Osmosis is defined as the movement of solvent from a region of its higher concentration to a region of its lower concentration when the two are separated by a semipermeable membrane.
Osmosis is of two types:
- Endosmosis. When a cell is kept in a hypotonic solution (concentration of solution is low as compared to cell sap) movement of water or solvent takes place towards the cell sap, it is called endosmosis.
- Exosmosis. When a cell is kept in hypertonic solution (concentration of solution is high as compared to the cell sap) water molecule comes out from cell sap as a result of osmosis, it is called exosmosis.
Permeability and Membranes: Permeability is the degree of diffusion of gases, liquids and dissolved substances through a membrane. The following four types of membranes have been recognized on the basis of permeability.
- Permeable. This type of membrane allows a free passage of water, other solvents and most of the dissolved substances. For example, plant cell wall.
- Impermeable. The membrane which has a heavy deposit of cutin and suberin and does not allow the entry of water, dissolved substances and gases is called impermeable. For example, cell wall with thick layer of cutin.
- Semipermeable. A membrane which is impermeable to solute molecules while it is permeable to the solvent is called a semipermeable membrane.
- Selectively or differentially permeable membrane. This membrane allows some molecules or ions to enter readily, while allowing others more slowly and does not allow certain molecules at all.
Osmotic pressure of a solution is the pressure which must be applied to it, in order to prevent the flow of solvent due to osmosis. In other words, the maximum amount of pressure that can be developed in a solution separated from solvent by a semipermeable membrane is called osmotic pressure. Osmotic pressure is measured in atmospheres, bars, or pascals. The instrument used for measuring osmotic pressure is called osmometer. Osmotic pressure is directly proportional to the numbers of solute particles in a given amount of solvent. More is the number of solute molecules in a solution, more would be its osmotic pressure.
FACTORS AFFECTING OSMOTIC PRESSURE
- Temperature: The osmotic pressure of a solution increases with increase in temperature.
- Concentration of solute particles: Osmotic pressure is influenced by the ratio of solute and the solvent particles. More are the solute particles, more would be the osmotic pressure.
- Ionization of solute particles: An electrolytic compound or a compound that can ionize would have more solute particles. Therefore, when a cell is kept in 0.5 M solution of sucrose, its volume does not change because it is isotonic to the cell contents. However, if the same cell is placed in 0.5 M solution of sodium chloride, which is an electrolyte, the osmotic pressure of the solution increases.
SIGNIFICANCE OF OSMOSIS FOR PLANTS
- Absorption of water from the soil by root hairs is carried out by osmosis.
- Osmosis helps in maintaining the turgidity of plant organs. Due to their higher osmotic concentration, cells become resistant to freezing and desiccation.
- Various cell organelles like mitochondria and chloroplast will collapse if they are not able to maintain a proper osmotic concentration.
- The opening and closing of stomata is dependent on the turgidity of guard cells which is due to osmosis.
- Many plant movement like the folding and drooping of leaves in Mimosa are brought about by osmosis.
- Osmosis plays an important role in the growth of radicle and plumule during the germination of seeds.
- Seeds and spores are able to pass through the unfavourable condition due to high osmotic pressure.
TYPES OF SOLUTION
- Hypertonic solution: A solution whose concentration is more than that of the cell sap is known hypertonic. If a cell is placed in such a solution, water diffuses out of it and the protoplasm shrinks.
- Hypotonic solution: When the concentration of a solution is less than that of the cell sap it is known hypotonic. If a cell is immersed in hypotonic solution, water diffuses into the cell increasing the size of the cell.
- Isotonic solution: A solution with concentration equal to that of the cell sap is known as isotonic. If a cell is placed in isotonic solution, there would be no diffusion of water. As a result, there would be no change in volume and weight of the cell.
TURGOR PRESSURE (T.P)
Plant cells remain surrounded by rigid cell wall. When a plant cell is kept inside water, it swells due to endosmosis. As a result, a pressure is developed in protoplasm which is exerted on the cell wall. The actual pressure which is exerted by protoplast on the cell wall is the turgor pressure (T.P).
The cell wall is rigid and elastic. It exerts an equal and opposite pressure against the expanding protoplasm due to turgor pressure. This pressure is called wall pressure. Hence, at a given time, TP=WP.
DIFFUSION PRESSURE DEFICIT
The term was given by Meyer (1938). Originally, the diffusion pressure deficit (D.P.D.) was described as suction pressure by Renner (1915).
The pressure exerted by diffusing particle is known as diffusion pressure. This pressure is proportional to the concentration of the diffusing particle. Diffusion pressure of a solvent in a solution is always lower than that of pure solvent. The amount by which the diffusion pressure of a solution is lower than that of its pure solvent is known as diffusion pressure deficit (D.P.D).
The difference in concentration results in the flow of water from its higher concentration to its lower concentration. As the water enters into the cell, the turgor pressure of the cell increases. Consequently, the cell wall exerts a pressure called wall pressure against the turgor pressure. The wall pressure is equal in magnitude but opposite in direction to the turgor pressure at a given temperature. The external force responsible for the entry of water will be, therefore O.P-T.P. The relationship of O.P, T.P, W.P and D.P.D is as follows: