Tonicity describes the capacity of a cell to lose or gain water. It depends on the quantity of solute that does not penetrate the membrane. Tonicity delimits the magnitude and direction of osmosis and results in three possible scenarios that alter the volume of a cell: hypertonicity, hypotonicity, and isotonicity. Due to differences in structure and physiology, tonicity of plant cells is different from that of animal cells in some scenarios.
Unlike animal cells, plants thrive when there is more water in their surrounding extracellular environment compared to their cytoplasmic interior. In hypotonic environments, water enters the cell via osmosis and causes it to swell because there is a higher concentration of solutes inside plant cells than outside. The force, that is generated when an influx of water causes the plasma membrane to push against the cell wall, is called turgor pressure. In contrast to animal cells, plant cells have rigid cell walls that limit the osmosis-induced expansion of the plasma membrane. By limiting expansion, the cell wall prevents the cell from bursting and causes plants to stiffen (i.e., become turgid). Turgidity allows plants to hold themselves upright instead of wilting.
Plants wilt if they cannot take up sufficient water. In such a scenario, their extracellular surrounding becomes hypertonic, causing water to leave the interior via osmosis. As a result, vacuoles decrease in size and the plasma membrane detaches from the cell wall causing the cytoplasm to constrict. This process is called plasmolysis and is why plants lose turgor pressure and wilt.
In isotonic environments, there is a balance of water both inside and outside plant cells. Therefore, like in animal cells, no changes occur in plant cell volume.
A variety of different plant cell structures and strategies help maintain appropriate osmotic balance in extreme conditions. For instance, plants in dry environments store water in vacuoles, limit the opening of their stoma and have thick, waxy cuticles to prevent unnecessary water loss. Some species of plants that live in salty environments store salt in their roots. As a result, osmosis of water occurs into the root from the surrounding soil.
