Osmosis ?

Marc Roussel roussel at henri.chem.uleth.ca
Mon Jul 14 11:59:56 EST 1997

David wrote:
> Uptake of solutes by a cell from an isotonic medium will be followed
> by osmosis. This statement is true but why?  Can someone explain it to
> me?

It's basic physical chemistry.  If we have two aqueous solutions
separated by a semi-permeable membrane (one that only lets water
through) then the equilibrium state, other things being equal, has the
same concentration of solutes on both sides of the membrane.  Thus, if
solutes are transported into a cell, water will flow in to dilute them. 
In practice, many factors contribute to determining the eventual
equilibrium state of such a system, including the nature of the solutes
and the elasticity of the cell membrane so the concentrations inside and
outside of the cell are not made equal.  Nevertheless, some water will
flow in when solutes are transported unless an equal number of waste
molecules are ejected at the same time.
     The detailed theory is a little complicated and involves the
concept of chemical potential.  (The equilibrium condition is that the
chemical potential of the water is equal on both sides.)  For an
elementary treatment, dig up an introductory chemistry book.  For a more
detailed treatment, try a standard physical chemistry text.

				Marc R. Roussel
				(roussel at henri.chem.uleth.ca)
				Department of Chemistry and Biochemistry
				University of Lethbridge

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