Osmosis is 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.

The plasma membrane of the cell acts as a semi-permeable membrane.  It allows only certain types of molecules to pass through it but not all.  Therefore it is also known as selectively permeable membrane.

Plant cell contains a solution inside it called cell sap.  The concentration of cell sap is higher than that of water.  So when it is immersed into water diffusion of water molecules into the cell takes place by osmosis.  Gradually the cell sap gets diluted and the cell is stretched.

Permeability and Membranes:

Permeability is the degree of diffusion of gases, liquids and dissolved substances through a membrane.  The four types of membranes are as follows:

  1. Permeable. This type of membrane allows a free passage of water, other solvents and most of the dissolved substances.  For example, plant cell wall.
  2. 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.
  3. Semipermeable. A membrane which is impermeable to solute molecules while it is permeable to the solvent is called a semipermeable membrane.
  4. 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.

Experiment to demonstrate the process of osmosis:

A peeled potato tuber is taken and a hole into the tuber is made by means of a cork borer.  Sugar solution is poured into the hole.  A glass tube is fitted into the mouth of the hole, such that it touches the sugar solution.  The whole thing is kept inside a beaker of water.

The potato tuber is half immersed in the water.  The potato tuber acts as a semi-permeable membrane through which only water molecules can pass, but not the sugar molecules.  The solution inside the tuber is more concentrated.

As a result water molecules from the beaker diffuse into the tuber.  After sometime the volume of the liquid inside the tuber increases. So this experiment demonstrates the process of osmosis taking place through the potato tuber.


Fig: Process of Osmosis

Osmotic pressure

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 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


The osmotic pressure of a solution increases with increase in temperature.

2. Concentration of solute particles:

The ratio of solute and the solvent particles influences the osmotic pressure .  More are the solute particles, more would be the osmotic pressure.

3. Ionization of solute particles:

An electrolytic compound has more solute particles. A cell kept in 0.5 M solution of sucrose, does not change the volume because it is isotonic to the cell contents.

However, if the same cell is kept in 0.5 M solution of sodium chloride, which is an electrolyte, the osmotic pressure of the solution increases.

Significance of Osmosis for plants

  1. Osmosis helps in the absorption of water from the soil by root hairs.
  2. Osmosis helps in maintaining the turgidity of plant organs.
  3. Various cell organelles like mitochondria and chloroplast will collapse if they are not able to maintain a proper osmotic concentration.
  4. The opening and closing of stomata is dependent on the turgidity of guard cells which is due to osmosis.
  5. The folding and drooping of leaves is due to osmosis as in Mimosa.
  6. Osmosis plays an important role in the growth of radicle and plumule during the germination of seeds.
  7. Seeds and spores are able to pass through the unfavourable condition due to high osmotic pressure.
  8. Due to their higher osmotic concentration, cells become resistant to freezing and desiccation.

Turgor Pressure (T.P)

Plant cells remain surrounded by rigid cell wall.  A plant cell when kept inside water, swells due to endosmosis.  As a result, a pressure developes in protoplasm. The actual pressure which the protoplast exerts on the cell wall is the turgor pressure (T.P).

Wall Pressure (W.P)

The cell wall is rigid and elastic.  It exerts an equal and opposite pressure against the expanding protoplasm due to turgor pressure. The pressure exerted by the cell wall is the wall pressure.  Hence, at a given time, TP=WP.

Diffusion pressure deficit (D.P.D)

Meyer in the year 1938 coined the term diffusion pressure deficit (D.P.D).   Renner in the year 1915 described  diffusion pressure deficit (D.P.D.) as suction pressure.

The diffusing particle exert a pressure which is termed 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 difference between the diffusion pressures of a solution and a pure solvent, is termed 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 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:




Difference between Diffusion and Osmosis

Diffusion Osmosis
1.The molecules always move from the region of their higher concentration to the region of their lower concentration. 1. The water molecules always move from less concentrated solution to the more concentrated solution.
2.  Diffusion occurs in solids, liquids and gases. 2.  Osmosis occurs only in solvent (water).
3.  Diffusion does not require semipermeable membrane. 3.  The process of osmosis takes place through a semipermeable membrane.
4.  It is only dependent on the free energy of the diffusing substance. 4.  Osmosis is dependent upon the degree of free energy of one solvent over that of another.
5.  Diffusion helps in equalizing the concentration of the diffusing substance throughout the available space. 5.  It does not equalize the concentration of solvent on the two sides of the system.