Cytolysis happens when cells are placed in hypotonic solutions.
Hypotonic solutions are solutions whose concentrations are lower than the concentration of the cell sap of any cell suspended in them. Hence, water molecules enter the cell by the process of osmosis resulting in the cell becoming turgid.
If the movement of water continues, the cells eventually burst and spill their contents. This process is known as reverse plasmolysis or cytolysis.
