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Caleb Gu AP Lab 4: Diffusion, Osmosis, and Water Potential Before you begin this lab, review the relevant information in the content and in your textbook. Also prepare by reading over this lab in the AP Biology Investigative Lab Manual on pages S51-S54. Prelab (IMPORTANT): Watch the following video throughout these labs https://www.youtube.com/watch?v=LeS2-6zHn6M Lab 1: Effects of Osmotic Potential Differences Across a Membrane Materials: ● 6 ~ 20 cm strips of dialysis tubing ● 0.0 M sucrose solution – distilled 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 ● Distilled water ● Timer ● Digital scale ● 6 clear plastic cups Lab 1 Hypothesis : Think about what your variables are and construct a hypothesis predicting the relationship between them. As the independent variable of the concentration of the sucrose solution increases, the dependent variable of rate of osmosis will also. Procedure: You will conduct the experiment using the virtual lab here (don’t worry about recording this data, you’ll be provided data): https://video.esc4.net/video/assets/Science/Biology/Gateway%20Resources/cell%20homeostasis%20vi rtual%20lab%20-%20activity/index.html Answer the questions after the virtual lab here. 1. Which dialysis tubes had little or no change in mass after a 24-hour period? A and D 2. Why was pure water used as a control group? The control group provides a standard for comparison for the experimental group 3. Why didn’t the mass of dialysis tubes “A” and “D” change over the 24-hour time frame? The concentration both inside and outside the dialysis tube is the same

Data Concentration of sucrose solution inside dialysis bags (outside always dH2O) 0.0 M (water) 0.2 M 0.4 M 0.6 M 0.8 M 1.0 M Class mean 1.80% 7.8% 13.8% 18.9% 23.4% 26.2% Class standard deviation 8.0% 8.9% 9.9% 12.2% 14.6% 7.6% Class sample size (n) 27 27 27 27 27 24 Graph the above data (remember what your variables are and where they go!) and paste your graph below:

Analysis Questions

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1. What is the general trend that you see between variables in the graph? Why does this occur? They are generally rising because there is more concentration outside of the sucrose bag than on the inside. Due to osmosis, the water moved from the higher concentration of solute (outside) into the lower concentration inside the dialysis bags. 2. Identify a case where solutions where isotonic to each other in the above data. Also identify one case where a solution is hypertonic (be clear which solution you are referring to). A case where the solutions are isotonic to each other is the Class mean at 0.0 M because the percentage is so close to 0 (1.8%). Class Standard deviation at .4 M is a hypertonic solution because its percentage of (9.9%) is greater than 0. 3. Identify the general rule for predicting water movement if the solute concentrations on both sides are known (and the solute is impermeable). The general rule is water potential where the higher concentration of water will move to the lower concentration. 4. These bags all spent the same amount of time in the distilled water (20 mins). So you could divide the % changes in mass by 20 minutes to calculate a rate of osmosis. As the concentration gradient increases between outside and inside, what happens to the rate of osmosis? As the concentration gradient increases between outside and inside, osmosis will eventually become an isotonic solution. The rate of osmosis will increase by using, -iCRT. Lab 2: Determining the Water Potential of Potato Cells In animal cells, the movement of water into and out of the cell is influenced by the relative concentration of solute on either side of the cell membrane. If water moves out of the cell, the cell will shrink. If water moves into the cell, the cell may swell or even burst. In plant cells, the presence of a cell wall prevents the cells from bursting, but pressure does eventually build up inside the cell and affects the process of osmosis. When the pressure inside the cell becomes large enough, no additional water will accumulate in the cell even the though cell still has a higher solute concentration than does pure water. The movement of water through the plant tissue cannot be predicted simply through knowing the relative solute concentrations on either side of the plant cell wall. Instead, the concept of water potential is used to predict the direction in which water will diffuse through living plant tissues. In a general sense, water potential is the tendency of water to diffuse from one area to another under a given set of parameters . Water potential is expression in bars, a metric unit of pressure equal to about 1 atmosphere and measured with a barometer. Water potential is abbreviated by the Greek letter psi ( Ψ ) and has two major components:

Materials: ● 0.0 M sucrose solution – distilled 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 ● Distilled water ● Timer ● Digital scale (food scale works great) or triple-beam balance ● Potato ● Potato peeler ● 6 clear plastic cups ● Plastic wrap Procedure: Watch the video starting at 5:35 Hypothesis: As the independent variable of sugar in each cup, increases, the dependent variable of mass will increase, but eventually decrease. Data Concentration of sucrose solution outside the potatoes (inside unknown solute concentration) 0.0 M (water) 0.2 M 0.4 M 0.6 M 0.8 M 1.0 M Class mean 21.88% 7.25% -7.91% -11.82% -25.88% -30.95% Class standard deviation 15.6% 25.4% 8.8% 21.8% 19.5% 6.5% Class sample size (n) 15 13 13 14 9 10 Analysis: Prepare a corresponding graph of your findings. Be sure your graph indicates positive and negative changes in mass as indicated above and below the x axis. Remember to follow good graphing procedures by labeling each axis including units and giving the graph a title. Use a best fit line to determine where the line crosses the X axis. The black lines represent the x and y axes. This point of intersection represents the point at which the potato cells do not change in mass, meaning that the solute concentration of the potato cells is isotonic to the external solution .

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Determine the molar concentration of the potato cube. This would be the sucrose molarity in which the mass of the potato cube does not change. To find this, look at graph above and find the point at which your data line crosses the x-axis. This represents the molar concentration 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 = .32 M Calculation of Water Potential from Experimental Data: The solute potential of the sucrose solution can be calculated using the following formula: Ψ S = -iCRT ● i = Ionization constant (for sucrose this is 1.0 because sucrose does not ionize in water) ● C = Molar concentration (determined above) ● R = Pressure constant (R = 0.0831 liters bars/mole K) ● T = Temperature K (273 + o C of solution) (**Room temperature water is 25 degrees C) Please take a moment to review help information on page 52-53 of your student lab manual as well as the recorded synchronous session on water potential to help you make the following calculations. First calculate the solute potential (Ψ S ) of the sucrose solution. Show answers and work here. Knowing the solute potential (Ψ S ) of the sucrose solution and that the pressure potential of the solution is zero (Ψ P = 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 + Ψ S or Ψ = Ψ S The water potential of the solution at equilibrium will be equal to the water potential of the potato cells. What is water potential of the potato cells? Ψ potato cells = 7.924 bars Reasoning paragraph – Please construct a reasoning paragraph explaining the following: 1. Identify your independent and dependent variables 2. What is the relationship between variables (discuss the trend you see) 3. Why does this trend occur? 4. Discuss the concept of water potential and the two physical factors that affect it. The independent variable is the molarities of sucrose in the beaker while the dependent variables were the percent increases in mass of potato cubes. The relationship was that as the molarity of sucrose increased, the mass of the potato decreased. This trend occurs because water potential forces water to move from high to low concentration. In this case, when the sucrose is low, the water from the outside goes into the potato to move into lower concentration. However, after .32 M, the potato becomes more concentrated, forcing the the

potato to lose water. The concept of water potential, as mentioned before, is when water moves from a higher value to a lower value. The two physical factors that affect it are osmotic potential and pressure potential.