Potato Osmosis and the Isotonic Point
A student investigates osmosis in potato tissue. Cylinders are cut from one potato and trimmed to equal length. Each cylinder is blotted dry, weighed, and placed in a labelled sucrose solution for 30 minutes: 0.0, 0.2, 0.4, 0.6 and 0.8 mol/dm3. There are three cylinders per solution. After incubation, the cylinders are blotted and reweighed. Mean percentage mass changes are: 0.0 mol/dm3 +6.5%; 0.2 mol/dm3 +1.8%; 0.4 mol/dm3 -3.2%; 0.6 mol/dm3 -8.9%; 0.8 mol/dm3 -10.1%. One cylinder from 0.4 mol/dm3 lost 22.0% mass and is excluded from the mean. The student claims the potato tissue is isotonic with 0.2 mol/dm3 sucrose because that solution gave the smallest change. The teacher asks for a more careful estimate and an evaluation of the method.
1. Estimate the sucrose concentration at which the potato tissue would show no net change in mass. Show your reasoning and explain why the student's claim is not fully supported.
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The data show a trend from gain to loss as sucrose concentration rises. Between 0.2 and 0.4 mol/dm3 the mean changes from +1.8% to -3.2%, a fall of 5.0 percentage points over 0.2 mol/dm3. Assuming a straight-line change over this short interval, the rate is 5.0/0.2 = 25 percentage points per mol/dm3. To go from +1.8% to 0% needs a further 1.8/25 = 0.072 mol/dm3. The estimated isotonic concentration is therefore about 0.2 + 0.07 = 0.27 mol/dm3. The student's claim is not fully supported: 0.2 mol/dm3 still gave a positive mean change, so water entered the tissue on average; it was the smallest observed change, not zero. A zero change would be expected nearer 0.27 mol/dm3, although replicate variation means a range should be quoted.
2. Suggest one likely cause of the anomalous cylinder and explain how the method could be improved to reduce such anomalies.
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The 22.0% loss is far outside the pattern for 0.4 mol/dm3 (-3.2%), so it is likely to be an error rather than normal biological variation. Possible causes: the cylinder was damaged or bruised, so more cells were cut open; it was not blotted in the same way; it was left in the solution longer; or it had a much smaller initial mass, making the same absolute water loss a larger percentage. To reduce this: cut all cylinders from the same potato with the same diameter and length, use a cork borer and scalpel, blot all cylinders in the same way for the same time, use a top-pan balance of suitable precision, keep timing identical, and use more replicates so one outlier has less effect. The outlier should be investigated, not simply deleted without comment.
3. Explain why the student used percentage change in mass rather than change in mass in grams.
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Percentage change compares each cylinder with its own starting mass. Cylinders cut by hand may differ slightly in mass, and the same absolute water loss will be a larger percentage in a smaller cylinder. Using percentage change reduces the effect of initial size and allows means from different cylinders to be compared fairly. Absolute change in grams would depend on the initial mass and on surface area, so it would be harder to compare treatments. However, percentage change can exaggerate differences for very small cylinders, so standardising size is still important.