Osmosis- Introduction, Types, Solution, Osmotic pressure, Factors, Significance


Osmosis is biological process that refers to the movement of free water molecules from a region of high concentration to a region of low concentration across a partially-permeable membrane until they are evenly spread out. It may define as the diffusion of the solvent molecules into the solution through a semipermeable membrane.

Types of membrane:

Impermeable membrane

  • The membrane which doesn’t allow to pass for both solute and solvent molecules.
  • For e.g.  cutinized cell wall.

Permeable membrane

The membrane which allows to pass for both solute and solvent molecules. For e.g., cell wall

Semi-permeable membrane

The membrane which allows to pass for the solvent molecules but don’t allow to pass for solute molecules.  For e.g., animal bladder, gee membrane

Selectively permeable membrane

The membrane which allows to pass for some solute molecules along with solvent molecules. For e.g., cell membrane, membrane of cell organelles.

Types of solution:

On the basis of solute concentration of solution, solution is classified into three types

Hypotonic solution

A solution which has lower concentration of solute as compared of solute as compared to cell sap of cell.

Isotonic solution

A solution which has equal concentration of solute as compared to cell sap of cell.

Hypertonic solution

A solution which has higher concentration of solute as compared to cell sap of cell.

 Water potential:

The amount of potential energy that water molecules have in a system is known as water potential, and it controls the speed and direction of water movement in a system. It is denoted by the symbol Ψ (psi) and is expressed in units of pressure, typically kilopascals (kPa).

Water potential is influenced by several factors, including pressure potential (Ψp) and solute potential (Ψs). Pressure potential is the physical pressure exerted on the water, such as from cell walls or the atmosphere, and it can be either positive or negative. Solute potential is the effect of solute concentration on water potential and is always negative. The water potential of a solution decreases with increasing concentration.

The overall water potential (Ψw) of a system is determined by the equation:

Ψw = Ψp + Ψs

If a system has no pressure and no solute concentration, the water potential is defined as zero, which is the reference point. In most cases, water moves from an area of higher water potential to an area of lower water potential, driven by the tendency to reach equilibrium.

Water potential is important in biology, especially in processes like how plants take up water from the soil, how water is lost through plant leaves (transpiration), and how water moves in and out of cells. Understanding water potential helps us understand how water behaves in different living systems.

Types of osmosis:


Exit of water molecules from a living cell when it is placed in high osmotic pressure solution or hypertonic solution is called exosmosis.


Entrance of water molecules inside the living cell from outside when it is kept in lower osmotic pressure or lower concentration (hypotonic solution) is called endosmosis. It is a real osmosis from which water absorbs by plants.

Osmotic relation in plant cell:

As the living cells in plant consist of cell membrane which is semipermeable and cell sap which has certain osmotic pressure, these cells in plants constitute an osmotic system. Plasma membrane in reality is not semipermeable as it allows to pass through it small solute articles so it is called as a selectively permeable membrane. The cell wall is permeable membrane. The osmotic connection is essential for sustaining cell shape and function in plant cells. Water enters a plant cell through osmosis when it is submerged in a hypotonic solution. The central vacuole expands as a result of the water influx, providing turgor pressure on the cell wall. As a result, the cell tightens up, supporting and supporting the plant. On the other hand, in a hypertonic solution, water leaves the cell, causing the central vacuole to contract and the cell membrane to separate from the cell wall. The cell becomes weak and wilted as a result of this process, known as plasmolysis. The cell maintains its equilibrium in an isotonic solution because there is no net water movement. Maintaining cell form, turgor pressure, and adjusting to changing environmental conditions all depend on the osmotic connection in plant cells.

Osmosis and Turgor Pressure in Plant Cell

Fig: Osmosis and Turgor Pressure in Plant Cell


The protoplast is closely adpressed against the cell wall in normal condition. If this cell is kept in s hypertonic solution, then water comes out from the cell sap into the outer solution due to osmosis and the protoplast contracts and shrinks away from the cell wall. This process is called incipient plasmolysis, if the outer hypotonic solution is too much concentrated as compared with that of cell sap, then the concentration of protoplast continues which finally results in the separation of protoplast from cell wall and forms a round shape. This process is called plasmolysis.

When plants are exposed to drought or are placed in high-salt environments, for example, or in other circumstances, plasmolysis can be observed. Plasmolysis has significant effects on plant physiology. The osmotic characteristics of plant cells and their capacity for environmental adaptation can be studied using this phenomenon.


  • It proves semipermeable nature of cell membrane.
  • It proves that cell is either dead or living because it occurs in living cells.
  • It checks the growth of microbes in jams, jelly, pickles because microbes (bacteria and fungi) get plasmolyzed in sugar or salt solution.
  • A prospective strategy to improve post-harvest agricultural storage by lowering microbial growth, delaying spoiling, and prolonging the life of the food has been experimentally investigated using this process.
  •  It is used to determine the osmotic pressure.
  • This process is utilised in salting of meat and fishes to check decaying.


In plant cells, plasmolysis is reversed through a process called deplasmolysis, commonly referred to as rehydration. As a result of water loss in a hypertonic solution, plasmolysis causes plant cells to contract and the protoplast to peel away from the cell wall. When a plasmolyzed cell is exposed to a hypotonic solution, deplasmolysis does, nevertheless, take place. The protoplast expands and makes contact with the cell wall once more when water enters the cell through osmosis. The cell wall and cell membrane then press against each other, causing the cell to become turgid. This procedure enables the cell to re-establish its regular morphology, turgor pressure, and functional integrity.

Osmotic pressure:

The pressure developed in solution due to the presence of dissolved solutes in that solution as a result of separation from its solvent by semipermeable membrane is called osmotic pressure. It is measured in term of atmosphere and is directly proportional to the concentration of solute particles in the solution i.e., more concentrated solution has higher osmotic pressure. The movement of the solute’s molecules from the region of lower osmotic pressure to the region of higher osmotic pressure takes place during the process of osmosis.

Factor affecting osmosis:

Pressure: At constant temperature, if pressure of water or solvent on both sifes of semipermeable are different, the water molecules move from region of higher pressure to the region of lower pressure.

Temperature: At constant pressure and concentration of solution, if solvent at different temperature is placed on either side of semi permeable membrane, the solvent moves from the side of higher to the sides of lower temperature.

Nature of the membrane: Osmosis rate is influenced by membrane permeability and selectivity. Water flow is influenced by a selectively permeable membrane, which allows just specific molecules to pass through. The rate of osmosis can also be influenced by the presence of membrane proteins, channels, or openings.

Concentration gradient: Increases in concentration gradient increase the rate of osmosis.

Significance of osmosis:

  • It maintains the shape of organs and make the origin rigid due to turgidity.
  • Plant roots absorb water by osmosis.
  • High osmotic pressure of cells protects the pants against drought and frost injury.
  • It helps the growth of tissue.
  • It regulates the opening and closing of stomata by turgidity and flaccidity of guard cells due to osmosis.
  • It helps in movement of water molecules from cell to cell
  • Osmosis enables cells to control water transport, prevent swelling or shrinkage, and maintain suitable internal conditions. It is essential for maintaining water balance and overall homeostasis in living organisms.

Reverse Osmosis (RO):

RO is a water purification technique that uses a semi-permeable membrane to remove pollutants and toxins from water. Water molecules are pushed through the membrane by applying pressure, leaving larger particles and contaminants behind. Water molecules flow across the barrier while dissolved salts, bacteria, viruses, and other undesirable things are blocked. Pre-treatment to remove bigger particles, pressurization to overcome osmotic pressure, separation of contaminants, and collecting of purified water as permeate are all steps in this extensively used process for creating clean drinking water. Typically, contaminated concentrated water or brine is discarded. Although reverse osmosis is an excellent approach for eliminating various contaminants, it produces waste water. Through innovative technologies, modern systems attempt to improve efficiency and reduce waste.


  • Cath, T.Y., Childress, A.E. and Elimelech, M., 2006. Forward osmosis: Principles, applications, and recent developments. Journal of membrane science, 281(1-2), pp.70-87.
  • Marbach, S. and Bocquet, L., 2019. Osmosis, from molecular insights to large-scale applications. Chemical Society Reviews, 48(11), pp.3102-3144.
  • Sourirajan, S., 1970. Reverse osmosis. London, UK: Logos Press Ltd..

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