What is Osmosis in a Cell in Biology? Examples, Osmosis vs Diffusion

Osmosis is part of the passive transport system that occurs in living eukaryotic cells and since antiquity; osmosis has been used to preserve foods by dehydrating them with salt or sugar. Therefore, the process of a solvent diffusing through a semi-permeable membrane is referred to as osmosis.

It can occur in any solvent, as well as gases and supercritical liquids. Therefore, let’s go into a detailed explanation of the subject.

Table of Contents


Osmosis is a process in which solvent molecules pass through a semi-permeable membrane from a low-concentration solution to a high-concentration solution.

What is osmosis in biology?

Osmosis in biology is a passive process that occurs without the use of any energy. Therefore, osmosis is the diffusion of a solvent (such as water) through a semipermeable membrane (such as that of a living cell) into a solution with a higher solute concentration.

Before we go any further, the difference between osmosis and diffusion needs to be clearly understood.

Osmosis vs Diffusion

It can only be used in a liquid medium.
It can be found in liquids, gases, and even solids.
A semipermeable membrane is required.
It is not necessary to use a semipermeable membrane.
It is determined by the number of dissolved solute particles in the solvent.
It is contingent on the presence of other particles.
Water is required for particle movement.
Particle movement does not necessitate the use of water.
Only the solvent molecules have the ability to diffuse.
Solute and solvent molecules are both capable of diffusing.
There is only one way for particles to flow.
Particles flow in all directions at the same time.
By increasing the pressure on the solution side, the entire process can be stopped or reversed.
This is a process that is not capable of being stopped or reversed.
It only happens between solutions of the same type.
This occurs between solutions that are similar and those that are dissimilar.
Only solvent molecules are moved from one side to the other in this process.
From one side to the other, molecules are transported. It entails the transfer of all particles from one location to another.
The solvent concentration does not balance out on both sides of the membrane.
The diffusion substance’s concentration balances out to fill the available space.
It is determined by the potential of the solute.
There is no relationship between solute potential, pressure potential, or water potential.
Only water or another solvent can move from a lower-energy or lower-concentration region to a high-energy or high-concentration region.
Any substance moves from a high-energy or high-concentration area to a low-energy or low-concentration area.
Mineral and nutrient absorption are not linked.
It aids in mineral and nutrient absorption.
Table showing the differences between osmosis and diffusion

Examples of osmosis

Osmosis plays an important role in the lives of plants, animals, and humans. Osmosis aids in the absorption of water from the intestines to the blood in animal cells.

More Osmosis examples can be found below;

  1. Osmosis is the process by which water is absorbed from the soil. Because the concentration of plant roots is higher than that of soil, water flows into the roots.
  2. It has an effect on the plant’s guard cells as well, resulting in a swelling up and the opening of the stomata when the plant cells are filled with water.
  3. When a freshwater or saltwater fish is placed in water with varying salt concentrations, the fish dies as a result of water entering or exiting the fish’s cells.
  4. It is also a problem for people who have cholera. Overpopulation of bacteria in the intestines reverses the absorption flow and prevents water from being absorbed, resulting in dehydration.
  5. The flow of water inside the cells causes the fingers to become pruney when they are submerged for an extended period of time.

Types of osmotic solutions

There are three (3) different types of solutions that exist when it comes to osmosis. They are as follows;

  1. Isotonic solution
  2. Hypotonic solution
  3. Hypertonic solution

When describing whether a solution will cause water to move into or out of a cell, three terms are used: hypertonic, hypotonic, and isotonic.

When a cell is placed in a hypertonic solution, it will lose volume due to a net flow of water out of the cell. If the concentration of solutes in a solution is higher than that inside the cell, and the solutes cannot cross the membrane, the solution is hypertonic to the cell.

However, when a cell is placed in a hypotonic solution, it will gain volume due to a net flow of water into the cell. The solution is hypotonic to the cell if the solute concentration outside the cell is lower than inside the cell and the solutes cannot cross the membrane.

Furthermore, when a cell is submerged in an isotonic solution, there is no net flow of water into or out of it, and the volume of the cell remains constant. The solution is isotonic to the cell if the solute concentration outside the cell is the same as inside the cell and the solutes cannot cross the membrane.

Osmotic solutions in living systems

Before a detailed examination of how osmotic solutions are used in the living systems, it is important to define a key term known as tonicity. Tonicity means that an extracellular solution’s ability to cause water to move into or out of a cell via osmosis. Now that a clear definition is given, a deep dive can commence into their effects on a living system.

When a cell is submerged in a hypertonic solution, water escapes, and the cell shrinks, and when then there is no net water movement in an isotonic environment, so the cell size does not change, whereas water will enter a cell when it is placed in a hypotonic environment, causing it to swell.

A hypotonic extracellular solution, on the other hand, is ideal for a plant cell. The plasma membrane can only expand to the rigid cell wall’s limit, preventing the cell from bursting or lysing. Water will enter a cell until its internal pressure (turgor pressure) prevents further influx. It is critical for the plant’s health to maintain this balance of water and solutes. If a plant isn’t watered, the extracellular fluid becomes isotonic or hypertonic, causing water to escape from the cells. This causes a drop in turgor pressure, which you’ve probably noticed as wilting. The cell membrane may detach from the cell wall and constrict the cytoplasm under hypertonic conditions, a condition known as plasmolysis.

Tonicity is a concern for all living things, but especially for those without rigid cell walls who live in hypertonic or hypotonic environments. Paramecia and amoebas, for example, are protists without cell walls that may have specialized structures known as contractile vacuoles. A contractile vacuole collects and pushes out excess water from the cell, preventing the cell from lysing as it absorbs water from its hypotonic environment.

Types of osmosis

There are two types of osmosis that occur in living cells namely; endosmosis and exosmosis.

Endosmosis occurs because the surrounding solution’s solute concentration is lower than that of the cytoplasm.

Exosmosis occurs because the surrounding solution’s solute concentration is higher than that of the cytoplasm.

Differences between Endosmosis and Exosmosis

Let’s look at the differences between endosmosis and exosmosis now that we’ve learned about osmosis, endosmosis, and exosmosis. The following are the main differences between endosmosis and exosmosis:

Differences between Endosmosis and Exosmosis
The solvent is able to penetrate the cell.   (Endo = within)The solvent leaves the cell.   (Exo means “outside”)
Occurs when the osmotic pressure is low.It happens when the osmotic pressure is high.
Cells undergo endosmosis when exposed to hypotonic solutions (distilled water is hypotonic because it contains no solute).Exosmosis occurs in cells when hypertonic solutions are used (Intravenous Fluid is often hypertonic as it has many solutes).
When compared to the cytosol, the environment has a higher water potential (the watery part of cytoplasm in a cell) during endosmosis.When compared to the cytosol, the water potential of the surroundings is lower during exosmosis.
The cell swells as a result of endosmosisAs a result of exosmosis, the cell becomes smaller.
For example, when raisins are placed in ordinary water, they swell.For example, when raisins are placed in a concentrated salt solution, they shrivel.

As a result, we can see how the two types of osmosis, endosmosis, and exosmosis, differ.

Mechanism of osmosis

Osmosis is an example of diffusion where water, like many other substances, migrates from a low-concentration area to a high-concentration area. What causes water to move in the first place? Consider two full glasses of water, one containing a single teaspoon of sugar and the other containing a quarter cup of sugar, and if the total volume of the solutions in both cups is the same, which cup contains more water? The first cup has more water in it because a large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup.

The solute cannot diffuse through the membrane in this case, but water can in this system, as water has a concentration gradient, resulting in water being diffused down the concentration gradient, crossing the membrane to the less concentrated side. Water will continue to diffuse through the membrane (osmosis) until the concentration gradient of water reaches zero.

Osmotic pressure

It is the minimum pressure that must be applied to a solution in order to stop the flow of solvent molecules through a semipermeable membrane and it’s a colligative property that’s influenced by the concentration of solute particles in the solution.

Important note: The semipermeable membrane just allows solvent molecules to pass through it; solute particles are not allowed to pass through.

The osmosis process can be stopped if enough pressure is applied to the solution side of the semipermeable membrane. The osmotic pressure is the minimum amount of pressure required to stop the osmosis process.

The following formula can be used to determine osmotic pressure:



Π = osmotic pressure

I = the van’t Hoff factor

C = molar concentration of the solute in the solution

R = the universal gas constant

T = temperature.

It should be noted that Jacobus van’t Hoff, a Dutch chemist, proposed a relationship between a solution’s osmotic pressure and the molar concentration of its solute. It’s worth noting that this equation only applies to solutions that behave like ideal ones.

Application and example of osmotic pressure

Plants rely on osmotic pressure to keep their upright shape. When the plant receives enough water, its cells (which contain several salts) absorb the water and expand. Plant cells expand, increasing the pressure on their cell walls and causing them to stand upright. Whenever a plant receives insufficient water, its cells become hypertonic (they shrink due to loss of water). They wilt and lose their firm, upright posture as a result. The molecular weights of compounds can also be determined by measuring osmotic pressure.

Desalination and purification of seawater using reverse osmosis is another important application of osmotic pressure.

Osmosis in living cells

Osmosis has a different effect on the cells. In comparison to a plant cell, an animal cell will lyse when placed in a hypotonic solution, but due to the fact that the plant cell’s walls are thick, it requires more water. Hence, when placed in a hypotonic solution, the cells will not burst. In reality, a plant cell flourishes in a hypotonic solution. Only an isotonic solution can support an animal cell. Plant cells become turgid and the plant’s leaves droop in an isotonic solution.

By applying external pressure to the sides of the solute, the osmotic flow in living cells can be stopped or reversed.

Factors affecting osmosis

  • Pressure is one of the factors that affect the rate of osmosis.
  • Surface Area
  • Water potential.
  • The gradient of concentration.


As biological membranes are semi-permeable, osmosis is an important process in biological systems. Therefore, the following are some useful reasons why osmosis is important in living cells.

  • The transport of nutrients and the release of metabolic waste products are both influenced by osmosis.
  • It is in charge of absorbing water from the soil and transporting it to the plant’s upper parts via the xylem.
  • Keeps the balance between water and intercellular fluid levels in a living organism’s internal environment stable.
  • It keeps the turgidity of cells in check.
  • It’s a method for plants to maintain their water content despite constant water loss from transpiration.
  • This process regulates water diffusion from cell to cell.
  • It causes cell turgor, which controls plant and plants part movement.
  • Fruit and sporangia dehiscence is also controlled by osmosis.
  • Plants are protected from drought injury by higher osmotic pressure.

What is reverse osmosis?

Reverse osmosis is a type of filtration that employs a porous, semi-permeable membrane that allows only pure water to pass through while filtering larger molecules and impurities. It is a process in which pressure is applied to overcome the colligative property and osmotic pressure of a solvent, which is controlled by a thermodynamic parameter and a chemical difference.
This application is primarily used in water plants and industries to produce potable water. It occurs when a pure solvent is allowed to follow one end of a membrane, allowing a solute to remain on the membrane’s permissible side. Reverse osmosis removes bacteria and other suspended and dissolved species from water.

Reverse osmosis principles

The principle of osmosis is reversed in reverse osmosis. For instance, a salt solution is pressed against the semi-permeable membrane under pressure and the applied pressure is higher than the osmotic pressure in this case. As a result, the molecules transition from a highly concentrated to a less concentrated solution.


Diffusion is known to be the movement of molecules from a region of higher concentration to a region of lower concentration. There is a net movement, which means that more molecules are moving in one direction than in the other. On the other hand, in osmosis water molecules and a concentration gradient pass through a semipermeable membrane, allowing water to pass through but blocking the passage of ions and other larger molecules such as sodium, chlorine, bacteria, glucose, and so on.

Therefore, the process of removing ions, mineral chemicals, and other impurities from drinking water is known as reverse osmosis. The water is forced to travel through the semipermeable membrane in the opposite direction of natural osmosis in this process, which uses more pressure.

Osmosis works in the same way as reverse osmosis but in the opposite direction. The direction of water flow is reversed in this process by applying more pressure.

Consider a semipermeable membrane that separates freshwater from a concentrated aqueous solution. Freshwater crosses the semipermeable membrane in natural osmosis and dilutes the concentrated solution. Pressure is applied to the concentrated aqueous solution in reverse osmosis, forcing the water molecules to cross the membrane and into freshwater.

Reverse osmosis (RO) water filters

From feedwater, reverse osmosis removes 99 percent of dissolved salts, colloids, bacteria, and pyrogens. The RO membrane separates the contaminants based on their size and charge. The smaller the contaminant’s charge, the more likely it is to pass through the RO membrane. Sodium and calcium, for example, are monovalent and divalent, respectively. They (RO) can easily pass through the membrane due to their smaller charges. Because carbon dioxide and other gases are not highly ionized, RO cannot remove them from water.

Reverse osmosis system

A number of steps are usually included in such systems:

  1. A sediment filter for particles such as rust and calcium carbonate to be trapped.
  2. A second sediment filter with smaller pores, if desired.
  3. An activated carbon filter is the next step in the system and its function is to trap organic chemicals and chlorine, which can damage and break down thin-film composite membranes.
  4. A reverse osmosis filter is a thin-film composite membrane that filters water.
  5. Alternatively, an ultraviolet lamp sterilizes any microbes that evade the reverse osmosis membrane’s filtering.
  6. If need be, a second carbon filter to capture chemicals that the reverse osmosis membrane did not remove.

A cellulose triacetate membrane is used instead of a carbon prefilter in some systems.

Differences between osmosis and Reverse osmosis

The major differences between osmosis and reverse osmosis are as follows:

Reverse osmosis (RO)
This is the process by which solvent molecules move from a low-concentration region to a higher-concentration region through a semi-permeable membrane.
Occurs when a pressure greater than the osmotic pressure is applied to a semi-permeable, the molecules of a solvent pass through from a region of higher concentration to a region of lower concentration.
It’s a completely normal process.
It’s a man-made process.
This happens along the gradient of the potential.
This occurs in the opposite direction of the potential gradient.
This can be seen during the opening of stomata and the roots’ absorption of water from the soil.
Water purification systems make use of this process.


What is the definition of osmosis?

The movement of a solvent through a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration is known as osmosis.

What are the three different types of osmotic conditions that have an impact on living cells?

Hypertonic, isotonic, and hypotonic osmotic conditions are the three types of osmotic conditions.

Is osmosis active or passive?

Osmosis is a passive transport process in which water moves from areas with low concentrations of solutes to areas with higher concentrations.

What are the various kinds of osmosis?

Endosmosis occurs when a substance is placed in a hypotonic solution, causing the solvent molecules to move inside the cell, causing the cell to become rigid.
Exosmosis occurs when a substance is immersed in a hypertonic solution, causing the solvent molecules to leave the cell and the cell to become flaccid.

What is the significance of osmosis for cells?

Osmosis is beneficial to cells for a variety of reasons. It aids in the movement of vital nutrients and electrolytes both inside and outside the cell. Osmosis is the process by which nutrients, water, and other solutes move into and out of the cell.

What is the difference between osmosis and diffusion?

Osmosis is the movement of solvents from a lower solute concentration region to a higher solute concentration region through a semi-permeable membrane. Diffusion, on the other hand, does not require a semi-permeable membrane to occur, and molecules move from a higher concentration region to a lower concentration region.

Is osmosis present in dead cells?

Yes, osmosis occurs in dead cells as well. Water moves inside a dead cell when it is placed in a hypotonic solution, causing it to bulge.

What is the primary purpose of osmosis?

By balancing the levels of water and intracellular fluids, osmosis aids in the stabilization of the organism’s internal environment. Furthermore, nutrients and minerals enter the cell via osmosis, which is required for cell survival.

What is the definition of osmotic pressure?

The minimum pressure applied to a solution to stop the flow of solvent molecules through a semipermeable membrane is known as osmotic pressure. The molar concentration of the solute particles in a solution determines the osmotic pressure of the solution and the formula for determining the osmotic pressure of a solution is Π= iCRT.

What is a semipermeable membrane, and how does it work?

A semipermeable membrane is a biological membrane that allows certain molecules or ions to pass through it.

What is reverse osmosis, and how does it work?

Natural osmosis occurs in one direction, but reverse osmosis occurs in the opposite direction. This type of osmosis removes the majority of contaminants from water by forcing it through a semi-permeable membrane under pressure.

What is forward osmosis, and how does it work?

Forward osmosis is a natural process that occurs all around us every day. It’s a type of osmosis that separates water from dissolved solutes using a semi-permeable membrane. Osmosis of this type is widely used in wastewater treatment, osmotic power generation, and other applications.

Why is osmosis important?

Osmosis is important for a cell’s survival and also needed for the germination of seeds.

What are some osmosis examples?

The following are some real-life examples of osmosis:
– After eating salty food, and you feel thirsty.
– Kidney dialysis in the excretory system.
– When resins and other seeds are soaked in water, they swell.
– The movement of saltwater across our cell membrane in an animal cell.
– Water and minerals are transported from root nodules to various parts of the plant.