The plant cell wall can either shrink or become turgid in response to the movement of water. It is the surrounding isotonic, hypotonic and hypertonic solution outside the cell that decides the direction in which water flows.
If the concentration of the external solution is more than that of the cytoplasm, that is, if it has more solutes, it is said to be hypertonic. If a plant cell is placed in a hypertonic solution, water moves out of the cell cytoplasm and then the vacuole due to osmosis. The cell membrane shrinks away from the cell wall. This phenomenon is called plasmolysis, while the cell is said to be plasmolysed.
This movement of water takes place from a cell which has a higher water potential to an area outside the cell that has a lower water potential. However, plasmolysis is a reversible process.
If the concentration of the external solution is lower than that of the cytoplasm, it is said to be hypotonic. When plasmolysed cells are placed in a hypotonic solution, that is, a solution with less solutes and higher water potential; water moves from the solution into the cell due to osmosis. This causes the cytoplasm to build a pressure against the cell wall. This pressure is called turgor pressure, which enables the plant to be erect. This turgor pressure exerted by the protoplast against the cell wall due to the entry of water is called pressure potential, Ψp. Since plant cells have a rigid cell wall, the cell does not rupture despite the turgor pressure.
If the concentration of the external solution is the same as that of the cell cytoplasm, the solution is said to be isotonic. Now if the cell is placed in an isotonic solution, there is no net flow of water either from inside or outside the cell. When the flow of water from and into the cell is in equilibrium, the cell is said to be in a flaccid state. Flaccid cells are found in a wilted plant that has not been watered for a long time.