Plasmolysis is the separation of plant cell cytoplasm from the cell wall as a result of water loss. It is unlikely to occur in nature, except in severe conditions. Plasmolysis is induced in the laboratory by immersing a plant cell in a strongly saline or sugary solution, so that water is lost by osmosis.

If onion epidermal tissue is immersed in a solution of calcium nitrate, cells rapidly lose water by osmosis and the protoplasm of the cells shrinks (Web Figure 3.8.A). This occurs because the calcium and nitrate ions freely permeate the cell wall and encounter the selectively permeable plasma membrane. The large vacuole in the center of the cell originally contains a dilute solution with much lower osmotic pressure than that of the calcium nitrate solution on the other side of the membrane. The vacuole thus loses water and becomes smaller. The space between the cell membrane and the cell wall enlarges and the plasma membrane and the protoplasm within it contract to the center of the cell. Strands of cytoplasm extend to the cell wall because of plasma membrane-cell wall attachment points. Plasmolysed cells die unless they are transferred quickly from the salt or sugar solution to water.

Web Figure 3.8.A   Plasmolysis of an epidermis cell of Allium cepa after addition of calcium nitrate. Both chloroplast and vacuole become unevenly shaped. Strands of cytoplasm remain between the shrinking protoplasm and the neighboring cells. (Courtesy of Dr. Peter v. Sengbusch, University of Hamburg.)