and water movement.

Activity 3 is similar to Activity 2, except that potato cores will be used instead of dialysis bags to imitate a living cell system. The water potential of the potato cells will also be calculated.

Activity 4 uses microscopes to observe plasmolysis in living tissue.

Materials

Safety Precautions

Iodine solutions are irritating to eyes and skin, mildly corrosive and toxic by ingestion. Scalpels and knives are sharp instruments, use caution when cutting. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all normal laboratory safety guidelines.

Procedure

Activity 1—Diffusion

1. Obtain 1 mL of the glucose–starch solution in a graduated pipet.
2. Record the color of the glucose–starch solution on the Diffusion Worksheet
3. Place 2 drops of the glucose–starch solution onto a glucose test strip. Set the test strip aside on a clean piece of paper for 5 minutes.
4. Using a clean graduated pipet, add 2 drops of iodine solution to the remaining glucose–starch solution into the small weighing dish and mix.
5. Observe the color of the glucose–starch–iodine solution and record the color on the Diffusion Worksheet.
6. After the 5 minutes, observe the color of the glucose test strip and note this color on the Diffusion Worksheet.
7. Obtain a 20-cm piece of pre-soaked dialysis tubing.
8. Twist and roll approximately 3 cm of the end of the dialysis tubing and secure the end with a dialysis clamp to form a bag.
   {10766_Procedure_Figure_3}
9. Open the other end of the dialysis tubing bag by rubbing the free end between your fingers until the edges separate.
10. Use a graduated cylinder to measure 15 mL of the glucose–starch solution.
11. Place a funnel into the open end of the dialysis bag and pour the 15 mL of glucose–starch solution into the dialysis bag. Rinse the graduated cylinder with water.
12. Twist and roll the open end of the bag and use a dialysis clamp to close off the end of the dialysis bag, leaving sufficient space for the expansion of the bag’s contents.
    {10766_Procedure_Figure_4}
13. Rinse the dialysis tube bag thoroughly under gently running water so that there is no glucose–starch solution on the outside of the dialysis bag.
14. Fill a 9-oz cup approximately ¾ full with deionized water.
15. Place the dialysis tube in the water-filled cup. Note: The dialysis bag should be completely immersed in water.
16. Using a clean graduated pipet, add 4 mL of iodine solution to the deionized water.
17. Record the color of the deionized water–iodine solution on the Diffusion Worksheet.
18. Using a clean graduated pipet, remove 2 drops of the deionized water–iodine solution from the cup and drip onto a new glucose test strip. Set the test strip aside on a clean piece of paper for five minutes.
19. After five minutes, observe the color of the glucose test strip and record this color on the Diffusion Worksheet.
20. Allow the dialysis experiment to continue for approximately 30 minutes or until a distinct color change occurs in the bag or beaker.
21. Record the final color of both the solution in the bag and the solution in the cup on the Diffusion Worksheet.
22. Using a clean graduated pipet, remove 2 drops of solution in the cup and drip onto a fresh glucose test strip. Set the test strip aside on a clean piece of paper for 5 minutes.
23. After 5 minutes, observe the color of the glucose test strip and record this color on the Diffusion Worksheet.
24. Using a clean graduated pipet, place 2 drops of the bag solution onto a new glucose test strip. Set the test strip aside on a clean piece of paper for 5 minutes.
25. After 5 minutes, observe the color of the glucose test strip and record this color on the Diffusion Worksheet.

Activity 2—Osmosis

1. Obtain and label six 9-oz cups, each with one of the following labels:

   0.0 M sucrose solution (deionized water)
   0.2 M sucrose solution
   0.4 M sucrose solution
   0.6 M sucrose solution
   0.8 M sucrose solution
   1.0 M sucrose solution

2. Fill each of the six 9-oz cups with 80 mL deionized water.
3. Obtain six 20-cm strips of presoaked dialysis tubing.
4. Twist and roll the open end of one dialysis bag and use a dialysis clamp to close off the end of the dialysis tubing to form six bags.
5. Open the other end of the bag by rubbing the free end between your fingers until the edges separate.
6. Use a graduated cylinder to measure 15 mL of deionized water.
7. Place a funnel into the open end of the dialysis bag and pour 15 mL of deionized water into the dialysis bag. Rinse the graduated cylinder with tap water.
8. Remove most of the air from the bag by drawing the dialysis bag between two fingers. Use a dialysis clamp to tie off the other end of the dialysis bag, leaving sufficient space for the expansion of the bag’s contents.
9. Rinse the dialysis tubing bag thoroughly under gently running water so that there is no sucrose solution on the outside of the dialysis bag.
10. Carefully blot the outside of the dialysis bag by using a paper towel.
11. Mass (weigh) the dried bag with its contents and record the mass in grams on Table 1 on the Osmosis Worksheet. This is the initial mass of the bag. Set the bag aside on a clean piece of paper.
12. Repeat steps 4–11 five more times using each of the remaining solutions.
13. Immerse the sucrose dialysis bags into their labeled cups of deionized water. Be sure to completely submerge the bags.
14. Let the dialysis bags stand for 30 minutes.
15. After 30 minutes, remove the bags from the water.
16. Carefully blot the outside of the bags dry and determine the final mass of each bag.
17. Record the mass of each bag on the Osmosis Worksheet.
18. Calculate the mass difference by subtracting the initial mass from the final mass for each dialysis bag. Record the mass difference on the Osmosis Worksheet.

    Mass difference = Mass_final – Mass_initial

19. Calculate the percent change in mass for each dialysis bag by dividing the mass difference by the initial mass and multiplying the result by 100. Record the percent change in mass on the Osmosis Worksheet.

    % change in mass = (Mass difference/Mass_initial) x 100%

20. Obtain data from the other lab groups to complete Table 2 on the Osmosis Worksheet.

Activity 3—Water Potential

1. Use a graduated cylinder to measure 100 mL of the assigned molar sucrose solution into a 9-oz cup labeled with the concentration of sucrose solution and the group’s names or numbers.
2. Slice the potato into 3 cm thick disks. Note: Use caution when cutting with a sharp instrument.
   {10766_Procedure_Figure_5}
3. Use the potato slicer to cut four cores cylinders from the center of the discs. Do not include any skin as the skin impedes osmosis.
4. Place the four potato cores into a second cup and cover the cup with plastic wrap while waiting to mass them on the balance.
5. Determine the mass of all four cores together on the balance and record the combined mass on the Water Potential Worksheet.
6. Place the four cores into the cup of sucrose solution.
7. Cover the potato–sucrose cup with plastic wrap to prevent water from evaporating from the cup. Secure the plastic wrap with a rubber band.
8. Let the potato soak in the sucrose solution overnight or as directed by the instructor.
9. After soaking, carefully remove the four potato cores from the sucrose solution.
10. Gently blot the excess liquid from the outside of the potato cores with paper towels.
11. Determine the mass of all four cores together on the balance and record the combined mass on the Water Potential Worksheet.
12. Calculate the mass difference and record in Table 1 on the Water Potential Worksheet. Note: Only fill in the data from your assigned solution. The remaining cells will be filled in during step 14 of the procedure.
13. Calculate the percent change in mass and record in Table 1 on the Water Potential Worksheet.
14. Obtain data from the other lab groups to complete Table 1 on the Water Potential Worksheet.

Activity 4—Plasmolysis

1. Using lens paper, clean a slide and cover slip.
2. Place a small piece of the epidermis of the plant tissue onto the center of the slide.
3. Place 1–2 drops of deionized water on the specimen.
4. Hold the cover slip by its sides and lay its bottom edge on the slide close to the specimen. Hold the cover slip at a 45° angle.
5. Slowly lower the cover slip so that it spreads the water out. If air bubbles (looking like little black doughnuts) form, gently press on the cover slip with a pencil eraser to move them to the edge. If there are dry areas under the cover slip, add a little more water at the edge of the cover slip. Too much water can be dabbed off with a piece of paper towel.
6. Focus on several cells at low magnification (40X).
7. Rotate the nose piece to 100X magnification, focus the microscope, and observe the cells.
8. Sketch and describe the appearance of the plant cells on the Plasmolysis Worksheet.
9. Add 3 drops of 4 M sodium chloride solution to one edge of the cover slip.
10. Draw the sodium chloride solution under the cover slip by touching a piece of paper towel to the water under the opposite edge of the cover slip. Note: Capillary action will draw the water into the paper towel and move the sodium chloride solution under the cover slip.
11. Sketch and describe the appearance of the plant cells on the Plasmolysis Worksheet.
12. Add 3 drops of deionized water to one edge of the cover slip.
13. Draw the deionized water under the cover slip by touching a piece of paper towel to the sodium chloride solution under the opposite edge of the cover slip.
14. Sketch and describe the appearance of the plant cells on the Plasmolysis Worksheet.
15. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

10766_Student1.pdf