[snip] at around 22 minutes, the cell in the 0.75M sucrose solution sunk to the bottom of the test tube. The only reason I can think of to explain this is that water flowed back into the now hypertonic cell, causing it to increase in mass and density. However, comparing the size of the potato tuber piece (1cm) and the volume of the test tube (20mL), this doesn't make sense. Can someone explain what would have really happened and also why the cell in the 1M solution did not ultimately sink as well?
The best explanation that I can come up with - aside from experimental error, which is something you should seriously consider - is that there was an eventual net movement of H2O back into the plant cell sample. Let's try to rationalise this:
• Plant cell is placed in 0.75M solution.
• The solution is hypertonic to the cell, causing net movement of water OUT of the cell into the solution. The solution is now hypotonic to the cell.
• The net flow of water into the solution has diluted the solution to an extent that it is sufficiently hypotonic relative to the cell's tonicity that there is now net H2O movement back into the cell.
The issue here is that the plant cell and the solution did not reach an equilibrium, which would occur after a while unless the solute concentration of the solution is extremely high.
The fact that this net back-flow of H2O didn't happen with the 1M sample somewhat supports this theory, because even with the dilution of the solution by the cell's water content, the solution may still have been to hypertonic relative to the cells that no net H2O flow back into the cells can occur.
N.B. This is all conjecture given that I did not observe your lab practice and cannot readily reproduce the experiment (at home haha). As I mentioned above, consider how experimental error may have played a part here; perhaps the 0.75M solution was prepared incorrectly and was not, in fact, 0.75M?
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