A Cell's Response to a Salt Challenge
Students place equal samples of peeled potato tissue into five beakers: distilled water, 0.1 M, 0.3 M, 0.5 M, and 1.0 M sodium chloride solutions. After 30 minutes they measure mass change and find: distilled water +8%, 0.1 M +3%, 0.3 M -2%, 0.5 M -9%, 1.0 M -18%. A student also notices that the 1.0 M cylinders feel limp and that a red dye added to the 1.0 M beaker later appears inside some cells when viewed under a microscope. The class must explain the trend, estimate the isotonic concentration, and address the dye observation and one anomalous cylinder that gained mass in the 1.0 M beaker.
1. Explain the mass-change trend in terms of water movement, and estimate the solution concentration at which potato tissue would neither gain nor lose mass.
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Potato cells contain solutes and are separated from the bathing solution by a selectively permeable membrane. In distilled water the external solute concentration is near zero, so water enters by osmosis and mass rises 8%. As external concentration increases, the water potential gradient shrinks; at 0.1 M the gain is only 3%. Above the cell's internal solute concentration, water leaves and mass falls, reaching -18% at 1.0 M. The limp texture at 1.0 M reflects loss of turgor as cells lose water and the vacuole shrinks.
The isotonic point lies where mass change is zero, between 0.1 M (+3%) and 0.3 M (-2%). Linear interpolation across that interval gives roughly 0.22 M. This is an estimate: the relationship is not strictly linear, and tissue type, temperature, and immersion time affect the result. A tighter estimate would use narrower concentration steps around 0.2 M and multiple replicates, reporting variability rather than a single cylinder per beaker.
2. The red dye appears inside cells from the 1.0 M beaker. Does this show the membrane is freely permeable? Explain.
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Not necessarily. In 1.0 M salt the cells lose water severely, and severe dehydration can damage membranes, allowing dye entry through breaches rather than through normal transport. The observation may therefore indicate injury, not ordinary permeability.
A valid permeability claim needs controls: dye applied at isotonic concentration, where healthy cells should exclude it, and a check that cells remain intact (for example, observing whether the dye is confined to the cell wall space versus the cytoplasm). If dye enters healthy cells at isotonic concentration, that supports permeability to the dye. If entry occurs only after extreme shrinkage, the result is evidence of membrane failure under stress. The class should also consider that the dye may bind to cell walls, producing apparent internal signal. Interpreting dye entry as proof of free permeability without these controls overreads the evidence.
3. One cylinder in the 1.0 M beaker gained mass. How should the class treat this, and what does it imply about their measurement plan?
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A single gain in a strongly hypertonic solution contradicts the trend and should be treated as an outlier pending investigation, not averaged in silently. Plausible causes: the cylinder had surface water not blotted before weighing, it was misassigned to the wrong beaker, it had a bruise or crack allowing solution uptake, or the balance reading was recorded incorrectly. Each has a different fix, so the class should check the original data sheet and reweigh.
If the anomaly cannot be explained, it should be reported and excluded with justification, and the analysis should note that n=1 per condition cannot reveal variability. The implication is that the plan needs replicates (at least three cylinders per concentration), standardized blotting and timing, randomized assignment to beakers, and coded labels to prevent mix-ups. A single unexplained point does not overturn the trend but does limit confidence, and hiding it would misrepresent the data.