AP BIOLOGY – LAB 1: DIFFUSION & OSMOSIS

Pre-Lab: Read the introductory information (handed out in class) and the procedures and answer the following:

  1. Formulate a hypothesis on the outcome of part A. If dialysis tubing with a sucrose/starch solution is placed into a beaker of distilled water then ______.
  2. Based on the procedures for part B, what would you expect your results to be? Be specific and articulate in your answer. ______
  3. After reading the procedures for part C, predict what you think will happen to the mass of the potato cores in each of the solutions: ______
  4. Fill in the blanks below:

If a cell is placed in a ______solution, it has ______solute in solution than the surrounding fluid, and will therefore experience a ______of water to its surroundings. This cell has ______water potential since there is a great deal of ______pressure causing water to leave the cell. Conversely, a cell sitting in a ______solution has a ______water potential, and since it will experience a ______of water, there will be little osmotic pressure causing water to ______the cell.

Procedures: Part A / DIFFUSION (Day 1) ~ 45 minutes

  1. Prepare a piece of dialysis tubing that has been soaking in water by tying a knot in one end of the tubing and then placing 15mL of glucose/starch solution into the tubing. Test the solution in the tubing with a glucose test strip and then tie off the other end of the tubing (be sure to leave enough room for some bag expansion). Note the color of the solution and the test strip in the bag and record them in the data table.
  2. Prepare a beaker with 250 mL of water and add 1mL of potassium iodide (IKI) to the beaker. Note the color of the solution and record it in the data table.
  3. Determine if glucose is present in the beaker using a glucose test strip. Record the results in the data table.
  4. Completely immerse the dialysis bag in the solution in the beaker and wait 30 minutes.
  5. Remove the bag from the beaker and record the final colors of the solutions in the data table.
  6. Determine the presence of glucose in the beaker and the dialysis tubing bag using the indicator strips. To test the solution in the bag, make a small cut in the bag with a pair of scissors, and insert the indicator strip through this hole. Record the presence or absence of glucose in the data table.
  7. Be sure work space is clean before leaving the lab.

Color / Glucose content
Time / Dialysis Bag / Beaker / Dialysis Bag / Beaker
Start
30 minutes

Procedures: Part B / OSMOSIS (Day 2) ~ 90 minutes

  1. Obtain six plastic cups and label them as follows: water, 0.2 M, 0.4 M, 0.6 M, 0.8 M, and 1.0 M.
  2. Obtain six pieces of dialysis tubing and tie off one end of the tubing. Pour 25 mL of distilled water into one piece of tubing. Be sure to leave enough room for expansion and then tie off the other end. Blot the tubing dry, find the mass and place it in the cup labeled “water.” Be sure to record the mass in the data table.
  3. Repeat step 2 for each of the other solutions.
  4. Fill each of the cups approximately ¾ full of distilled water and completely immerse each bag in the appropriately labeled cup.
  5. Wait 30 minutes. Remove the bags from the cups. Blot them dry, and weigh each one again.
  6. Record final masses in the data table.
  7. Calculate the percent change in mass for each of the dialysis bags using the following formula:

%change = (Final mass – Initial mass) / Initial Mass x 100

  1. Record the % change in the data table.
  2. Graph both your individual and class average results (on the same graph).

Solution / Dialysis Bag
Initial Mass / Dialysis Bag
Final Mass / Change in mass (grams) / Individual % Change in Mass / Class Average % Change in Mass
Water
0.2 M
0.4 M
0.6 M
0.8 M
1.0 M

Procedures Part C / WATER POTENTIAL (Day 3 & 4) ~ 30 min & 15 min

Day 3

  1. With the cork borer, cut four cylinders from a potato for each solution you will be using. Cut each cylinder to a length of 3 cm for greater accuracy; remove any skin from the cylinders. Place the cylinders in a covered cup or beaker.
  2. Weigh four cylinders together. Record the initial mass of the cylinders in Table 3.
  3. Pour 100 mL of one solution you have been assigned into a beaker or plastic cup. Take the initial temperatures and record them in Table 3. Insert the four potato cylinders and cover the beaker or cup with plastic wrap and let the beaker sit over night.

Day 4

  1. Measure the final temperature in the beaker; record in the data table.
  2. Remove the four cylinders from the beaker. Blot them carefully with a paper towel and get a final mass. Record the mass in the data table.
  3. Calculate the percent change in mass for the four cylinders. Record the percent change in the data table.
  4. Graph the class results and canned data on the same graph. Place the percent change in mass on the y-axis and the sucrose molarity on the x-axis. Using the graph, determine the molar concentration of the potato cylinders. This is equivalent to the sucrose molarity in which the potato core mass is constant.

Solution Temperature / Potato Cylinders
Solution / Initial Temp / Final Temp / Initial Mass / Final Mass / Change in Mass / % Change in Mass / Canned Data % Chg.
Water
0.2 M
0.4 M
0.6 M
0.8 M
1.0 M

Procedures: Part D / WATER POTENTIAL CALCULATION (Homework) ~ 30 minutes

  1. Using the data from part C and the following formula, calculate the osmotic potential of the sucrose solutions in bars.

Osmotic potential can be calculated using the following formula:

Ψπ = -iCRT

i = ionization constant

C = osmotic molar concentration (determined in part C)

R = pressure constant (0.0831 liters•bars / mol•K

T = temperature (Kelvin)

Procedures: Part E / PLANT CELL PLASMOLYSIS (Day 5) ~ 40 minutes

  1. Cut an onion in half lengthwise, from top to bottom, and remove the center if this has not already been done. Note the layers of the scale leaves in the onion. The epidermal layer, only one cell thick, between each scale is the layer that will be used.
  2. Make several shallow cuts approximately 1 cm apart on one of the scale leaves. Make more cuts perpendicular to the first set, resulting in 1 x 1 cm squares.
  3. Using forceps, carefully remove the epidermal layer from one of the squares. If the layer tears while being removed, take another layer from one of the other squares.
  4. Place the later in two to three drops of distilled water on a microscope slide. Place a coverslip on top.
  5. Examine the cell layer under 100 X magnification and note the characteristics of the cells. Draw several representative cells.
  6. Remove the slide from the microscope and add two to three drops of 15% NaCl solution to one side of the coverslip.
  7. Carefully touch the edge of appear towel to the other side of the coverslip in order to draw the NaCl solution across the sample.
  8. Allow the slide to sit for two or three minutes in the salt solution and re-examine it under the microscope.
  9. Note the appearance of the cells; draw several representative cells.

Analysis/Conclusions:

  1. Compare and contrast osmosis and diffusion.
  2. In part A, did osmosis occur? Did diffusion occur? Explain your reasoning.
  3. Did the dialysis tubing serve as a selectively permeable membrane? Explain your reasoning.
  4. In part B, what caused the mass of the dialysis bags to change? Was there more or less water in the bag at the conclusion of the experiment? Explain.
  5. Was the distilled water in the beakers hypertonic or hypotonic in relation to the sucrose solution found in the dialysis bags?
  6. Suppose the dialysis bags were placed in beakers containing a 0.6 M sucrose solution as opposed to distilled water. How do you think your results would change? Sketch a graph below to show how the mass of each of the bags would be affected.
  7. Study the graph you have plotted for part C of the experiment. What is important about the point where the best fit line crosses the x-axis? What is the concentration of sucrose in your potato?
  8. The water potential of a solution is equal to the osmotic potential plus the pressure potential. Since there is no differential pressure action on the solution the pressure potential is equal to zero, making the water potential equal to the osmotic potential. If the equilibrium point between the solutions and the potato cylinders indicates the point where the two water potentials are equal, what is the water potential of the potato cells?
  9. Would the water potential of the potato cells change if the cylinders were allowed to dry out? In what way?
  10. What are the effects on cells when they are placed in a hypotonic solution, a hypertonic solution, and an isotonic solution?

NOTES:

  • The pores in the dialysis tubing are extremely small, and can be easily clogged by any oil or dirt on your fingers. Wash your hands before handling the tubing and keep physical contact to a minimum.
  • To prevent leaks, consider tying two knots, about ¼ inch apart on each end of the tubing.
  • IKI solution is an irritant; it affects skin and eyes, and can stain clothing. Handle with caution.
  • If at any time the dialysis tubing becomes too dry, simply rewet it by placing it in water for several seconds.
  • When working with several solutions, try to keep the lengths of the dialysis bags the same.

Adapted from Ward’s Natural Science: AP Biology Lab 1 / Osmosis & Diffusion 36W7100 / 36W7111 9/11/06