Osmotic Pressure

Just a difference in concentration of solutions on opposit sides of a membrane, so far as I know. If you need something real technical, I guess we'll both wait.

http://en.wikipedia.org/wiki/Osmotic_pressure

i have already been there, it was of no help!

this statement helped a lot, thank you!

sorry...

maybe someone will be able to explain.

As I remember, the cell membranes are semi-permeable, thus molecules can flow through, and they flow from a higher concentration to a lower one. Just how the semi-permeable membranes are worked out in structure I've no idea.

I guess I might add that I once took a histology class taught by an electron microscopy research person, and the class was heavily oriented to biophysics.
That was decades ago, and I forget how cell membranes are formed. However, it might help answer your question if you look up on google words such as "biophysics of cell membranes", or just "semipermeable cell membranes".