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 3cm 3 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 bromthymol 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 labeled by color only, but each solution is one of six sucrose molarities: Distilled water, 0.2 M sucrose, 0.4 M sucrose, 0.6 M sucrose, 0.8 M sucrose, and 1.0 M sucrose. a. NOTE: Before the lab is over I will reveal to you the answers of which molarity is represented by which color. HOWEVER, as a non-graded test, see if you can use your knowledge and reasoning skills to determine based on the results of this experiment which color is which. (Think about would potatoes in 1.0 M sucrose GAIN or LOSE the most mass? How about 0.0 M sucrose?).
2. Obtain your food from Mr. Marshall. Depending on your class period, you will obtain a russet potato, a Yukon potato, a red potato, or a yam. 3. Slice your food into cubes between 1-3 cm 3 in volume. 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 Red Solution in Table 4.2. Place these four cubes in the container of red solution. 5. Cover the cup with plastic wrap to prevent evaporation 6. Repeat steps 4-5 for each colored solution (orange, yellow, green, blue, and purple). 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. c. For at least your group s potato you must also include the standard deviation error bars for the % change in mass at each molarity. You may choose to do this with the other three potatoes as well for practice, but it is optional. Part C: Osmosis 1. Obtain two containers. Fill each container ½-way with 0.5 M salt solution. Label one container solution A and solution B. *NOTE: You will only need enough to submerge your bags 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. Let them stand for 20 minutes. 9. Remove the bags from the water. Carefully blot dry and determine the mass of each bag. 10. 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 the key feature in the design of the winning cell that allowed it to be efficient? 2. Why do you calculate % change in mass rather than simply the change in mass of the potatoes and the bags? 3. Which food sample has the highest molarity of sucrose naturally? Explain using evidence from the lab, how you know. 4. Which solution in part C is distilled water and which is sucrose? Explain, using evidence from the lab, how you know. 5. 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. 6. Give two specific reasons why cells are small. Be sure to reference at least one part of this lab with each answer. 7. Do any of your error bars on your graph cross another line on the graph? Whether yes or no, what does this tell you about the data you calculated for your potato molarity? Lab Book Requirements 1. Date of Lab 2. Title of Lab 3. Objective of AP Lab 4 overall 4. Sketch of your cell design (I should be able to recreate your cell) 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 Molarity Graph (with error bars for your group s potato, as well as any of the other three potatoes you choose to calculate the standard deviation for). 13. Conclusion for Part B only 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 Color Initial Mass (g) Final Mass (g) Mass Difference Your Group % Change in Mass Molarity of (Your guess) Red Orange Yellow Green Blue Purple Table 4.3 Food Core Results Class Data Food 0.0 M Dist. H2O 0.2 M 0.4 M 0.6 M 0.8 M 1.0 M Class Averages, Percent Change in Mass of 4 Food Cores Russet Potatoes Yukon Potatoes Red Potatoes Yams Table 4.4 Dialysis Bag Results Individual Group Data Contents in Dialysis Bag Initial Mass (g) Final Mass (g) Mass Difference % Change in Mass Solution A Solution B