Potato-cell mass change and a controlled concentration comparison
A class analyzes a supplied dataset from equal-sized potato cylinders placed in several labeled solutions. The cylinders began with the same average mass, and the observation interval, temperature and blotting convention were matched. The solution labels are concentrations in a hypothetical exercise, not a preparation recipe. Mean mass changes were plus 6 percent in solution A, plus 2 percent in B, approximately zero in C and minus 5 percent in D. The class is told that the external water potential becomes progressively lower from A to D. One student says that a zero mass change means no water molecules crossed the membranes. Another claims that every mass decrease must mean complete plasmolysis. The analysis must separate net water movement, dynamic exchange and the limits of what whole-cylinder mass can establish.
1. Explain the direction of net water movement and the meaning of solution C.
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A and B gained mass, consistent with net water entry under the supplied matched conditions; D lost mass, consistent with net exit. Near-zero change in C suggests approximately balanced net water exchange over the observation interval. Water molecules can still move in both directions, so zero net change does not mean the membranes became impermeable.
2. Can the mass data prove complete plasmolysis or an exact intracellular solute concentration?
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No. Whole-tissue mass change alone does not show every cell membrane separating from its wall, and water potential includes pressure as well as solute effects. A mass loss supports net water exit under the supplied assumptions, while microscopic evidence and a specified pressure/solute model would be needed for stronger cellular conclusions.