Exercise 1C: Water Potential


Materials: As needed

Procedure:

  1. Pour 25-30 ml of the assigned solution into a labeled 50 ml beaker. Slice a potato into discs which are approximately 3 cm thick.
  2. Use a cork borer (approximately 5 mm in inner diameter) to cut four potato cylinders. Do not include any skin on the cylinders. You need four potato cylinders for each beaker.
  3. Keep your potato cylinders in a covered beaker until it is your turn to use the balance.
  4. Determine the mass of the four cylinders together and record the mass in Table 1.4. Put the four cylinders into the beaker of sucrose solution.
  5. Cover the beaker with plastic wrap to prevent evaporation.
  6. Let it stand overnight
  7. Remove the cores from the beakers, blot them gently on a paper towel, and determine their total mass.
  8. Record the final mass in Table 1.4 and calculate percentage change as in Exercise 1B. Do this for both your individual results and class average.
  9. Graph both your individual data and the class average for the percentage change in mass in Table 1.4.

 

Table 1.4: Potato Core: Individual Data

Contents of Beaker
Initial Mass
Final Mass
Mass Difference
Percent Change in Mass
Class Average Percent Change in Mass

a) Distilled Water

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b) 0.2 M Sucrose

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c) 0.4 M Sucrose

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d) 0.6 M Sucrose

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e) 0.8 M Sucrose

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f) 1.0 M Sucrose

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Table 1.5: Potato Core Results: Class Data (see board)

 

Graph 1.2: Percent Change in Mass of Potato Cores at Different Molarities of Sucrose.

 

Determine the molar concentration of the potato core. This would be the sucrose molarity in which the mass of the potato cores does not change. To find this, Draw the straight line on Graph 1.2 that best fits your data. The point at which this line crosses the x-axis represents the molar concentration of sucrose with a water potential that is equal to the potato tissue water potential. At this concentration there is no net gain or loss of water from the tissue. Indicate this concentration of sucrose in the space provided below.

 

Molar concentration of sucrose = _______________ M