What is Osmosis?

The process by which solvent molecules pass from a solution of lower concentration to a solution of higher concentration through a semipermeable membrane. It is a passive process which takes place without any expenditure of energy. The transport of solvent molecules continues from low to high concentration regions till the concentration on either side of the membrane is equal.

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What is a Semipermeable Membrane?

It is a thin barrier between two solutions that permits certain components of the solutions, generally the solvent, to pass through.

Types of Osmosis

Generally, there are two types of osmosis. These are

Endosmosis: If a cell is placed in a hypotonic solution, water moves inside the cell making it swell or deplasmolysed. This happens because the solute concentration of the solution is less than the concentration inside the cell. This process is known as endosmosis.

Example: Raisins swell when placed in normal water.

Exosmosis: If a cell is placed in a hypertonic solution, the water inside the cell moves outside and thus the cell plasmolysis (becomes flaccid). This happens because the solute concentration in the solution is more than the concentration inside the cytoplasm. This process is known as exosmosis.

Example: Raisins placed in a concentrated salt solution shrivel.

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There are also Two other Variants of Osmosis which has Been Observed:

Reverse Osmosis: It can be defined as a separation process which uses pressure to force a solvent through a semipermeable membrane that retains the solute on one side and makes the solvent pass through the other side.  It uses pressure to force the solvent to move from a high solute concentration region to a low solute concentration region. So reverse osmosis can be referred to as the opposite of general osmosis.

Application: It is used to remove major contaminants from water by pushing water through a semipermeable membrane under pressure.

Forward Osmosis: It is a natural phenomenon which uses a semi-permeable membrane to separate dissolved solutes from water.

Application: Desalination of water, waste-water treatment, osmotic power generation.

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 Osmosis in a Plant Cell

If a cell is put in a hypertonic solution which is more concentrated than the cell, it will shrink due to loss of water and eventually die. For example, if a piece of carrot is put in a solution of salty water it will become soft and limp as the cells would shrivel

In contrast, if the carrot piece is put in an isotonic solution, it would swell and expand. Generally, a normal cell would burst, but the rigid cell wall in the carrot cell protects it from rupturing.

As the water enters the cell, it expands, until it creates a maximum pressure on the cell wall to expand more. However, the cell wall pushes back with equal pressure and no more water can enter.

Osmosis plays an important role in the transport of water in plants. Solute concentrations increase as they move from soil to root cells and then to leaf cells. The difference in pressure helps to push water upwards. Osmosis also controls the evaporation of water from leaves by regulating the size of the stomata on the leaf surface.

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The Importance of Osmosis:

In Plants:

  • Osmosis helps in maintaining water content within a plant cell

  • It provides turgidity to softer cells of a plant body.

  • Osmosis controls the absorption of water by root hairs from the soil.

  • It controls the conduction of water from xylem elements to adjacent cells

  • Higher osmotic pressure provides resistance to plants against drought injury

In Animals:

  • Osmosis regulates the flow of dissolved solids, liquids and gases across cells.

  • The semi-permeable membrane that encloses the cell selectively allows substances to pass in and out of the cell. This assists in releasing toxic metabolic waste products such as urea.

  • Osmosis also helps in absorbing water from the intestines to the blood.

The Significance of Osmosis:

  • Osmosis plays an important role in the transportation of nutrients and release of metabolic waste products within a living cell.

  • It stabilises the internal movement of water and intracellular fluid levels within a cell.

  • Osmosis also controls cell to cell diffusion and maintains the mechanical structure of a cell.

  • In plants, growing root tips remain turgid and can penetrate easily into the soil because of osmosis.

  • Osmosis plays a major role in the germination of seeds.

Examples of Osmosis and Diffusion:

Examples of Osmosis:

  • The absorption of water by plant roots from the soil.

  • The guard cells of a plant cell are affected by osmosis. When a plant cell is filled with water the guard cells swell up for the stomata to open and let out excess water

  • If you keep your fingers in water for a long time, they become prune. The reason behind this is that the skin absorbs water and expands.

Examples of Diffusion

  • A drop of food colouring diffuses throughout the water in a glass.

  • If you put a sugar cube in water it will eventually dissolve and sweeten the water.

FAQ (Frequently Asked Questions)

Q1: What is Osmotic Pressure?

Ans: It is the amount of pressure required to stop water from diffusing through a membrane by osmosis. The concentration of the solute in a solution determines the value of pressure. It can be calculated using the equation below.


Where Π denotes osmotic pressure

R is the gas constant

M denotes the molar concentration of the solute

T is the temperature

Osmotic pressure plays a role in the transport of solutes from the goes in and out of the cell. It also prevents the inward flow of water across a semipermeable membrane and thereby nullifying osmosis.

Q2: What are Hypertonic, Hypotonic and Isotonic Solutions?


Hypertonic Solutions: It is a type of solution which has a greater concentration of solute than another solution. Hypertonic solutions have high osmotic pressure.


Example: A solution of 5%sugar and 0.45% salt


Hypotonic Solution: It is a type of solution which has a lower solute concentration than another solution. Hypotonic solutions have low osmotic pressure.


Example: 0.45% saline solution. Hypotonic solutions are used to treat patients suffering from diabetic ketoacidosis


Isotonic Solutions: Two solutions which have the same concentration of solute particles and the same osmotic pressure.


Example: 0.9% of normal saline solution, lactated ringers ( a sterile solution composed of Sodium Chloride, Sodium Lactate, Potassium Chloride and Calcium Chloride)