AP BIOLOGY LAB: Osmosis and Diffusion

Procedure 2: Diffusion and Osmosis in a Synthesis Cell (dialysis tubing)

    Part A: Demo in front of class

    Drawing of Set Up 1A:

    Result Table 1A:

    Analysis of Results 1A:
    1. Which substance(s) are entering the bag and which are leaving the bag? What experimental evidence supports your answer?

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

    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?

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

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

    Part B: Comparing Affect of Various Concentration of Sucrose on Synthesis Cells

      Drawing of Set Up 1B:

        Result Tables 1B:

            Percent Change in Mass = Final Mass - Initial Mass/ Initial Mass   X 100   

Analysis of Results 1B:
    1. Graph the results for your individual data. For this graph you will need to determine the independent and dependent variables to label the axis with and make a title for your graph.

    2. Determine the molarity of each color and explain the relationship between the change in mass and the molarity of sucrose within the dialysis bags.  (Relationship statement, using data to support).

    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 distilled water. Explain your response.

    4. Why do you think you were asked to calculate the percent change in mass rather than simply using the change in mass?

    5. A dialysis bag is filled with distilled water and then place in a sucrose solution. The bag's initial mass is 20 g and its final mass is 18 g. Calculate the percent change of mass, showing your calculations.

    6. The sucrose solution in the beaker would have been _____________ to the distilled water in the bag. (Write in the word that best completes the sentence.)  isotonic     hypertonic     hypotonic

Exercise 1C: Water Potential

      Drawing of Set Up 1C:

        Result Tables 1C:

        Temperature of Room ______________   

        Potato Squares Individual Data

            Percent Change in Mass = Final Mass - Initial Mass/ Initial Mass   X 100   

Analysis of Results 1C:

    1. Graph both your individual data and the class average for the percentage change in mass on one graph. For this graph you will need to determine the independent and dependent variables to label the axis with and make a title for your graph.

    2. Determine the molar concentration of the potato square. This would be the sucrose molarity in which the mass of the potato squares does not change. To find this, draw a straight line on the graph you made above through your data points (best fit line). The point at which this line crosses the x-axis represents the molar concentrations of sucrose with a water potential that is equal to the potato tissue water potential. At this concentration there is no net gain or loss of water from the tissue. Indicate this concentration of sucrose in the space provided below.

        Molar concentration of sucrose = _______________________M

Exercise 1D: Calculation of Water Potential from Experimental Data

    1. Calculate the solute potential of this sucrose solution using the following formula:

                                    ^ψ_s = -iCRT

            where i = Ionization constant (for sucrose this is 1.0 because sucrose does not ionize in water)
                     C = Molar concentration (determine at the end of Exercise 1C)
                     R = Pressure constant (R = 0.0831 liter bars/mole K)
                     T = Temperature K (273 + °C of solution)
            Potential of pure water is zero because it doesn't want to move, if you put a solute in it, the water will move so negative experience...

    2. Calculate the water potential of the potato cells, knowing that the pressure potential of the solution is zero. ( remember  ^ψ = ^ψ_p + ^ψ_s )

    3.Water potential values are useful because they allow us to 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. What direction did water flow in each of the solutions?

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

    5. 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 gain water or lose water? Explain you response.

    6. Zucchini cores placed in sucrose solution at 27°C resulted in the percent changes below after 24 hours. Make a graph of this data with independent and dependent variables labeled and a title on the graph. Determine the molar concentration of solutes within the zucchini cells.
                            % Change in Mass            Sucrose Molarity
                                    20%                            Distilled Water
                                    10%                                  0.2 M
                                    -3%                                  0.4 M
                                    -17%                                0.6 M
                                    -25%                                0.8 M
                                    -30%                                1.0 M

    7. Calculate solute potential (^ψ_s ) of the sucrose solution in which the mass of the zucchini cores does not change.  Also calculate the water potential ^(ψ) of the solutes within the zucchini cores.

    8. What effect does adding solute have on the solute potential component (^ψ_s ) of that solution? Why?

    9. Explain what would happen to a red blood cell (RBC) placed in distilled water. Be sure to include which solution has the higher water potential in your explanation and why?