Osmosis Definition, Process, Types, Examples, and Osmotic Pressure

Osmosis Definition

Osmosis is a biophysical process that typically occurs in biological systems where the molecules of the solvent move towards a region of high solute concentration in a semi-permeable membrane.

  • It is a type of passive transport and is directed towards a direction that equates solute concentrations across a semi-permeable membrane.
  • It is a common process occurring in most biological membranes in organisms.
  • In a biological system, the solvent is mostly water; However, osmosis can also occur in other liquids and gases.
  • Since it is a means of passive transport, it does not require any energy.
Osmosis Definition, Process, Types, Examples, and Osmotic Pressure

What is osmosis?

Osmosis is a type of proliferation that, in biology, is usually related to cells. Diffusion occurs when molecules or atoms move from a region of high concentration to a region of low concentration. Osmosis occurs when a substance crosses a semiconductor membrane to balance the concentrations of another substance. In biology, this usually occurs when a solvent such as water depends on the concentration of a solute in the form of salt. Osmosis occurs spontaneously and without any energy on the part of the cell.

The process of osmosis

  • To describe the process and mechanism of osmosis, we take two solutions separated by a semi-permeable membrane.
  • One of the solutions is pure water while the other one is a solution to solution and water.
  • According to the definition of osmosis, in such a case, the purified water moves towards the membranous solution.
  • Several descriptions have been made to understand the force of osmosis. One such theorem states that the movement of water across the membrane is caused by the concentration gradient of water in the two solutions.
  • This theorem, however, does not explain the process of reverse osmosis, which occurs against the concentration gradient with the circulation of the solvent towards a solution of low concentrations.
  • Another theorem put forward by many other scientists is the fact that solute molecules in solution attract solvent molecules across the membrane. This theorem is also not true because the size of solute molecules does not affect the motion of a solvent across the membrane.
  • Therefore, the process of osmosis is explained with the concept of chemical potential.
  • The chemical potential of pure water in a solution is different from the chemical potential of water in a solution with solute molecules.
  • The interaction between solute and water molecules reduces the pressure exerted by the water molecules in the solute solution. Consequently, water molecules in pure water exert more pressure toward the solution with lower solvent concentration.
  • This pressure results in a loss of water in the membrane. This process continues until the pressure on both sides becomes equal for equilibrium.
Osmosis Definition, Process, Types, Examples, and Osmotic Pressure

Factors affecting osmosis

Osmosis is a result of various factors, and thus the rate of osmosis is influenced by many such factors:


  • The rate of osmosis increases with an increase in system temperature.
  • This happens because the energy of the molecules also increases with the increase in temperature.
  • As the molecules become more energetic, their speed also increases, and thus the process of osmosis increases.

Concentration gradient

  • Since the concentration of solute molecules in the driving force of osmosis is necessary, any change in concentration directly affects the rate of osmosis.
  • Osmosis shoots in a situation where there is a large difference in the concentration of solutes across the membrane.
  • Since the number of solute molecules is greater in one solution than the other, the pressure is reduced by the molecules of the solvent, thus accelerating the process of osmosis.
  • Once equilibrium is maintained across the membrane, the process of osmosis stops.

Water potential / Solvent Capacity

  • The capacity of water across a semi-permeable membrane also affects the rate of osmosis.
  • As the water potential of a solution is high, water molecules can cross the membrane, as the pressure is increased by the particles.
  • Eventually, the water capacity on both sides becomes the same, creating equilibrium.
  • Once equilibrated, water continues to flow across the membrane, but it flows the same amount in both ways, thus stabilizing the solutions.

Membrane Surface Area and Thickness

  • With the increase in surface area, molecules will have more space available for their movement which in turn will increase the rate of osmosis.
  • Likewise, if the surface area decreases, the molecules will have less room to move, which will restrict their movement.
  • The rate of osmosis also decreases as the membrane thickness increases.


The pressure is an essential factor affecting the process of osmosis because it can also change the direction of osmosis.

If the pressure is exerted more than the pressure exerted by the molecules of the solvent, the direction of osmosis can change, and the molecules of the solvent begin to move toward the region with greater solvent concentration.

However, if less pressure is applied than the pressure exerted by the solvent molecules, it does not change direction but reduces the rate of osmosis.

The pressure applied in the same direction of the concentration gradient also increases the rate of osmosis.

Types of osmosis

There are some types of osmosis depending on the direction of the motion of the molecules of the solvent.

  • Reverse osmosis is a method of separation used to force a solvent through a semi-permeable membrane resulting in solute molecules on one side and solvent molecules on the other side.
  • Reverse osmosis differs from forwarding osmosis in that reverse osmosis used hydraulic pressure to force the solvent against octane pressure.
  • Forward osmosis is another variant of osmosis where the osmotic pressure gradient is used to induce the flow of water from the sample solution to separate the solutes.
  • Forward osmosis uses a draw solution with a high concentration of solutes, which removes solvent molecules from the sample solution; Thus, as a result of the separation of solutes and solvent in the sample solution.
  • Endosmosis is the movement of water in a cell that occurs when a cell is placed in a solution with a higher concentration of water than the cell.
  • Exosmosis is the movement of water from the cell that occurs when the cell is placed in a solution with a higher concentration of solutes than the cell.
  • Cells swell to size after endosmosis while cells shrink after exosmosis.

Osmotic solutions (Tonicity)

Tonicity is the ability of extracellular solutions to induce the movement of water in and out of a cell through the process of osmosis.

The tonicity of a solution is determined by the concentration of solute and solvent molecules in the solution.

Based on the tonicity of the solutions, they are called hypotonic, hypertonic, or isotonic solutions.

Hypotonic solution

  • If an extracellular solution has a lower concentration of solute than what is inside the cell, the solution is called a hypotonic solution.
  • When a cell is placed in a hypotonic solution, the movement of water occurs within the cell and causes endosmosis.
  • The cell is a condition that will swell and could even explode.

Hypertonic solution

  • If an extracellular solution has a higher concentration of solute than what is inside the cell, the solution is called a hypertonic solution.
  • When a cell is placed in a hypertonic solution, the movement of water occurs outside the cell and exosmosis occurs.
  • The cell contracts, losing the ability to divide and even function.

Isotonic solution

  • When an extracellular solution has the same concentration of solute as that inside the cell, the solution is called an isotonic solution.
  • When a cell is placed in an isotonic solution, there is no movement of water through the cell membrane.
  • In this case, the cell size is not affected since there is no movement of water.

Osmotic Pressure

  • Osmotic pressure is the pressure applied by a hypotonic pressure that results in the movement of solvent molecules through the semipermeable membrane.
  • It is the minimum pressure that must be applied to the solution to prevent the internal flow of pure solvent through the semipermeable membrane.
  • Osmotic pressure is the driving force of osmosis, and the rate of osmosis increases as osmotic pressure increases.
Osmosis Definition, Process, Types, Examples, and Osmotic Pressure

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

∏= 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

  • Osmosis is responsible for the transport of nutrients within the cell and waste materials outside the cell.
  • Osmosis influences the transport of water from the soil to the roots of plants, which is then conducted to different parts of the cell through the xylem tissue.
  • The internal environment of the cell in living organisms is stabilized by balancing water and intracellular fluid levels.
  • Osmosis is also responsible for maintaining the turgidity of the cell.
  • Osmosis in plants prevents cells from drying out as a result of water loss through perspiration.
  • The diffusion of water and other cellular fluids from cell to cell is also maintained by osmosis.
  • The movement of plants and plant parts is regulated by the turgor of the cell, which in turn is balanced by osmosis.
  • Osmosis also prevents the drying of fruits and sporangia among other plant structures.
  • An increase in osmotic pressure supports plants in desert areas against drought and other similar injuries.
  • Reverse osmosis and direct osmosis are separation methods used in drinking water purification, desalination, wastewater purification, concentration of liquid foods such as juices, production of maple syrups, low alcohol beer and hydrogen peroxide.

Examples of osmosis

In the animal cells

  • Osmosis influences the shape and size of animal cells, since there is no cell wall in animal cells.
  • Red blood cells in humans are highly influenced by the osmotic pressure of the blood. If the blood is too thin, the red blood cells decrease in size as they swell and even explode if the blood becomes concentrated.
  • Thus, in animals, the concentration of body fluids; Blood plasma and tissue fluid must be kept within strict limits.
  • Another example of osmosis in animals is the reduction of slugs by exposure to salt.
  • The skin of slugs is a semipermeable membrane that, when exposed to salt, draws water from the cells and contracts the cell and, in turn, the animal.

In the plant cells

  • The root system in plants absorbs water from the soil through osmosis.
  • Cells in the roots of plants have a semi-permeable membrane that allows water in the soil to infiltrate the roots that influence the protective cells.
  • Another classic example of osmosis in plants is the swelling and shrinkage of potato cells when potato slices are dipped in hypotonic solution and hypertonic solutions, respectively.

Osmosis Definition, Process, Types, Examples, and Osmotic Pressure


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Osmosis Definition, Process, Types, Examples, and Osmotic Pressure

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