|MadSci Network: Cell Biology|
As you know, some chemicals are permeable to the cell membrane freely, say water, oxygen, alcohol and so on, while others need to be selectively transported if they want to get across the membrane at all.
The other thing to be understood, is that water tends to go from solution of low concentration (where there is a whole lot excess of water) to solution of high concentration (where water is relatively in smaller amount). Other molecules have the same tendency too, only sometimes they are not allowed to go as they want by the block of the membrane.
It is exactly the case when you see plasmolysis. When the cell is put into a high concentration solution which contains a molecule that does not go through cell membrane freely, say, Na ion, the water inside of the cell will go out of the cell so as to balance the concentration of the two sides of the membrane. Na would like to go in the cell too but the membrane is not taking it. As a result, the cellplasm together with the membrane will shrink. By the way, animal cell membrane has the same quality, only that animal cells don't have cell wall so you are not able to tell if they have shrinked or not without a proper comparison. Hope this helps. Cheng Huang
Try the links in the MadSci Library for more information on Cell Biology.