Osmosis-Definition, Types, Examples, (Osmosis Vs Diffusion)

Osmosis Definition

  • Osmosis is a frequent biophysical phenomenon observed in biological systems in which solvent molecules migrate through a semipermeable barrier towards an area with a high solute concentration.
  • It is a form of passive transport that attempts to equalise the concentration of solutes throughout a semipermeable barrier.
  • It is a frequent mechanism that occurs in the majority of biological membranes in animals.
  • In a biological process, water is the primary solvent, although osmosis may often happen in various liquids and gases.
  • As it is a passive mode of transportation, it requires no energy.

Mechanisms of osmosis (How does osmosis work?)

  • To demonstrate the mechanism and process of osmosis, we use a semipermeable membrane to separate two liquids.
  • One of the solutions consists of pure water, while the other contains a solute.
  • In accordance with the concept of osmosis, in this instance, pure water travels through the membrane towards the solute solution.
  • Many explanations have been provided to comprehend osmosis’ driving power. Due to one of these theorems, due to the concentration differential of water in the two solutions, water is transported across the membrane.
  • The process of reverse osmosis, which includes the movement of solvent towards a solution having a lower concentration and against the concentration gradient, is not explained by this theorem.
  • A number of different scientists have presented the additional theory that solute molecules in a solution draw solvent molecules across a membrane. The size of the solute molecules has no effect on the transport of a solvent over a membrane, hence this theorem is also false.
  • Consequently, the mechanism of osmosis has been characterised by utilising the concept of chemical potential.
  • The chemical potential of pure water in one solution differs from that of water, including solute molecules, in another solution.
  • In a solution comprising the solute, the interaction between the solute and water molecules reduces the pressure exerted by the water molecules. Therefore, the water molecules in pure water apply more pressure on the solution with a lower solvent content.
  • This pressure causes water to be forced through the membrane. This procedure is repeated until the pressure on both sides is equalised, resulting in equilibrium.

Factors affecting osmosis

Osmosis is caused by a variety of variables, and its rate is regulated by a number of these elements:


  • The speed of osmosis rises as the system’s temperature rises.
  • This occurs because when the temperature rises, the power of the molecules also rises.
  • When the molecular activity rises, so does their movement, hence accelerating the osmosis process.

Concentration gradient

  • While the concentration of solute molecules is essential to the driving force of osmosis, alterations in concentration have an instantaneous influence on osmosis rate.
  • As the contrast in solute concentration along the membrane grows, so does the rate of osmosis.
  • As the concentration of solute molecules in one solution grows compared to that of the other, the pressure exerted by the solvent molecules reduces, resulting in an increase in osmosis.
  • As soon as equilibrium is reached across a membrane, osmosis ends.

Water potential/ Solvent potential

  • Furthermore, the water potential across a semipermeable membrane has an effect on the osmosis rate.
  • As the water potential of a solution rises, water molecules can move across the membrane as the pressure generated by the particles increases.
  • Ultimately, both sides’ water potentials equalise, culminating in equilibrium.
  • If equilibrium is reached, water keeps flowing across the membrane, but in equal quantities in both directions, stabilising the solution.

Dimensions of membrane surface area as well as thickness

  • With a rise in surface area, there will be extra available space for the movement of molecules, hence speeding up the process of osmosis.
  • Likewise, once the surface area is diminished, there will be insufficient room for molecules to move, thus restricting their movement.
  • In addition, the rate of osmosis decreases as the membrane’s thickness grows.


  • The pressure is a crucial aspect in the osmosis process. Since that might modify the path of osmosis, it is possible.
  • If the applied pressure is higher than the pressure imposed by the solvent molecules, the path of osmosis might change and the solvent molecules will start to move towards the place with the higher solvent concentration.
  • However, if a pressure smaller than that exerted by the molecules of the solvent is applied, the path does not alter, but the speed of osmosis is retarded.
  • Moreover, osmosis is accelerated by pressure exerted in a similar direction as the concentration gradient.

Variation/ Types of osmosis

Based on the migration direction of solvent molecules, there are a number of osmosis variants and kinds.

Reverse osmosis (RO) and Forward osmosis

  • Reverse osmosis is a separation technique in which a solvent is forced across a semipermeable membrane to separate solute molecules from solvent molecules.
  • Reverse osmosis differs from forward osmosis in that hydraulic pressure is applied to drive the solvent against the osmotic pressure.
  • Forward osmosis is a form of osmosis wherein the osmotic pressure gradient is used to promote the movement of liquid from the sample solution to isolate the solutes.
  • Forward osmosis utilises a draw solution with a higher solute concentration and eliminates solvent molecules from the sample solution, therefore separating the solute and solvent in the sample solution.

Endosmosis and exosmosis

  • While a cell is placed in a solution with a higher concentration of water than the cell itself, endosmosis develops.
  • Exosmosis is the movement of water out of a cell when it is placed in a solution containing a higher concentration of solutes than the cell.
  • Endosmosis causes cells to inflate, whereas exosmosis causes them to contract.

Osmotic solutions (Tonicity)

  • Tonicity is the potential of extracellular solutions to induce osmotic water movement into and out of a cell.
  • In a solution, the tonicity is determined by the concentration of solute as well as solvent molecules.
  • In accordance with their tonicity, solutions are classified as hypotonic, hypertonic, or isotonic.

Hypotonic solution

  • If an extracellular solution contains a lower concentration of solutes than the intracellular solution, it is called hypotonic.
  • Endosmosis occurs when a cell is exposed to a hypotonic solution and water migrates inside the cell
  • In such a circumstance, the cell will enlarge and may possibly rupture.

Hypertonic solution

  • A hypertonic solution is one that contains a larger solute concentration outside of the cell than within.
  • While a cell is placed in a hypertonic solution, the cell loses water by exosmosis.
  • Once the cell contracts, it loses the ability to proliferate and even operate.

Isotonic solution

  • Once an extracellular solution possesses the same concentration of solutes as the internal solution, it is known as an isotonic solution.
  • While a cell is placed in an isotonic solution, no transmembrane water movement occurs.
  • In this instance, the size of the cell is unaffected because there is no water flow.

Osmotic Pressure

  • Osmotic pressure is the pressure exerted by a hypotonic force that causes solvent molecules to migrate across a semipermeable membrane.
  • It is the least pressure required to prevent pure solvent from passing through a semipermeable barrier when administered to a solution.
  • Osmotic pressure drives osmosis, and the speed of osmosis rises as osmotic pressure rises.

The osmotic pressure of a solution can be calculated as follows:

∏= MRT

Where ∏ is the osmotic pressure

M is the molar concentration of the solute.

R is the gas constant.

And T is the temperature of the system.

Significance of osmosis

  • Chemical and biological systems are affected by osmosis in the following ways:
  • Osmosis is crucial for transporting nutrients into cells and waste products out of cells.
  • Osmosis controls the transfer of water from the soil into the roots of plants, where it is subsequently transported to other cell components via the xylem tissue.
  • In living organisms, the interior environment of the cell is stabilised by the equilibrium between water and intracellular fluid levels.
  • Additionally, osmosis is crucial for maintaining the cell’s turgor.
  • In plants, osmosis protects the cells from drying up due to transpiration-induced water loss.
  • Osmosis also maintains the movement of water and other cellular fluids between cells.
  • The movement of plants and plant components is controlled by the cell’s turgidity, which is in turn maintained by osmosis.
  • Osmosis also protects fruits and sporangia, among other plant structures, from drying out.
  • An increase in osmotic pressure protects plants in arid regions from dehydration and other stresses.
  • Reverse osmosis and forward osmosis are separation processes utilised in the filtration of drinking water, desalination, wastewater purification, concentration of liquid foods such as juices, manufacturing of maple syrup, low alcohol beer, and hydrogen peroxide.

Examples of osmosis

In the animal cells

  • Osmosis alters the form and size of animal cells due to the absence of a cell wall.
  • Human red blood cells are significantly affected by the osmotic pressure of blood. If the blood is too dilute, the RBCs shrink in size, and if the blood is excessively concentrated, they inflate and potentially rupture.
  • Thus, the concentration of bodily fluids in animals, including blood plasma and tissue fluid, must be maintained within stringent limits.
  • Another example of osmosis in animals is the salt-induced shrinkage of slugs.
  • The skin of slugs is a semi-permeable membrane that, when exposed to salt, pulls water from the cells, causing the cells and the animal to shrivel.

In the plant cells

  • Through osmosis, the root system of plants absorbs water from the soil.
  • The root cells of plants feature a semipermeable barrier that permits soil water to get into the roots and influence the guard cells.
  • The expansion and contraction of potato cells when dipped in hypotonic and hypertonic solutions, respectively, is an additional famous example of osmosis in plants.

Osmosis vs Diffusion

Basis for comparison Diffusion Osmosis
Definition Simple diffusion聽is a type of passive transport in which the movement of solute occurs when its electrochemical potentials on the two sides of a permeable barrier are different. Osmosis is a type of passive transport occurring commonly in biological systems where solvent molecules move across a semi-permeable membrane towards a region of high solute concentration.
Nature of the membrane Diffusion occurs through any permeable membrane. Osmosis requires a semi-permeable membrane.
Nature of the process Diffusion is a passive process. Osmosis is also a passive process.
Medium Diffusion can take place in all mediums (solid, liquid, and gas). Osmosis only occurs in a liquid medium.
Type of diffusing molecules The moving molecules can be either of solid, liquid or gases. The moving molecules in osmosis are always liquid molecules.
Rate of the process Diffusion is faster than osmosis. Osmosis is slower than diffusion.
Driving force The driving force of diffusion is the concentration gradient. The driving force of osmosis is osmotic pressure.
Direction of movement Diffusion takes place in all directions. Osmosis takes place in one direction.
Control of the process Diffusion cannot be stopped or reversed by any pressure. Osmosis can be stopped and even reversed by applying pressure equal to or more than the osmotic pressure.
Types of solution Diffusion can take place between similar or dissimilar solutions Osmosis takes place only between two similar solutions.


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