CBSE Class 9 Biology The Fundamental Unit of Life Notes Set 08

Osmosis

Osmosis is the movement of water (the solvent) through a semi-permeable membrane from a region where water is more abundant (a dilute solution or pure water) to a region where water is less abundant (a more concentrated solution). It continues until a balance is reached. It is a special case of diffusion.

Osmosis in Plant Cells

A living plant cell behaves like a small osmotic system. The cell membrane and the thin layer of cytoplasm act as a semi-permeable membrane and the cell sap in the vacuole is a concentrated solution.
1. Endosmosis: water moving into a cell. It occurs when the cell is kept in a solution weaker than its cell sap. The cell swells and becomes turgid.
2. Exosmosis: water moving out of a cell. It occurs when the cell is kept in a solution stronger than its cell sap. The cell loses water and becomes flaccid.

Isotonic, Hypotonic and Hypertonic Solutions

1. Isotonic solution: the outside solution has the same concentration (osmotic pressure) as the cell sap. There is no net movement of water.
2. Hypotonic solution: the outside solution is more dilute than the cell sap. Water enters the cell.
3. Hypertonic solution: the outside solution is more concentrated than the cell sap. Water leaves the cell.

Plasmolysis and Deplasmolysis

If a plant cell is kept in a strongly hypertonic solution, so much water comes out that the protoplasm shrinks away from the cell wall and becomes rounded. The gap between the wall and the shrunken protoplasm fills with the outside solution. This is called plasmolysis. The stage at which the first signs of shrinking appear is called incipient plasmolysis.
If the plasmolysed cell is put in pure water or a hypotonic solution, water re-enters and the cell regains its original shape. This is deplasmolysis.

Uses of Plasmolysis

1. It shows that the cell membrane is semi-permeable while the cell wall is freely permeable.
2. It helps to tell a living cell from a dead cell, since plasmolysis occurs only in living cells.
3. Salting meat, fish, pickles and jams preserves them because microbes lose water by exosmosis and die.
4. Common salt is used to kill weeds or to stop plants from growing in the cracks of walls.

Osmotic Pressure

Osmotic pressure is the pressure that must be applied to a solution to stop water entering it through a semi-permeable membrane from pure water. The more concentrated the solution, the greater its osmotic pressure.

Simple Experiments

1. Raisins: dry raisins placed in water swell because water enters them (endosmosis). When the swollen raisins are put in a strong sugar solution, they shrink (exosmosis).
2. Potato osmoscope: a cavity is made in a peeled potato and filled with strong sugar or salt solution. The potato is placed in a beaker of water. After some hours the level of the solution in the cavity rises, showing that water has entered through the potato tissue. If the potato is kept in a strong solution instead, the level in the cavity falls.

Importance of Osmosis in Plants

1. Roots absorb water from the soil through root hairs by osmosis.
2. Water is carried from cell to cell, keeping cells turgid; leaves, flowers and young stems stay firm. Without water they droop.
3. The osmotic pressure of cells in the roots helps to push water upward (root pressure).
4. A high concentration of cell sap helps plants to resist freezing.
5. The opening of stomata depends on the guard cells becoming turgid.