Exercise 1: Plant Cells and Osmosis
Data Table 1. Potato Masses.
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Test Tube
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Solution
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Initial Mass (g)
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Final Mass (g)
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Mass Difference (g)
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Percent Change in Mass (%)
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Class Average: Percent Change in Mass (%)
(If Available)
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a
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Distilled water
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b
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0.2 M sucrose
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c
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0.4 M sucrose
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d
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0.6 M sucrose
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e
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0.8 M sucrose
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f
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1.0 M sucrose
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Questions
A. Define osmosis. How did you observe osmosis in Exercise 1?
B. List molar concentrations (0.0 M, 0.2 M, etc.) at which water entered the potato strips. Why did water move into the potato strips? Were these solutions hypotonic, hypertonic, or isotonic?
C. List the molarities at which water exited the potato strips. Why did water move out of the potato strips? Were these solutions hypotonic, hypertonic, or isotonic?
D. Was the hypothesis you recorded in Data Table 1 supported or refuted by your experimental results? Discuss your results in relation to your hypothesis. (It is acceptable if your hypothesis was refuted, as long as you can explain WHY.)
E. If you were asked to place potato strips in a solution that would cause the potato to neither gain or lose mass, what molarity would you choose and why? Would this solution be hypotonic, hypertonic, or isotonic?
Exercise 2: Diffusion across a Membrane
Questions
A. Define diffusion.
B. IKI indicator tests for the presence of which substance? Benedict's reagent tests for the presence of which substance?
C. In the first part of Exercise 2, a glucose/starch solution was placed in dialysis tubing. Then, the tubing was placed in a cup containing IKI indicator. Did starch stay within the tubing or move out of the tubing? Did IKI indicator stay in the solution in the cup or move into the tubing? Support your answer with the observations you recorded in Data Table 3.
D. In the second part of Exercise 2, the IKI solution in the cup and the contents of the dialysis tubing were heated with Benedict's reagent. Did sugar stay within the dialysis tubing or move out of the tubing? Support your answer with the observations you recorded in Data Table 4.
E. What was the purpose of testing distilled water with Benedict's reagent?
F. How is a cell membrane similar to the dialysis tubing used in this experiment?
G. Is the movement of substances in this exercise active or passive?
H. From your data, might it be inferred that a) starch and glucose molecules are the same size, b) starch molecules are larger than glucose molecules, or c) starch molecules are smaller than glucose molecules. Use your data to explain your answer.