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Osmosis is a natural phenomenon in which water moves across the membrane into the solution from a high concentration of water to a lower water concentration. Osmosis is a process that allows the small solvent molecules, like water to pass through a semi-permeable membrane but prevents the larger solute such as sugar molecules, from passing through. Solvent passes from a more dilute solution through the semi-permeable membrane into the more concentrated solution.
Osmosis is an important process in any biological system, that is plants and animals including ourselves. Many of the cell processes depend upon the ability of cell walls to act as semi-permeable membranes and allow the passage of fluids depending upon the concentrations of solutions inside and outside the cells.


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"Water is a small, polar molecule, which can easily diffuse across cell membranes through intermolecular spaces.This simple diffusion of water down its concentration gradient is called osmosis."

Osmosis occurs from an area of low solute concentration to an area of high solute concentration and results in the displacement of volume. Hence, osmosis provides a mechanism whereby the cells can regulate their volume.

Osmotic Pressure

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"The osmotic pressure of a solution is defined as the excess pressure which must be applied to a solution in order to prevent the flow of solvent into it through a semipermeable membrane."

Osmotic pressure concept can also be explained on the basis of fugacity or escaping tendency. The osmotic pressure of pure solvent is zero. Solutions have osmotic pressure, pure solvents do not. The net flow of solvent through the semipermeable membrane is from the solution of lower osmotic pressure into the solution of higher osmotic pressure.

Osmotic Pressure Formula

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By Van't Hoff's law, osmotic pressure can be calculated as

Osmotic pressure = iRTc


i = number of ions formed by dissociation of a solute molecule.
R = ideal gas constant
T = absolute temperature(measured in K)
c = molal or molar concentration

Calculating Osmotic Pressure

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To calculate the effective osmotic pressure ($\pi_{eff}$), the Van't Hoff equation for osmotic pressure is modified by the value for $\sigma$ as follows.

$\pi_{eff}$ = gC$\sigma$RT


$\pi_{eff}$ = effective osmotic pressure (atm)
g = number of particles /mol in solution
C = Conentration in mmol/L
R = gas constant
T = absolute temperature
$\sigma$ = reflection coefficient (no units, varies from 0 to 2)

Diffusion and Osmosis

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Diffusion is the process by which particles moving in a random matter spread evenly throughout an available space, moving from regions of high concentration to low concentration. For instance perfume or smoke pervade an entire room due to this process. A specific type of diffusion is the diffusion of water, osmosis. Water moves from an area of low particle concentration to high particle concentration.

The difference between diffusion and osmosis are listed below.`

S.No Diffusion Osmosis
1 Diffusion can operate in any medium like solid, liquid, gas etc.
Osmosis operates only in a liquid medium.
2 It does not require any semi-permeable membrane.
It requires a semi-permeable membrane.
3 It is a rapid process in gases and slow process in liquids. It is a slow process.
4It helps in equalizing the concentration of the diffusing substances on the two sides of the system.
It does not equalize the concentration of solvent on the two sides of the system.

Importace of Osmosis

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The significance of osmosis are listed below.
  • Entry of soil water into root occurs through osmosis.
  • Osmosis plays a key role in speed germination.
  • Cell to cell movement of water occurs through osmosis.
  • Living cells remain turgid by osmosis.
  • The stomata open and close in response to increase or decrease in osmotic pressure of the guard cell.

Osmosis Potential Energy

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Osmotic potential is defined as the amount by which the water potential is reduced as a result of the presence of solute. It is also known as solute potential.Adding solutes always lowers the water potential.

The water potential of a plant cell is negative owing to the presence of dissolved solutes. Roots contain more dissolved materials than does soil water, unless the soil is extremely dry. This means that under normal conditions water potential of the root is more negative than water potential of the soil. Thus water moves by osmosis from soil into the root.

High Osmotic Pressure

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The osmotic pressure depends on the concentration of osmotically active particles in solution. The osmotic pressure increases when the number of particles in solution increases.
For example, a 1M CaCl2 solution has a higher osmotic pressure than a 1M NaCl solution. The lower the osmotic pressure of a solution, the greater the H2O flow into it. For high solute concentrations, we can calculate the osmotic pressure from the mole fraction of water without assuming a dilute solution by identifying the value of $\pi$.
$\pi$ = - $\frac{RT}{\overline{V_{w}}}$$ lnX_{w}$


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Osmosis is a particular type of diffusion that refers specifically to the movement of water across a semipermeable membrane. Osmosis is simply an operation in which water diffuses through a membrane to an area with a large quantity of dissolved substances or solutes.

Reverse Osmosis

A process called reverse osmosis is employed by industrial chemist to achieve the purification of water. When pressure is applied greater than osmotic pressure to the more concentrated solution solvent flows from it to the more dilute solution by a process we call reverse osmosis. Reverse osmosis is used to make drinkable water from seawater or brackish water.


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Red blood cells provide a good example of osmosis. When these cells are in a solution of the same concentration, or an isotonic solution, the amount of water that leaves and enters is equal. In the case in which solution is more concentrated or hypertonic, water leaves the cell due to osmosis. Thus the cell shrinks. If the concentration outside the cell is less than that inside the cell, or hypotonic water flows into the cell and the cell bursts.

Osmosis Example