AP Biology
AP Lab Four: Water Potential and Osmosis
Name_________________________________________
Atoms and molecules are constantly in motion, bumping off of membranes, barriers, each other,
without end. The results of this—among other phenomena—are the processes of diffusion, osmosis and
water potential. Water potential is the movement of water from high energy (high potential) areas to
low energy (low potential) areas.
Procedure
Part A: Cell Races
1. For this lab you and your team will compete against the other teams in class to see who can
design the most efficient cell. Your goal will be to have a cell that maximizes volume and mass
while, at the same time, minimizing the amount of time it takes a liquid to diffuse throughout
the cell.
2. Your cell will be carved out of a 3cm3 cube. Obtain a piece of graph paper and sketch a rough
draft of what your cell will look like. Include this sketch with your lab when you turn it in.
a. *Your cell may be any shape you like, with one exception. You cannot “donut-hole” your
cell. Whatever your structure is, it must be intact in the center of the cell.
3. Using a scalpel, carefully carve out your cell according to your design. You will not be given
“redo” cubes if you mess up.
4. Once your cube is carved, use a balance or scale to mass your cell. Record this mass in Table 4.1.
5. Carefully place your cell in a beaker and have your timer ready. Fill your beaker with enough
vinegar to cover your cell. When the vinegar first touches the cell, start your timer.
6. Monitor your cell until you can visually see all bromothymol blue disappear in your cube. Once
the entire cube is yellow, stop the timer. Record the time in Table 4.1.
7. Calculate the ratio of the mass of the cubes (grams) divided by the total number of seconds to
diffuse. Record your ratio on the front board.
8. The team with the highest ratio in their class will win 10 points extra credit, and the team with
the highest ratio of all classes will win 30 points extra credit for this lab.
Part B: Water Potential
1. Pour 100 ml of each solution into a separate 250-ml cup. The solutions are:
a. Distilled water
b. 0.2 M sucrose
c. 0.4 M sucrose
d. 0.6 M sucrose
e. 0.8 M sucrose
f. 1.0 M sucrose
2. Obtain your food from Mr. Marshall. Depending on your class period, you will obtain either a
russet potato, a Yukon potato, a red potato, or a yam.
3. Slice your food into cubes that are approximately 3-cm thick. Make sure not to include any skin.
You need a total of four cubes for EACH cup (24 total). Keep your cubes together in a covered
container unless they are being massed or transferred to the solutions.
4. Determine the mass of the four cubes together and record as the initial mass for distilled water
in Table 4.2. Place these four cubes in the container of distilled water.
5. Cover the cup with plastic wrap to prevent evaporation
6. Repeat steps 4-5 for each cup of sucrose solution (0.2, 0.4, 0.6, 0.8, 1.0 M).
7. When all cups are filled and covered, let stand overnight.
8. The next day, remove the cubes from the beakers. Blot them gently on a paper towel and
determine their total final mass. Record in Table 4.2.
9. Calculate the % change in mass for both individual group results and class results. Negative
numbers are ok. Record this values in Table 4.3. The % change in mass is:
Final mass – Initial Mass X 100
Initial Mass
.
10. Using the graph provided, determine the molar concentration of the cube cores for each food
type.
a. Plot the class averages of the four cube cores % change in mass for each container.
b. Find the best-fitting line for each set of data. Where the lines cross the X-axis represents
the molar concentration of sucrose for that particular food.
Part C: Osmosis
1. Obtain two containers. Fill each container with 0.5 M sucrose solution. Label one container
“solution A” and “solution B”.
2. Obtain two 30-cm strips of presoaked dialysis tubing
3. Tie a knot in one end of each piece of tubing to form the bags.
4. Fill one bag with 10-15 ml of solution A. Remove most of the air from the bag. Tie off the other
end of the bag, leaving sufficient space for the expansion of the contents in the bag (the solution
should only fill about 1/3 to 1/2 of the tubing)
5. Repeat step 3-4 for the other bag and solution B.
6. Rinse each bag gently with distilled water to remove any solution spilled during the filling.
7. Carefully blot the outside of each bag and record the initial mass of the bag in Table 4.4.
8. Place the bag of solution A in the solution A container. Repeat with the B bag. Be sure to
completely submerge the bag.
9. Let them stand for 20 minutes.
10. At the end of 20 minutes, remove the bags from the water. Carefully blot dry and determine the
mass of each bag.
11. Record your group’s data in Table 4.4. Calculate the % change in mass as you did in part B for
each bag.
a. NOTE: One of the solutions is distilled water. The other is 2.0 M sucrose solution.
Post-Lab Questions
1. What was your rationale for how you designed your cell?
2. What was the key feature in the design of the winning cell?
3. Why do you calculate % change in mass rather than simply the change in mass of the potatoes
and the bags?
4. Which food sample has the highest molarity of sucrose naturally? Explain how you know.
5. Which solution in part C is distilled water and which is sucrose? Explain, using evidence from the
lab, how you know.
6. You are in the hospital and you need IV fluids. You read the label on the bag and notice that it
says it has salts in it. Explain why.
7. If a plant cell has a lower water potential than its surrounding environment, and the pressure is
equal to zero, is the cell hypertonic or hypotonic to its environment? Will it gain or lose water?
Explain your response.
8. Give two specific reasons why cells are small. Be sure to reference at least one part of this lab
with each answer.
Lab Book Requirements
1. Date of Lab
2. Title of Lab
3. Objective of AP Lab 4
4. Sketch of your cell design.
5. Manipulated and Responding variables for Part B only
6. At least three controls for Part B only
7. Hypothesis for B
8. Table 4.1
9. Table 4.2
10. Table 4.3
11. Table 4.4
12. Food Sucrose Molarity Graph
13. Conclusion
14. What is the next experiment you could perform?
15. Answers to Post-Lab Questions
Table 4.1—Cell Race
Mass of your Cube
Time to Diffuse Completely
Ratio
(Mass of Cube)
(Total Seconds)
Table 4.2—Food Cube Individual Group Data
Contents in
Beaker
Initial Mass (g)
Final Mass (g)
Mass Difference
Your Group
% Change in
Mass
Class Average
% Change in
Mass
0.0 M Dist. H2O
0.2 M Sucrose
0.4 M Sucrose
0.6 M Sucrose
0.8 M Sucrose
1.0 M Sucrose
Table 4.3—Food Core Results Class Data
Food 
Russet Potatoes
Class Averages, Percent Change in Mass of 4 Food Cores
Yukon Potatoes
Red Potatoes
Yams
0.0 M
Dist. H2O
0.2 M
Sucrose
0.4 M
Sucrose
0.6 M
Sucrose
0.8 M
Sucrose
1.0 M
Sucrose
Table 4.4—Dialysis Bag Results Individual Group Data
Contents in
Dialysis Bag
Solution A
Solution B
Initial Mass (g)
Final Mass (g)
Mass Difference
% Change in Mass