Potato Osmosis Lab: When One Outlier Changes the Story
Students place equal-mass potato cylinders in four salt solutions: 0%, 1%, 3%, and 5% NaCl. Each cylinder is blotted and weighed before and after 20 minutes. The class averages show mass changes of +8%, +3%, −2%, and −9%. One lab group's 3% cylinder lost 15% of its mass; the teacher notes they left it in the solution for 45 minutes and blotted it only once before weighing. Another group's 0% cylinder gained 12%, but their potato had been peeled while the others were unpeeled. A student writes, 'The 3% solution is isotonic because one cylinder lost mass, and the potato cells must have actively pumped salt out in the 5% solution.' The class must decide what the data support about osmosis and what must be retested.
1. Use the class averages to estimate the salt concentration at which potato mass would not change, and explain the direction of water movement in the 0% and 5% solutions.
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The averages move from +8% at 0% salt to +3% at 1%, then to −2% at 3% and −9% at 5%. The zero-change point is therefore between 1% and 3%, closer to 2% if the trend is roughly linear. That estimate is approximate because only four concentrations were tested and the data include outliers.
In the 0% solution, the external solute concentration is lower than inside the potato cells, so water enters the cells by osmosis and mass increases. In the 5% solution, the external solute concentration is higher, so water leaves the cells and mass decreases. The direction of water movement is determined by the water concentration gradient, not by the potato 'pumping' salt.
2. Critique the student's statement that the 3% solution is isotonic and that potato cells actively pumped salt out in the 5% solution.
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The 3% solution is not necessarily isotonic. The class average at 3% is −2%, which suggests the solution is slightly hypertonic, not isotonic. The 15% loss is an outlier: that cylinder stayed in the solution for 45 minutes instead of 20 and was blotted differently, so it should not be used to define the isotonic point. Even the −2% average is close enough to zero that the true isotonic concentration may lie between 1% and 3%.
The claim about active salt pumping is also unsupported. In 5% salt, water leaves the potato cells by osmosis, which explains the −9% mass change without invoking active transport. Active transport may occur in living cells, but the mass data alone do not show it, and the student confuses one trial with the class average.
3. Identify procedural inconsistencies in the investigation and describe a revised procedure that would reduce error.
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Inconsistencies include unequal soaking times (45 min vs 20 min), different blotting methods, and one group using peeled potato while others used unpeeled potato. Peeling removes the outer tissue and changes the surface available for water movement, so it is a real confound, not just a minor detail.
A revised procedure should use cylinders cut to the same size from the same potato, keep the peel treatment the same for all samples, soak for exactly 20 minutes, blot each cylinder the same way (for example, roll gently on a paper towel and weigh immediately), and test each concentration with at least three replicates at the same temperature. Adding a 2% solution would help locate the isotonic point more precisely. Means and ranges should be reported, and outliers should be investigated rather than deleted.