Osmotic Pressure Equation: Significance & Examples

Osmotic pressure can be explained as the pressure that is exerted to the solution side to prevent fluid movement when a semi-permeable membrane differentiates a solution from pure water. The term ‘osmose’ was introduced in 1854 by a British chemist, Thomas Graham. Jacobus Henricus van’t Hoff, a Dutch chemist showed in 1886 that if the solute is dilute then its partial vapour pressure obeys Henry’s law that is the osmotic pressure varies with the concentration and temperature nearby as it would if the solute were gas occupying the required volume. This relation led to equations for finding molecular weights of the solutes in the dilute solutions through effects of freezing point, boiling points and vapour pressure of a solvent. The process of Osmosis is quite important in biology and was thoroughly examined in 1877 by a German plant physiologist, Wilhelm Pfeffer.

Key Takeaways: Osmosis, Osmotic Pressure, Reverse osmosis, water potential, xylem, types of osmosis, osmotic solutions, semipermeable membrane, Transpiration  

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What is Osmosis?

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Osmosis can be stated as a movement of a solvent through a semipermeable membrane as a living cell into a solution of higher concentration solute that tends to balance the concentrations of solute on both sides of the membrane. It is typically a binding property and is completely dependent on the concentration of solute particles in the solution.

Osmosis

Types of osmosis

There are basically two types of osmosis:

• Endosmosis

It is a process in which when a substance is placed in a hypotonic solution, its molecules in the solvent move inside the cell and the cell becomes swollen. This occurrence is called endosmosis.

• Exosmosis

It is a process in which when a substance is placed in a hypertonic solution, the molecules in the solvent move outside the cell and the cell becomes loose or undergo plasmolysis. This occurrence is called exosmosis.

Osmotic solutions

There are three types of osmotic solutions:

1. Isotonic solution: It is the solution having an equal concentration of solutes both inside as well as outside of the cell.
2. Hypertonic solution: In this solution, the concentration of solute is higher outside the cell membrane.
3. Hypotonic solution: In this solution, the concentration of solute is higher inside the cell membrane.

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What Is Osmotic Pressure?

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Osmotic pressure in simple terms can be explained as the minimum pressure put on the solution to prevent the flow of solvent molecules through a semipermeable membrane. However, the flow continues till the equilibrium is achieved. This flow of solvent that wants to penetrate the membrane can be stopped if some extra pressure is applied to the solution. This very pressure that helps in preventing the flow of solvent is termed osmotic pressure.

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Osmotic Pressure Equation

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The quantitative relationship between the osmotic pressure and solute concentration was first given by Dutch chemist Jacobus, expressing the following equation for osmotic pressure.

\(\pi\) = iCRT

Where,

\(\pi\)- osmotic pressure,

i - dimensionless van’t Hoff index,

C - molecular concentration of solute in the solution,

R - ideal gas constant,

T - temperature in kelvin.

Moreover, it is crucial to understand that the derived osmotic pressure equation holds only true for solutions that act the same as ideal solutions.

Note: Osmotic pressure for pure water is ‘0’ as it has ‘0’ osmotic pressure.

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Understanding Osmotic Pressure

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Osmotic pressure can be explained easily with the help of the following steps given below:

• For this assume a U-tube showing an osmotic pressure diagram,

U-tube showing Osmotic Pressure.

1. The left portion of the U-tube contains an aqueous solution, and right portion consists of pure water. 
2. Here, the pure water tries to dilute the solution by penetrating through the semipermeable membrane but the weight added by the water on the left tube caused pressure to hold osmosis.
3. This goes on till the equilibrium is maintained.
4. Now to attain osmotic pressure, increase the hydrostatic pressure on the solution side of the membrane. This then squeezes the solvent molecules closer, increasing the tendency to escape.
5. This escaping tendency of a solution can be ascended until it becomes equal to the molecules in the pure solvent.
6.  And at this very point, osmosis will cease. Osmotic pressure is the pressure required to attain osmotic equilibrium.

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Reverse Osmosis

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Reverse osmosis is a natural occurrence that happens in the opposite direction of the natural osmosis. This is basically used for extracting the major contaminants from water by pushing the water under pressure through a semipermeable membrane.

Reverse Osmosis

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Significance of Osmosis

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Osmosis plays an important role as:

• Osmosis holds the transport of nutrients and the release of metabolic waste products.
• It plays a vital role in the absorption of water from the soil and distributing it to the upper parts of the plant via xylem.
• It maintains the equilibrium in the internal environment of the living organism by maintaining the balance between water and intercellular fluid.
• It maintains cell turgidity.
• Osmosis controls cell-to-cell diffusion.
• Osmosis prevents plants from drought injury due to high osmotic pressure.
• Osmosis also prevents the partial opening of fruits and sporangia.
• Osmosis induces cell turgor which regulates movements of plants and part of it.
• Osmosis maintains the continuous water supply to plants even though there is a loss of water due to transpiration.

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Examples of Osmosis

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Osmosis plays a vital role in plants, animals, and humans.

• The assimilation of water through soil occurs due to osmosis.
• The guard cells of the plants are affected due to osmosis. When the plant cells are filled with water, the guard cells swell up and thus opening the stomata.
• If the fish from salt water or fresh water is placed in different salt concentrations, the fish dies due to the entry and exit of water in the cells of the fish.
• Humans suffering from cholera faces osmosis, as the bacteria overpopulate the intestine hence reversing the flow of absorption and thus preventing the absorption of water and leading to dehydration.
• When fingers are placed inside the water, they get wrinkled due to the flow of water inside the cell.

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Things to Remember

• The semipermeable membrane is a biological membrane that helps in the movements of certain molecules or ions through it.
• Osmosis is applied for preserving foods. As in fruits are commonly kept in honey to keep them from spoiling. Honey acts as a hypertonic solution thus sucking water out and preventing bacteria to prevent microbial metabolism.
• Osmotic pressure has a colligative (binding) property which prevents the solvent from entering the solution through a membrane barrier.
• The concentration of solute and temperature are two factors affecting osmotic pressure.
• The pressure that moves water in and out of cells is known as water potential.
• The water moves from high potential to low. Pure water has high potential thus it moves inside the cell if they have water around them.
• Plant cells absorb water by osmosis until the cell wall forces back on the cell’s content with equal pressure.

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