Let's now consider osmosis in a more complex system. Two compartments containing identical solutions are separated by a membrane permeable only to water. Such a system is simulated at the top of the next page. Note both water molecules and solvated solute particles exhibit Brownian motion, but the larger, solvated solute particles are moving less rapidly and traversing a shorter path than the smaller water molecules.
Occasionally, water diffuses across the membrane, but solute diffusion is restricted by
the membrane to one compartment or the other. A membrane that restricts the flow of one
substance - in this case, solute - relative to that of another such as solvent is said to
be selectively permeable.
The selectively permeable membrane in the simulation on the next page exhibits one more
interesting feature: it moves! And its relative location reflects the volume of
each compartment, that is, the total number of particles in each. Initially, both
compartments contain the same number of particles and are therefore at equilibrium with
respect to each other. Ideally no
net movement of solvent occurs in either direction across the membrane: on
balance, an equal amount of water moves out of a compartment as moves into it.