OSMOTIC PRESSURE OF POTATO TUBER CELL
SBI4U – Fall 2007
PROBLEM: What osmotic pressure is exerted by the cytoplasm of a cell of the potato tuber?
REASONING:
Gain a basic understanding of the terms: Isotonic, Hypotonic and Hyperonic
Understand differences between molar solutions and percent solutions.
Draw graphs and lines of best fit using computer programs.
BACKGROUND INFORMATION:
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
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
slightly and the potato will be 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
scalpel test tube rack
ruler electronic balance
PROCEDURE:
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. Pat each potato strip dry and determine the mass of each.
4. 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.
% sucrose solution

initial mass (g)

final mass (g)

mass change (g)

final mass
initial mass






5. 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.}
6. Remove the strips from the sucrose solutions, pat dry, and determine the mass of each. Record these final masses in the table of observations.
7. Complete the table of observations.
8. 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, WordPerfect 9.0, etc…).
All calculations must follow the rules for significant figures (see web page, Documents section, for rules).
QUESTIONS

Before answering question 2 & 3, you must calculate the line of best fit for your data points.

According to the graph, which sucrose concentration is isotonic with the cytoplasm?
3. Assuming that sucrose is the only osmotically active material in the cell, what is the sucrose concentration of the cytoplasm?
4. If the osmotic pressure of a 34.2% (1 molar) sucrose solution (34.2 g of sucrose in 100 mL of solution) is 25.69 atmospheres, what is the turgor pressure of the cells? 