Changing the concentration of solute in the right-hand compartment of the last simulation moved the entire system away from equilibrium. Diffusion of water then brought the system back into equilibrium. Such differential movement of water between compartments containing initially different solute concentrations is called osmosis. How can we explain osmosis?
The pattern of changes you observed suggests a correlation between the number of
solute particles in each compartment and its new volume when equilibrium is
re-established. Equilibrium occurs when both compartments contain the same concentration
of solute, and osmosis in all the simulations caused one compartment to lose water and the
other to gain it as a new equilibrium was reached. What, in turn, causes osmosis?
In the initial simulation, when both solutions contained the same concentration of
solute, the volumes of both compartments stayed more-or-less the same because essentially
the same amount of water diffused across the membrane in both directions. Why? Both
compartments contained approximately the same mole fractions of free and bound solvent,
and diffusion of free water was as likely to occur in either direction. Thus, when the
chemical activity of water in both compartments was the same, no osmosis occurred.
Such was not the case when solute concentrations differed between the compartments. In
every instance where there was a lower solute concentration, there was correspondingly a
lower fraction of bound water and a correspondingly higher fraction of freely diffusable
water. And you observed that more water always diffused out of the compartment with higher
solvent activity than into it. Recalling that solute lowers the chemical activity of
water in solution, you can now understand how the effect of solute on solvent behavior
causes osmosis. How does this explanation
of osmosis compare with others you've learned?
Cellular volume constantly fluctuates in responses to osmotic differences in the activities of cytoplasmic and extracellular water, as the solute concentrations in these two compartments change. Often, such changes in volume are difficult to measure.. It is also possible to envision osmosis as the pressure necessary to prevent changes in cellular volume. Go on to the next page and simulation to consider osmotic pressure..