This is something everybody with some experience from cooking knows. If a potato has lost water and become dehydrated, wrinkled and soft it will regain its form by putting it into fresh water for some hours. If you put a fresh potato into salt water it will behave opposite. It shrinks and becomes sloppy and soft. It is the water in the cellular juice that moves one way or the other through the cellular membrane and thus will enlarge/shrink the juice pressure in the cells.

This phenomenon is caused by osmosis. Osmosis does always act through a membrane where small molecules can pass while larger dos not. Water is small molecules while dissolved salts and organic substances are big clusters. Water will then pass through the membrane both ways while the solute does not. The net movement of the water will always be from the side with lowest concentrations of solutes (most water molecules per volume) to the highest (least water molecules per volume). The movements are random and the action is called diffusion. Next page will explain it further.