Exercise 3: Osmosis in Potatoes
Materials
1 medium to large white potato
1⁄4 cup table salt
3 clear glasses
Metric ruled graph paper OR ruler
Procedure
Prepare two salt solution:
Measure 1.5 teaspoons (tsp) of salt and dissolve it in 3 cups of room temperature water. Stir for a few minutes to get as much salt dissolved as possible. Transfer the solution into a clear glass big enough to hold 1 potato slices (~4 cm deep).
Measure 1⁄4 cup of salt and dissolve it in 3 cups of room temperature water. Stir for a few minutes to get as much salt dissolved as possible. Transfer the solution into a clear glass big enough to hold 1 potato slices (~4 cm deep).
Transfer some room temperature tap water to a third glass, at least 4 cm deep.
Label the three cups (No salt, low salt, high salt).
Carefully cut a white potato into three equal pieces approximately 1 cm X 1 cm X 5 cm. The size doesn’t have to be exact, but it is critically important that all 4 slices are exactly the same length. Record the length of each slice in Table 4.
Transfer one potato slice into each glass.
Immediately note your observations of both glasses.
Don’t get the slices mixed up. Allow the slices to stay submerged for 2 hours then remove them and compare their lengths. Using a ruler, measure to the closest mm. Record the length of each slice in Table 4.
Place the slices back into their glasses and leave them overnight (approximately 12 hours). The next day measure again. Record the length of each slice in Table 4.Also, note how each slice feels (soft, hard etc.).
Calculate the percent change in length of all 4 slices at each time point and record your results. To calculate % change as follows:
[(length (mm) after overnight – original length) / original length] x 100%
For example. After overnight length = 63 mm. Original length = 60 mm.
% change = [(63 mm – 60 mm) / 60 mm] x 100% = [3 / 60] x 100% = 5%.
Table 4. Potato Experiment Observations and Calculations
Water
Start (time 0)
2 Hours
~ 14 Hours
Observations
Potato Slice Size (mm)
% Change
0
Low Salt
Start (time 0)
2 Hours
~ 14 Hours
Observations
Potato Slice Size (mm)
% Change
0
High Salt
Start (time 0)
2 Hours
~ 14 Hours
Observations
Potato Slice Size (mm)
% Change
0
Q1. Which solution is considered to be …
Isotonic(ish) compared to the potato cells?
Hypotonic compared to the potato cells?
Hypertonic compared to the potato cells?
Q2. Explain with reference to your experimental data how you determined the tonicity of each solution with reference to the potato.
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