Experiment 2: Tonicity and the Plant Cell Plant cells are able to generate osmotic pressure while other cells cannot. This is due to specialized plant structures such as the cell wall which prevent lysis caused by osmosis. By taking advantage of this system, you will be able to look at the effects of tonicity in a biological system. Materials (1) 100 mL Graduated Cylinder *Kitchen Knife Permanent Marker *2 Potatoes (these must be different 2 Pipettes types; e.g., russet, Idaho, sweet, etc.) Ruler *Stopwatch 16 g Sodium Chloride (Salt), NaCl *Water 4 Test Tubes (Glass) Test Tube Rack *You Must Provide *Cutting Board Procedure 1. Use the permanent marker to label two test tubes as A, and two test tubes as B. Place the test tubes in the test tube rack. 2. Identify the two potato types in the first two cells of the first column in Table 3. Then, select one potato to test first and record observations about the physical characteristics in Table 3. Note: Be sure to include observations which acknowledge the texture, color, and any other distinguishing factors. 3. Use a knife to carefully cut two strips of the potato on a cutting board. These will be referred to as Sample A and Sample B. The strips should be as close to 10.0 cm. long and 1.0 cm. wide as possible to ensure that the strips fit in the test tubes. 4. Fill the 100 mL graduated cylinder with 50 mL of water. Place the first potato strip (Sample A) into the graduated cylinder and record the initial displacement in Table 3. ©eScience Labs, 2016 Note: Displacement is a measurement of change. It is calculated by subtracting the original volume (50 mL) from the final volume after the potato is added to the 50 mL of water. For example, 57 mL - 50 mL = 7 mL of displacement. 5. Remove Sample A from the graduated cylinder and place it in Test Tube A. If any water was lost in the graduated cylinder, refill it to the 50 mL graduation mark. 6. Place the second potato strip (Sample B) into the graduated cylinder. Record the initial displacement in Table 3. 7. Remove Sample B from the graduated cylinder and place it in Test Tube B. If any water was lost in the graduated cylinder, refill it to the 50 mL graduation mark. 8. Repeat Steps 2 - 7 for the second potato type, using the remaining test tubes in the test tube rack. 9. Use a pipette to add water to each of the test tubes with the A samples in them until the water covers the potato strips. 10. Refer to the instructions provided on the bottle with 16 g of sodium chloride in it to create a 20% sodium chloride solution. Use a pipette to add the 20% sodium chloride solution to each of the test tubes with the B potato samples in them until the solution covers the potato strips. 11. After an hour, pour out the liquid from the test tubes. 12. Repeat Steps 4 - 7 for each sample and type, and record the final displacement values in Table 3. 13. Complete the last column of Table 3 by subtracting the initial displacement from the final displacement. Table 3: Water Displacement per Potato Sample Potato Type Potato Sample Observations Initial Displacement (mL) Final Displacement (mL) Net Displacement (mL) A B A B ©eScience Labs, 2016 Post-Lab Questions 1. How did the physical characteristics of the potato vary before and after the experiment? Did it vary by potato type? 2. What does the net change in the potato sample indicate? 3. Different types of potatoes have varying natural sugar concentrations. Explain how this may influence the water potential of each type of potato. 4. Based on the data from this experiment, hypothesize which potato has the highest natural sugar concentration. Explain your reasoning. 5. Did water flow in or out of the plant cells (potato cells) in each of the samples examined? How do you know this? 6. Would this experiment work with other plant cells? What about with animal cells? Why or why not? 7. From what you know of tonicity, what can you say about the plant cells and the solutions in the test tubes? 8. What do your results show about the concentration of the cytoplasm in the potato cells at the start of the experiment? ©eScience Labs, 2016 9. If the potato is allowed to dehydrate by sitting in open air, would the potato cells be more likely to absorb more or less water? Explain. ©eScience Labs, 2016
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