Tonicity in Plants

Tonicity describes how the extracellular solute concentration changes plant cell volume by making the cell lose or gain water. There are three possible conditions.

In a hypotonic environment, solute concentration outside the cell is lower than inside, causing water to enter the cell via osmosis.

Water influx makes the plasma membrane push against the cell wall creating turgor pressure.

The rigid cell wall prevents the cell from bursting by limiting the expansion of the plasma membrane.

The resulting stiffening or turgidity allows the plants to remain upright.

When plants cannot uptake water, the extracellular surroundings have higher solute concentrations or hypertonicity, causing water to leave the cell.

As a result, vacuoles decrease in size, and the plasma membrane detaches from the cell wall as the cytoplasm shrinks. Such plasmolysis explains why plants wilt.

In isotonic environments, solute concentrations outside and inside the cell are equal. So, there is no net movement of water. The cells become flaccid, and the plant starts to droop.