Plasmolysis

PLASMOLYSIS, the shrinking of the protoplast, its separation from the cellulosic wall, observed when a plant cell surrounded by a rigid wall is immersed in a hypertonic solution of any substance. If the cell is then transferred to a more dilute solution or to water, the protoplast absorbs water and increases in volume again. During plasmolysis, the protoplast acts as a semipermeable membrane around the vacuole filled with cell sap, while the rigid cell wall serves as a mechanical support for the protoplast. H. de Vries used the plant cell as an osmometer in studying the osmotic pressure of various solutions (plasmolytic method). Solutions that are isotonic to each other cause the same change in the protoplast; thus, solutions whose osmotic pressure only slightly exceeds that of the cell sap (isotonic to it) cause barely noticeable separation of the protoplast from the wall, stronger solutions (hypertonic) cause plasmolysis, and weaker solutions (hypotonic) cause stretching of the cell wall (turgor pressure). Plasmolysis leads to a loss of turgor pressure. Plasmolysis was applied by F. Weber for the rough determination of the viscosity of protoplasm. With low viscosity, the protoplast during plasmolysis takes on a shape close to spherical, with higher viscosity an irregular, angular shape. Some substances do not cause prolonged plasmolysis: after some time the protoplast increases in volume again (deplasmolysis). This occurs when the dissolved substance penetrates from the external fluid into the cell, as a result of which the initial difference between the osmotic pressure of the cell contents and the external solution gradually equalizes! Plasmolysis should be distinguished from pseudoplasmolysis—the irreversible postmortem shrinking of the protoplast, which occurs even when the cell is surrounded by distilled water.