31.3: Tonicity in Plants

Plant cells 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, water osmosis occurs in the root from the surrounding soil.

Tonicity

Tonicity describes the capacity of a cell to lose or gain water depending on the solute concentration outside. Organisms such as plants, fungi, bacteria, and protists, have cell walls surrounding the plasma membrane. Three possible scenarios alter the volume of a cell: hypertonicity, hypotonicity, and isotonicity.

Hypotonic environment

In hypotonic environments, there is a higher concentration of solutes inside plant cells than outside. Water enters the cell via osmosis and causes it to swell. Because the cell wall limits the expanding plasma membrane, the cell does not lyse. By limiting expansion, the cell wall allows cells to become turgid, resulting in the stiffening of plants.

Plant cytoplasm is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available. The force generated when an influx of water causes the plasma membrane to push against the cell wall is called turgor pressure. Turgor pressure keeps non-woody plants upright.

Hypertonic Environments

Conversely, the extracellular fluid becomes hypertonic in a dry climate, causing water to leave the cell through osmosis. In this condition, the cell cannot shrink because the cell wall is not flexible. As a result, vacuoles decrease in size the cell membrane detaches from the wall and constricts the cytoplasm. This process is called plasmolysis. Thus, plants lose turgor pressure and wilt.

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.