Potato Strips and the Isotonic Point
Students at a school in Zliten investigate osmosis in potato tissue. Cylinders are cut to 3.0 cm length and 0.8 cm diameter, blotted, and placed for 30 minutes in sucrose solutions of 0.0, 0.2, 0.4, 0.6, and 0.8 mol/dm³. Final lengths are 3.3, 3.1, 3.0, 2.8, and 2.6 cm. One group's 0.6 mol/dm³ cylinder was left on the bench for 10 minutes before measurement and read 3.1 cm. The class must report the isotonic point, decide whether to include that reading, and predict the result at 1.0 mol/dm³. The teacher insists that the conclusion be based on the trend, not on a single reading, and that the method be criticised in one respect. Temperature is 22 °C and all cylinders come from the same potato.
1. Identify the isotonic concentration from the data and explain the direction of water movement at 0.0 and 0.8 mol/dm³.
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The isotonic point is at 0.4 mol/dm³, where the final length is 3.0 cm, equal to the starting length, so there is no net water movement. At 0.0 mol/dm³, pure water, the external water potential is higher than inside the potato cells, so water enters by osmosis through the partially permeable membranes; cells become turgid and the cylinder lengthens to 3.3 cm. At 0.8 mol/dm³ the external solution has a lower water potential than the cell contents, so water leaves the cells; the vacuoles shrink, tissue loses turgor, and the cylinder shortens to 2.6 cm. The trend across 0.0, 0.2, 0.4, and 0.8 is monotonic and consistent with the model. The isotonic point is a range, not a single exact value; 0.4 mol/dm³ is the best estimate because it is the only concentration with zero change, but the true value could lie between about 0.35 and 0.45 mol/dm³.
2. The 0.6 mol/dm³ cylinder from one group measured 3.1 cm after being left on the bench for 10 minutes. Should it be included? What should the class do?
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Exclude it from the class mean and investigate it. The value 3.1 cm is higher than the 0.4 mol/dm³ result and breaks the monotonic decrease (3.0 → 2.8 → 2.6). A 0.6 mol/dm³ solution should shrink the tissue, so 3.1 cm is biologically implausible for that treatment. Leaving the cylinder on the bench would normally cause evaporation and shrinkage, not elongation, so the stated cause does not explain the direction of the anomaly; likely explanations are a mislabelled solution, measuring a different cylinder, or a recording error. The class should not quietly drop it without comment. The correct action is to record the anomaly, repeat the 0.6 mol/dm³ treatment with a fresh cylinder and a timed blot, and report the repeat alongside the original. If the repeat agrees with the trend, use it and note the exclusion; if it does not, the method needs checking.
3. Predict the length at 1.0 mol/dm³ and criticise the use of length as the measured variable.
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Prediction: from 0.6 to 0.8 mol/dm³ the length falls by 0.2 cm per 0.2 mol/dm³, so at 1.0 mol/dm³ expect about 2.4 cm, with the caveat that shrinkage must flatten as cells approach plasmolysis; a value between 2.3 and 2.5 cm is reasonable, and the tissue may become flaccid rather than uniformly shorter. Criticism: length is a one-dimensional proxy for a three-dimensional water change, and it is affected by how the cylinder is cut, whether it is straight, and how it is blotted. Mass is the better variable because it responds directly to water gain or loss and can be measured on a balance to 0.01 g. The class should also control potato variety, cylinder diameter, temperature, and incubation time, and should blot all cylinders identically before measuring. Length can support the osmosis conclusion, but it cannot give a precise isotonic concentration on its own.