MadSci Network: General Biology


Date: Tue Feb 16 22:51:33 1999
Posted By: Phyllis Pugh, Post-doc/Fellow, Neurobiology, Medical College of Ohio
Area of science: General Biology
ID: 917362942.Gb

First, this will all make a lot more sense if you try the experiment with your fruits and vegetables and the salt water first. You might also want to try peeling them and seeing if it makes a difference. [It's especially interesting if you use a microscope and leafy green vegetables with varying amounts of salt water.] My prediction is that your veggies will shrink (unless you use a lot of salt, you will have to measure very carefully).

So, the question is: what's happening here?

For this, it's important to remember plants use water to help maintain their rigidity (the technical term is "torpor."). A good example is the fact that if you don't water a plant it begins to wilt (lose rigidity).

Well, perhaps this picture will help a little:
osmosis picture
See, on the left hand side, there are 14 molecules of water and 2 molecules of other stuff inside the cell and 11 molecules of water and 3 molecules of salt on the outside. If you do the math, that means that the inside is 87.5% water and 12.5% stuff, and the outside is 78.6% water and 21.4% salt. That's means that the inside isn't the same as the outside. Now, nature is driven by balance (also called equilibrium), which means that this inequality won't last for long.

Now, there are a couple ways to make things more equal on the two sides of the cell membrane:

  1. The amount of salt on the inside can go up; or
  2. The amount of water on the inside can go down.
Either manipulation makes it so that the "concentrations" are more equal. Because of the nature of the cell membrane (it's "semi- permeable"), the salts can't go through, but the water can (so we choose the second answer above).

In the drawing, you see that 4 water molecules from the inside of the cell move to the outside, making the inside now 10/12 water (83.3%) and the outside 15/18 water (also 83.3%). Since water is what gives the cell it shape, you also will notice that the cell is a little bit smaller.

I hope that helps! By the way, this very phenomenon is one of the reasons that you should not cook vegetables in salt (you can add it later). The heat speeds up the process!

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