Types of solution
A solution having a relatively lower solute concentration is called a hypotonic solution.
Solutions having exactly the same solute concentration are called isotonic solutions.
A solution having a relatively higher solute concentration is called a hypertonic solution.
Plant cell is kept in hypotonic solution
When a plant cell is kept in a hypotonic solution, endosmosis occurs, as the osmotic pressure (OP) of the solution is lower and the osmotic pressure of the cell is higher. As osmosis occurs from low OP to high OP, water starts entering the cell from the solution. (The OP of the cell decreases continuously as water enters it.)
As a result, the volume of the cell protoplast increases, which exerts an outward pressure on the cell wall. This pressure is called turgor pressure (TP). Turgor pressure pushes the cell wall outwards, and due to this the plant cell swells up. But as the cell wall has limited flexibility, the size of the cell increases only up to a certain extent and then stops. At this point, turgor pressure reaches its maximum value and becomes equal to osmotic pressure. As OP becomes equal to TP, the DPD of the cell becomes zero (DPD = OP − TP), and no further entry of water can occur into the cell.
A cell which experiences turgor pressure is called a turgid cell.
NCERT describes the same process using water potential: ψw = ψs + ψp. DPD is simply the negative of water potential (DPD = −ψw), and water always moves from higher to lower water potential. In a fully turgid cell, ψw = 0.
Advantages of turgor pressure
Turgor pressure has several advantages for the plant:
- Helps to maintain the shape of the cell.
- Responsible for nastic movements in plants.
- Helps in the opening and closing of stomata.
Plant cell is kept in an isotonic solution
An isotonic solution has the same concentration as that of the cell. As a result, no change in the volume of the cell is observed, as the net exchange of water between the cell and the solution is zero.
Plant cell is kept in a hypertonic solution
When a plant cell (suppose a turgid cell) is placed in a hypertonic solution, it starts to lose water due to exosmosis. This occurs because the osmotic pressure of the solution is relatively higher than that of the cell, and osmosis occurs from low OP to high OP.
As the cell loses water, its turgor pressure decreases until, at one point, it reaches zero. At this point, the protoplast exerts no pressure on the cell wall and the cell is called a flaccid cell.
With further loss of water, the protoplast shrinks and starts detaching from the cell wall. This is called plasmolysis.
Stages of plasmolysis:
- Incipient (limiting) plasmolysis – the protoplast just begins to separate from the cell wall (TP = 0).
- Evident plasmolysis – the protoplast visibly separates from the cell wall.
- Final plasmolysis – the protoplast shrinks completely and rounds up in the centre of the cell.
A cell whose protoplast has separated from the cell wall is called a plasmolysed cell. The space between the cell wall and the protoplast is occupied by the external hypertonic solution.
Plasmolysis is reversible. If a plasmolysed cell is placed back in a hypotonic solution, it takes up water and becomes turgid again; this is called deplasmolysis. The space between the wall and protoplast fills with the outside solution because the cell wall is freely permeable.
Real-life plasmolysis: salting pickles and adding sugar to jams kill microbes by plasmolysing them, and spraying salt on weeds makes them wilt. Animal cells have no cell wall, so in a hypotonic solution they swell and burst instead (e.g. haemolysis of RBCs).