Cytolysis, or osmotic lysis, occurs when a cell bursts due to an osmotic imbalance that has caused excess water to move into the cell. It occurs in a hypotonic environment, where water moves into the cell by osmosis and causes its volume to increase to the point where the volume exceeds the membrane's capacity and the cell bursts. The presence of a cell wall prevents the membrane from bursting, so cytolysis only occurs in animal and protozoa cells which do not have cell walls.