# solutions

December 8, 2015

Tonicity and the plant cells.

1. Label two test tubes (A and B) for each type of potato you will be testing. Be sure to write the
type of potato on the tube as well. Fill in the types of potato used in this experiment in the
�Type of Potato� column of Table 3.
2. Carefully cut two strips of each type of potato on a cutting board. The strips should be as close
to 10cm long and 1cm wide as you can cut them so they fit into the test tube.
Note: In the next step, you will account for any variability by measuring the volume of water
displaced when submerged into a beaker containing a known volume of water.
3. Fill the 100mL graduated cylinder with 50mL of water. Place one strip of the first type of potato
(Sample A) into the graduated cylinder and record the amount of water it displaced in the
�initial Displacement� column of Table 3 in the row corresponding with the sample tested.
Note: Displacement is a measurement of change and is calculated by subtracting the original
volume (50ml) from the final volume that you read after the potato is added to the 50mls of
water. (e.g. 57mL�50mL = 7mL)
4. Remove Sample A from the graduated cylinder and, if any water was lost, fill the graduated
cylinder up again with 50mL of water. Place Sample A into the corresponding test tube.
5. Place the second strip of the same type of potato (sample B) into the graduated cylinder and,
again, record the amount of water it displaced in the �Initial Displacement� column of Table 3
row corresponding with the sample tested.
6. Remove Sample B from the graduated cylinder and, if any water was lost, fill the graduated
cylinder up again with 50mL of water. Place Sample B into the corresponding test tube.
7. Repeat Steps 3-6 for each type of potato you will be testing.
8. Using the plastic dropper, add water to each of the test tubes with the A samples in them until
the water covers the potato strip. In a similar manner, add the 20% Sodium Chloride (NaCl)
solution to each of the test tubes containing the B samples.
Note: Make sure your test tubes are upright during the experiment. It may be useful to use the
test tube rack provided.
9. After an hour, drain the liquid from the test tubes containing your samples.
10. Fill the 100mL graduated cylinder with 50mL of water. Repeat Steps 3-6 for each sample and
record the displacement in the �Final Displacement� column of Table 3 row corresponding with
the sample tested.
11. Complete the last column of Table 3 by subtracting the initial displacement from the final displacement.

Potato type:

Potato A: Beginning displacement (ml) =

Ending displacement (ml) =

Potato B: Beginning displacement (ml) =

Ending displacement (ml) =

Potato type:

Potato A: Beginning displacement (ml) =

Ending displacement (ml) =

Potato B: Beginning displacement (ml) =

Ending displacement (ml) =

Tonicity of plant cells.

What is measured when looking at the change in displacement of the potato samples?

Different potatoes have different amounts of sugar. How may this influence the experiment?

From what you know of tonicity, what can you say about the plant cells and the solutions in the test tubes?

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