OSMOTIC PRESSURE OF POTATO TUBER CELL

PROBLEM:            What osmotic pressure is exerted by the cytoplasm of a cell of the potato tuber?

        Under normal living conditions, the cells of the tuber are inflated slightly due to the osmotically developed pressure of the
cytoplasm. This is called the cell's turgor pressure. Normally, a balanced situation is reached in which net inward movement of
water is equal to net outward movement of water, so the cells stay a constant size.
        It is this slight inflation of the cells which gives the potato its stiffness, just as an inflated air mattress is stiff but a deflated
one is soft and flabby.
        If the potato cells are placed in a solution which is isotonic with the cytoplasm, then the cells will remain the same size.
        If the potato cells are placed in a solution which is hypotonic to the cytoplasm, the size of the cells will swell slightly and
the potato will be crisp to the touch.
        If the potato cells are placed in a solution which is hypertonic to the cytoplasm, the size of the cells will shrink slightly and
the potato will be soft to the touch.
        Changes in texture are too subjective, but measurements of the mass of a large number of cells (a strip of potato like an
uncooked french fry) are objective enough to distinguish an increase or decrease in the quantity of water in the cells and
consequently a change in size.

MATERIALS:                   potato                                  6 test tubes
                             knife                                   sucrose solutions
                             test tube rack                          ruler
                             electronic balance

1.   Cut a 7 mm thick slice from the center of a large potato. The sides of the slice should be parallel.
2.   Lay the slice of potato flat on the desk and cut off the ends so that they are parallel. Cut the piece of potato into 6 strips
     about 5 mm wide. These cuts should be parallel. See the following diagram.

3.         Rinse each potato strip with distilled water and then pat each potato strip dry.
4.         Determine the mass of each to the nearest 1/100th of a gram and record the masses in your table as initial mass (g).
           Use the same electronic balance throughout the lab to ensure accurate results.
5.         Label 6 test tubes for 20%, 15%, 10%, 7.5%, 5%, 2%, and 0% sucrose solutions. Place a potato strip into each test tube
           with just enough sucrose solution to cover the strip. You should know the mass of the strip that went into each test tube.
           Record the masses in a table similar to the one below.

                                  initial mass (g)              final mass (g)              mass change (g)                  final mass
     % sucrose solution
                                          I                           F                         (F – I)                     initial mass
6.      Leave the strips to soak for at least 45 minutes, longer if possible. {if a change in mass does occur in any of the solutions,
        the longer the potato sits, the greater the change in mass there will be, therefore, let each piece sit as long as time permits.
        Pieces of potato were placed during Period 1, record their mass during the lunch time break. This will ensure a change in
        mass, if any has occurred.}
7.      Remove the strips from the sucrose solutions, pat dry, and determine the mass of each. Record these final masses in the
        table of observations.
8.      Complete the table of observations.
9.      It is unlikely that any of the solutions was exactly isotonic and produced a final mass/initial mass of 1.0.
        Therefore, a graph must be drawn and used to determine the sucrose solution concentration which is isotonic with the
        cytoplasm. Plot a graph of final mass/initial mass against % sucrose solution. Your graph must be produced by a
        computer program (i.e.: Microsoft Excel, Microsoft Word (98, 2000, 2002, 2003, or 2007), WordPerfect 9.0 or 10,
        etc…). To ensure accuracy of results, all calculations should follow the rules for significant figures (go back to the
        significant rules handout from grade 10 (Physics unit) or follow the link:

1.    Before answering question 2 & 3, you must calculate the line of best fit for your data points. To calculate the line of best
      fit, you must calculate a Linear Regression line (line will have the form: y = mx + b) using the computer program you
      used to plot your graph with (i.e.: Microsoft Excel, Microsoft Word(98, 2000, 2002, 2003, or 2007), WordPerfect 9.0 or
      10, etc…)..
2.    According to the graph, which sucrose concentration is isotonic with the cytoplasm? (be specific – must use: y = mx +
      b. The isotonic sucrose concentration will be calculated from this line of best fit.)
3.    Assuming that sucrose is the only osmotically active material in the cell, what is the sucrose concentration of the
      cytoplasm? (be specific – must use: y = mx + b.)
4.    If the osmotic pressure of a 1 molar sucrose solution (342 g of sucrose in 1000 mL of solution) is 25.69 atmospheres, what
      is the turgor pressure of the cells?
      Hint: A 1 molar solution is 34.2%. If the isotonic solution is 5%, then the turgor pressure will be 5/34.2 of the osmotic
      pressure of the 1 molar solution.
5.    Explain your results. For example, explain why some potato sections gained mass and others lost mass over the 1 – 3 hour
      Period they were left to soak in the differing sucrose concentrations.

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