When cells are put in a dilute salt solution, water move out of the cell and into the salt solution by osmosis. This happens because the solution is more concentrated than the cell (meaning the solution has less water molecules than the cell) and by definition, osmosis is the movement of water molecules from a region of high concentration (of water molecules) to a region of lower concentration (of water molecules). When the cells lose water, the become flaccid or plasmolysed.
When these cells are placed in distilled water (pure water with no dissolved salts in it), water molecules from the distilled water move into the cell by osmosis. This can lead to cell lysis.
