Plasmolysis and Deplasmolysis

Plasmolysis is what happens when a living plant cell is placed in a hypertonic external solution — one more concentrated than the cell's own sap. Because the surrounding solution now has a lower (more negative) water potential than the cell interior, water leaves the cell by exosmosis. As water leaves, the living protoplast (the cell membrane and everything inside it) shrinks and physically recedes away from the rigid cell wall, and the resulting gap between the shrunken protoplast and the wall becomes filled by the surrounding (hypertonic) solution seeping in. Because the protoplast has pulled away from the wall and no longer presses against it, a plasmolysed cell always has a turgor pressure of exactly zero.

Deplasmolysis is the reverse process: if a plasmolysed cell is transferred into a hypotonic solution — one more dilute than its own cell sap — endosmosis occurs, water re-enters the cell, the protoplast swells back outward, and the cell becomes turgid again, its protoplast once more pressing fully against the cell wall. …