is the diffusion of water molecules
It's called osmosis.   A selectively permeable membrane will only allow certain molecules  to pass trough. To illustrate: If you have a high concentration of  sugar mole â¦ cules on one side of the membrane, no sugar on the other  side, and the sugar molecules are too big to pass trough the  membrane. If this is the case, then water molecules will diffuse  over to the side of the membrane with a high sugar concentration.  This happens because the molecules try to distribute themselves  evenly in the solution. The water molecules move because the sugar  molecules can't. 3 people found this useful.