Potatoes, Salt, and Moving Water
Three potato cylinders are cut to 5.0 g each and soaked for 30 minutes: one in distilled water, one in a 3% salt solution, one in a 10% salt solution. After blotting, the masses are 5.8 g, 5.0 g, and 4.2 g. A student writes, 'The salt pulled water out and the plain water pushed water in, because water always moves toward wherever there is more salt.' A second student says, 'The potato in distilled water got heavier because the cells swelled and stretched their walls, so the cell wall is what controls the water.'
1. Explain the three mass changes in terms of water crossing the cell membrane, and say what the 3% result tells you.
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Water crosses the membrane from where water is more concentrated (less dissolved salt) toward where it is less concentrated (more dissolved salt).
- Distilled water: the solution outside has almost no salt, so water enters the potato cells. Mass rises 5.0 -> 5.8 g.
- 10% salt: the solution outside holds more salt than the potato cells do, so water leaves the cells. Mass falls 5.0 -> 4.2 g.
- 3% salt: the mass does not change, which suggests water entered and left at equal rates - the potato's internal solution is close to 3% salt.
The student's wording is loose but the direction is right: water moves toward the higher salt concentration. The mass data are the evidence.
2. Evaluate the second student's claim that the cell wall controls the water.
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The claim does not fit the evidence. The wall is a tough but open structure - water passes through it freely - so it cannot decide which way water goes. What decides direction is the difference in salt concentration on either side of the cell membrane, which is the barrier that actually regulates what crosses.
Why does the potato look firm rather than burst? The wall does limit how far a cell can swell; the cell presses against it. But that is a limit on swelling, not control of water movement.
Strong test of the claim: if the wall controlled water, a potato in 10% salt would not lose mass, since the wall is not dissolving. It lost 0.8 g. The change follows the salt concentration, so the membrane is the controlling structure.
3. Predict the result for a 1.5% salt solution, and use the same reasoning to explain why a cucumber salad goes limp in salty dressing.
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1.5% prediction: between the distilled-water and 3% results - the cylinder should end up slightly heavier than 5.0 g but under 5.8 g, because the outside solution is less salty than the potato's interior, so water enters, but only a little.
Salad: the dressing is much saltier than the cucumber cells. Water leaves the cucumber cells and enters the dressing, so the cells lose their internal pressure against the walls and the slices go soft and limp. The same rule as the 10% trial: water moves toward the higher salt concentration, across the membrane, and the tissue shrinks as it loses water.