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So I did a prac to identify the osmotic potential of potato tuber cells. There were 5 test tubes with different concentrations of sucrose (0M,0.25M,0.5M,0.75M,1M); a small slice of potato was placed in each of them and left for 25 mins. When placed initially, all pieces of potato were at the bottom of the test tube. At around 8 mins, I noticed that the potato cell in the 0.75M and 1M solution begun to float. I'm assuming this is because water had left the cell from regions of low solute concentration to regions of high solute concentration and the density of the potato tuber cells in both these solutions became less than the density of water (lower mass to volume ratio). However, 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?

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For me the first puzzle is why the slices didn't float to start with - in my (non-scientific) experience raw potato slices tend to float in water, and the density of your sucrose solutions (e.g 1M sucrose is approximately 1.13 g/ml) should have helped to make them float. –  Alan Boyd Feb 5 at 15:25

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