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I tried an experiment. First, I placed grapes in pure water(hypotonic solution). The grapes burst because of endosmosis. Then, I placed new set of grapes in strong sugar solution(fully saturated) but they did not shrink. And then when I placed it in strong salt solution(fully saturated), they did shrink a little. Why does this occur?

How is this possible?

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What is a strong sugar solution? Grapes contain ~15-20% of sugar. –  nico Sep 30 '12 at 10:50
    
According to Wikipedia fully saturated sucrose solution should be 2000g/L, way above the 15-20% sugar content of grapes. Interestingly the versions of this experiment that I can find online all use salt for a hypertonic solution. Of course this could be just because it is cheaper than sugar. –  Alan Boyd Sep 30 '12 at 13:42
    
Why are we so sure that osmosis can happen in both directions in grapes? –  rwst Sep 30 '12 at 13:47
    
@rwst - The grapes are probably dead, so any trans-membrane pumps would most likely be inactive. That leaves pores and other entry points for water to equilibrate. There's also evidence in the experiments above; because of the membrane potential maintained by ion concentrations (fewer in the cytosol) the saturated salt solution would cause water to leave the grapes and also cause the smaller ions to enter the grapes to dissipate the membrane potential. Sugar isn't small, though, and requires active transport (to the best of my memory). –  MCM Sep 30 '12 at 19:58

3 Answers 3

It is important to note that osmotic pressure is a colligative property, which means that it does not depend on the identity of the solutes, but simply their concentration. This also means that in considering the grape, we have to estimate the concentration of all solutes, not just the one you used to make the (possibly) hypertonic solution.

I'll assume the sugar used was sucrose, which is the common table sugar. I guess I should also mention that I'll assume the salt was NaCl, which is the common table salt.

Wikipedia says that the solubility of sucrose is 2000 g/L. As the molar mass is 342.3 g/mol, the concentration is approximately 5.8 M.

NaCl on the other hand, has a solubility of 359 g/L. At 58.44 g/mol, the concentration is approximately 6.1 M. However, the Na+ and the Cl- dissociate in water, so the concentration is effectively double when counting solutes. That gives 12.2 M, which is much higher than a sugar solution. This accounts for why grapes would shrivel in a salt solution more than in a sugar solution.

As to why the grapes did not shrink in a sugar solution - I don't think it is unreasonable for a grape to have total solute concentration in the range of 5.8 M. Estimates of the sugar concentration in a grape are given in % w/w. Clearly grapes are denser than water (you can test this by putting a grape in water and seeing it sink). Therefore if you were to calculate in % w/v, you would find it to be more concentrated than a the same % w/w solution in water. Moreover, according to this site grapes have a significant amount of glucose and fructose. These are monosaccharides, as opposed to sucrose which is a disaccharide. This means for the same mass of "sugars" there are more individual sugar molecules, resulting in more osmotic pressure.

In sum, I think the sugar solution is probably isotonic, not hypertonic.

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i tried this experiment but failed. The reason for our failure in this experiment might be as follows: 1. The grape coating might be immpermeable. 2. It may happen that the solution might not be hypertonic but isotonic(equal concen. of solutes in both) 3. The salt solution might be less dilute than the sugar one. i am just a student but giving my point of veiw....

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Well, without the details of your experiment to cross-check with the sugar and H2O content of your average grape, I'd say the saturated solution probably approximated the natural molarity of sugar in the grape. At least to the point it didn't make any advese effects noticeably on your time scale.

That seems the most apparent solution to me. I suppose it's also possible (though I don't know the mechanics behind it) that hypertonic solution might have acted as a preservative - similar to jam. It's not just water which will attempt to reach an equilibrium, but all compounds that can pass through the membrane - perhaps sucrose/fructose/glucose can do so, maintaining the structural integrity of the grape.

Both are guesses until you reveal more information, though.

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please see the question now .... I have added some more information and then help me! –  Pranit Bauva Sep 30 '12 at 13:00

protected by Chris Jun 25 at 15:28

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