Osmosis

What is Osmosis?

Osmosis is a special type of diffusion which occurs through membranes.

Osmosis is defined as the movement of water (solvent) from a region of its higher molar concentration to a region of its lower concentration through a semi-permeable membrane until a state of dynamic equilibrium is attained.

Conditions required for osmosis are as follows,

There should be two solutions with common solvent.

1. Pure water/weak solution
2. Concentrated solution

Mechanism of Osmosis

If two solutions of different concentration are separated by semi-permeable membrane then as per the laws of diffusion the movement of solvent molecules will be from the region of its higher concentration to the region of its lower concentration.

or

The solvent molecules will move from the dilute solution to the concentrated solution because the amount of water molecules will be more in dilute solution and less in concentrated solution.

Why osmosis takes place?

Water diffuses through the semi-permeable membrane due to difference in diffusion pressure.

Diffusion pressure of pure solvent is higher while diffusion pressure of solution is lower due to addition of solute. Addition of solute lowers the diffusion pressure.

Water diffuses from a region of higher DP to lower DP.

What is Osmotic Pressure (OP)?

The accumulation of water in the left arm of the U-tube increases the hydrolytic pressure on the semipermeable membrane to stop further inflow of water from the right arm. This hydrostatic pressure is known as Osmotic Pressure (OP).

The pressure required to prevent osmosis is termed as Osmotic Pressure (OP).

Osmotic Pressure is also known as Osmotic Potential.

Unit of osmotic pressure is Atmosphere or Bars and symbol of osmotic pressure is PSI (Ψ).

Osmotic solution

There are three different types of solutions named as follows,

1. Isotonic Solution
2. Hypertonic Solution
3. Hypotonic Solution

Hypertonic Solution

When the concentration of the outside solution is more as compared to the cell sap it is termed as hypertonic solution.

or

When the concentration of solute in a solution is more than that of the cell sap, it is called as hypertonic solution.

Hypotonic Solution

When the concentration of the outside solution is less as compared to cell sap it is termed as hypotonic solution.

or

When the concentration of solute in a solution is less than that of the cell sap, it is called as hypotonic solution.

Isotonic Solution

When the concentration of the outside solution is same as compared to the cell sap it is termed as isotonic solution.

or

When the concentration of solute in a solution is equal as of the cell sap, it is called as isotonic solution.

Types of osmosis

Exo-Osmosis – (Exo means Outside ) When a plant cell is placed in hypertonic solution water moves out from the cell into the external solution.

Endo- osmosis – (Endo means Inside) When a plant cell is placed in hypotonic solution water moves into the cell from the external solution.

What is Turgor Pressure (TP)?

When a plant cell which is in a flaccid condition is immersed in water or a hypotonic solution then water moves into (within) the cell by osmosis. As a result of the passage of water into the cell a pressure develops which prevails throughout the cell sap and is exerted against the protoplasm and the cell wall. This outward pressure that is produced due to entry of water is called as Turgor Pressure.

What is wall pressure (WP)?

The cell wall is elastic and it produces an equal and opposite pressure to resist the turgor pressure, this pressure is called as Wall Pressure (WP).

What is Diffusion pressure deficit (DPD)?

The term Diffusion Pressure Deficit DPD was coined by Meyer in early year of 1939.

Diffusion Pressure (DP) – The pressure exerted by the diffusing particles is known as Diffusion Pressure.

Diffusion Pressure is directly proportional to the number/concentration of diffusing particles.

Diffusion is the net movement of molecules from a region of their greater diffusion pressure to the region of lower diffusion pressure.

Pure water has maximum diffusion pressure.

Addition of solute lowers the DP.

Diffusion Pressure Deficit (DPD) – The difference between DP of the solution and its pure solvent at a particular temperature and atmospheric pressure is known as DPD.

or

Diffusion Pressure Deficit (DPD) – The amount by which the diffusion pressure of the solvent molecules in a solution is lower than that of pure solvent is called DPD.

Concentrated solvent – In a solution amount of the solute particles is more than amount of solvent particles this solution is known as concentrated solution. Concentrated solutio is having high Diffusion Pressure Deficit (DPD).

Dilute solution – In a solution amount of the solute particles is less than amount of solvent particles this solution is known as concentrated solution. Concentrated solution is having low Diffusion Pressure Deficit (DPD).

Concentrated solutions will have greater tendency to absorb water.

Diffusion Pressure Deficit DPD is basically the thirst of the cell by which it sucks water from the surrounding hence it is also called as Suction Pressure.

Relation of DPD, OP, TP and WP

Diffusion Pressure Deficit DPD is closely related to osmotic pressure (OP), turger pressure (TP) and wall pressure (WP).

At Flaccid Condition (TP=0)

DPD = OP – TP

DPD = OP – 0

DPD = OP

Because of this condition water enters into the cell.

Let us assume a plant cell (a) Flaccid with OP = 20 units is separated by another cell (b) Turgid (having pure water).

Initial absorbing capacity of cell A is 20 units because DPD = OP as TP = O.

Let us assume that 5 ml of water has entered into the plant cell (a) creating TP = 5 units.

DPD = OP – TP (WP)

DPD = 20 – 5

DPD = 15

As the cell imbibes water TP increases and DPD decreases.

As more and more water enters, the cell it becomes fully turgid (TP=20)

At Turgid condition (OP = TP)

DPD = OP – TP (WP)

DPD = 20 – 20

DPD = 0 (Cell is fully turgid)

When cell is flaccid TP is low and OP is high.

Gradually when cell imbibes water TP increases and DPD decreases.

At equilibrium OP=TP and DPD=O.

Hence, no entry of water takes place into the cell.

Importance of Osmosis

1. Entry of water into plant cells from the external environment depends upon osmosis i.e., absorption of water from the soil through the root hairs is by osmosis.
2. Movement of water from one living cell to another occurs due to osmosis.
3. Maintenance of turgidity which is essential to maintain the definite shape of the leaf, flower etc depends upon osmosis.
4. Variety of plant movements are related to osmosis, such as opening and closing of stomata, sleep movements of leguminous plants and seismonastic movements of touch-me-not.
5. Growth is influenced by turgor pressure and osmotic pressure.
6. Osmotic concentration of the cell plays an important role in increasing the resistance of plants towards drought and frost.

FAQs : Osmosis

References and sources

1. https://byjus.com/biology/osmosis/
2. https://en.wikipedia.org/wiki/Osmosis
3. https://www.vedantu.com/biology/osmosis
4. https://biologydictionary.net/osmosis/
5. https://www.scienceabc.com/pure-sciences/what-is-osmosis-definition-biology-diffusion.html

You may also like to read following articles

1. Plant Water Relations
2. Imbibition
3. Diffusion
4. Meristematic Tissue System
5. Fruit
6. Leaves
7. Modification in Leaves
8. Epidermal Tissue System