AP Biology - Osmosis and Diffusion Inquiry Lab

Procedure #1: Surface Area and Cell Size

Cell size and shape are important factors in determining the rate of diffusion. Think about cells with specialized functions, such as the epithelial cells that line the small intestine or plant root hairs.

Step #1: Prepare the Gelatin recipe:

1 pkg of gelatin (gelatine) 237 ml water 2.5 ml 1% phenalthelin drop 0.1M NaOH till pink Refrigerate until tomorrow

Step #2: Preparing the gelatin blocks: Using a knife, cut 3 blocks of gelatin of different sizes

o A: 1cm x 1cm x 1cmo B: 2cm x 2cm x 2cmo C: 3cm x 3cm x 3cm

Place the block in 0.1M HCl or white vinegar (whichever is provided to you) Time how long it takes each block to go clear. Graph your data of time vs. surface area

– volume ratio. Answer the questions in the lab report section.

Step #3: Designing and Conducting Your Investigation: Using the available material (gelatin, indicator, acid, etc.) design an experiment that will

allow you to test your predictions regarding the relationship of surface area and volume in the artificial cells to the diffusion rate. Record your proposed experiment in the lab report.

Once you have written a description of your experiment, have the teacher check the design to provide approval.

Run the experiment, collect data and analyze your results.

Procedure #2: Modeling Diffusion and Osmosis

Procedure: (REQUIRES 30 MINUTES AFTER SETUP)1. Obtain 6 30-cm strips of dialysis membrane2. Use a dialysis clip at one end of the tube and form 6 bags. Pour approximately 15-25 ml

of the following solutions into separate bags:a. Distilled waterb. 0.2 M sucrosec. 0.4 M sucrosed. 0.6 M sucrosee. 0.8 M sucrosef. 1.0 M sucrose

3. Remove excess air and attach a clip to the opposite end of the bag. 4. Use distilled water to carefully rinse any liquid that may have spilled on the outside of the

tube.5. Carefully dry the bag with a blotting action and measure the mass in grams.6. Place each bag in a cup and record the molarity of the bag.7. Add distilled water to completely submerge the bags.8. Let the system stand for 30 minutes and record the mass of each bag.9. Record your data and obtain class data from the other groups.

Procedure #3: Observing Osmosis in Living CellsThe interactions between selectively permeable membranes and water and solutes are important in cellular and organismal functions. For example, water and nutrients move from plant roots to the leaves and shoots because of the differences in water potentials. Based on what you know regarding osmosis, diffusion and water potential answer the question in the Procedure #3 section of the lab report.

Step #1: Observing Red Onion Skin Under a Microscope Look at a wet mount preparation of red onion skin under low power magnification using

a light microscope. Make an appropriate diagram labeling the magnification and approximate size in your

lab report. Add 2-3 drops of 15% NaCl solution to the edge of the cover slip and observe the onion

cells. Make a diagram and answer questions. Remove the cover slip and flood the onion epidermis with distilled water. Make a

diagram and answer the questions.

Procedure #4: Determining the Concentration of an Unknown Solution of Sucrose

Step #1: Preparing the potatoes (REQUIRES OVERNIGHT TIME) You will be assigned 1 or more sample solutions to work with. Pour 100 mL of the assigned solution into a labeled beaker. Slice a potato into discs that are approximately 3 cm thick. Use a cork borer with an approximate internal diameter of 5mm to cut potato cylinders. You need 4 cylinders for each solution. Determine the mass of the 4 cylinders together and record that in the data table. Place the 4 cylinders into the beaker of solution and cover it with plastic wrap or Parafilm

to prevent evaporation overnight. Let the beakers stand overnight. Remove the cores from the beakers and gently blot them with a paper towel and

determine the total mass of the 4. Record the % change from your samples and share your data with the class. Record the % change that was reported from the other groups. Graph the data for the % change in mass (both individual group data and class data) Answer all questions and complete the data tables and graphs in the lab report section.

Designing and Conducting your Investigation Design an experiment to identify the concentrations of the sucrose solutions and use

the solutions to determine the water potential of the plant tissues. Materials that are available:

o Potatoeso Potato cork borero Digital balanceso Metric rulerso Cups / beakerso Color coded unknown sucrose solutions (The teacher will prepare the solutions

with different concentrations of sucrose and will color code them to provide a visual difference.

Osmosis and Diffusion Lab Report

PART I: PRELAB - Calculation of Water Potential from Experimental Data

Ψs = -iCRTR

i = Ionization Constant (for sucrose this is 1.0 because sucrose does not ionize in water)C = Molar ContrationR = Pressure Constant (R = 0.0831 liter bars/mole oKT = Temperature oK (273 + oC

The units will cancel as in the following example:A 1.0M sugar solution at 22oC under standard atmospheric conditions.

Ψs = -(1)(1.0 Mole / Liter)(0.0831 liter bar/mole oK)(295 oK)Ψs = -24.51 bars

Knowing the solute potential of the solution (Ψs) and knowing that the pressure potential of the solution is zero (ψp = 0) allows you to calculate the water potential of the solution. The water potential will be equal to the solute potential of the solution.

Ψ= 0 + Ψs or Ψ = Ψs

1. The water potential of the solution at equilibrium will be equal to the water potential of the potato cells. What is the water potential of the potato cells? SHOW YOUR CALCULATIONS

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

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

4. The container in the above diagram is open to the atmosphere. What is the pressure potential (ψp) of the system?

5. Where is the greatest water potential?

6. What direction will water diffuse? Explain why.

Zucchini cores placed in sucrose solution at 27oC resulted in the following percent changes after 24 hours:

% Change in Mass Sucrose Molarity20 % Distilled Water10% 0.2 M-3% 0.4 M-17% 0.6 M-25% 0.8 M-30% 1.0 M

Initial Values Beaker Contents

(0.4% Sucrose solution)

Ψs= (-)4

Dialysis Bag

(with 0.1 sucrose solution)

Ψs= (-)1 , ψp = 0

Graph the results and calculate the molar concentration of the solutes within the zucchini cells.

7. What is the molar concentration of solutes within the zucchini cells?

8. Calculate the solute potential (Ψs) of the sucrose solution in which the mass of the zucchini cores does not change. Show your work.

% increase in the mass of the cores

% decrease in the mass of the cores

0.0

Sucrose Molarity within the Beaker

9. Calculate the water potential (Ψ) of the solutes within the zucchini cores. Show your work.

10. What effect does adding solute have on the solute potential (Ψs) component of that solution? Why?

11. Calculate the solute potential of a 0.1 M NaCl solution at 25O C. (show your work)

12. If the concentration of NaCl inside the plant cell is 0.15 M, which way will the water diffuse if the cell is placed into the 0.1 M NaCl solutions?

13. What must the turgor pressure equal if there is no net diffusion between the solution and the cell?

PART II. Procedure #1 Questions:14. What is the surface area to volume ratio for the 3 cells?

a.

b.

c.

15. If you put the blocks into the solution, into which block would that solution diffuse into the greatest percentage of the cell the fastest?

16. Which one would diffuse into at the slowest rate?

17. How do you explain the differences?

18. Graph your Time to go clear vs Surface Area to Volume ratio: (be sure to include a proper title, x & y axis labels and a legend)

19. Provide a detailed protocol to for your procedures for part #1 here: Be sure to include a simple sketch of your cell shape and why you chose that specific design.

20. What was the rate of absorption in your “GREAT CELL RACE”?

21. Did your cell design provide optimal transport of the substances or could you have made improvements? What changes would you incorporate if given the opportunity?

PART III: Dialysis Analysis

Individual Group Data – Dialysis Bag Results

Contents in Bag

Initial Mass Final Mass Mass Difference % Change in mass

0.0 M H2O

0.2 M Sucrose

0.4 M Sucrose

0.6 M Sucrose

0.8 M Sucrose

1.0 M Sucrose

Class Data – Percentage Change in Mass of Dialysis Bags

Contents 1 2 3 4 5 6 7 Total Average0.0 M

0.2 M

0.4 M

0.6 M

0.8 M

1.0 M

Graph the results of the Dialysis Membrane Experiment:

a. Independent Variable: _____________________________ (x-axis)

b. Dependent Variable: _______________________________ (y-axis)

22. Explain the relationship between the change in mass and the molarity of sucrose within the dialysis bags.

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

24. Why was it necessary to calculate the percentage change in mass rather than simply using the change in mass?

PART IV: Procedure #3 Questions:

25. What would happen if you applied saltwater to the roots of a plant?

26. Draw the red onion cell observations and label each: (Don’t forget magnification and size)

Before After Salt After Water

27. What impact did salt water have on the onion cells?

28. What impact did the fresh water have on the onion cells?

29. What is plasmolysis?

30. Why did the onion cells plasmolyse?

31. In the winter, grass often dies near roads that have been salted to control ice. What is the cause of this occurrence?

32. To answer the final 3 questions consider “What would happen to a red blood cell (RBC) placed in distilled water?”

33. What would have the higher concentration of water molecules?

34. Which would have the higher water potential?

35. What would happen to the red blood cells? Why?

Potato Core Individual Results:

Potato Core Class Data

Contents 1 2 3 4 5 6 7 Total Average1.0 M

0.2 M

0.4 M

0.6 M

0.8 M

2.0 M

Explain your procedures for the experiment:

Determine the molar concentration of the potato core. This would be the sucrose molarity in which the mass of the potato core does not change. To find this, draw the linear graph that best fits your data. The point where this line intersects the X-axis represents the molar concentration of the sucrose with a water potential that is equal to the potato tissue water potential. At this concentration there is not net gain or loss of water from the tissue.

MOLAR CONCENTRATION OF SUCROSE = _______________ M