Differences Between Osmosis and Diffusion

They are similar in many ways, too

Gummy candy
An example of osmosis: water traveling from an area of high water density through the gelatin to an area of low water density, swelling the candy.

Martin Leigh / Getty Images

Students are often asked to explain the similarities and differences between osmosis and diffusion or to compare and contrast the two forms of transport. To answer the question, you need to know the definitions of osmosis and diffusion and really understand what they mean.


  • Osmosis: Osmosis is the movement of solvent particles across a semipermeable membrane from a dilute solution into a concentrated solution. The solvent moves to dilute the concentrated solution and equalize the concentration on both sides of the membrane.
  • Diffusion: Diffusion is the movement of particles from an area of higher concentration to lower concentration. The overall effect is to equalize concentration throughout the medium.


  • Examples of Osmosis: Examples include red blood cells swelling up when exposed to freshwater and plant root hairs taking up water. To see an easy demonstration of osmosis, soak gummy candies in water. The gel of the candies acts as a semipermeable membrane.
  • Examples of Diffusion: Examples of diffusion include the scent of perfume filling a whole room and the movement of small molecules across a cell membrane. One of the simplest demonstrations of diffusion is adding a drop of food coloring to water. Although other transport processes do occur, diffusion is the key player.


Osmosis and diffusion are related processes that display similarities:

  • Both osmosis and diffusion equalize the concentration of two solutions.
  • Both diffusion and osmosis are passive transport processes, which means they do not require any input of extra energy to occur. In both diffusion and osmosis, particles move from an area of higher concentration to one of lower concentration.


Here's how they are different:

  • Diffusion can occur in any mixture, including one that includes a semipermeable membrane, while osmosis always occurs across a semipermeable membrane.
  • When people discuss osmosis in biology, it always refers to the movement of water. In chemistry, it's possible for other solvents to be involved. In biology, this is a difference between the two processes.
  • One big difference between osmosis and diffusion is that both solvent and solute particles are free to move in diffusion, but in osmosis, only the solvent molecules (water molecules) cross the membrane. This can be confusing because while the solvent particles are moving from higher to lower solvent concentration across the membrane, they are moving from lower to higher solute concentration, or from a more dilute solution to a region of more concentrated solution. This occurs naturally because the system seeks balance or equilibrium. If the solute particles can't cross a barrier, the only way to equalize concentration on both sides of the membrane is for the solvent particles to move in. You can consider osmosis to be a special case of diffusion in which diffusion occurs across a semipermeable membrane and only the water or other solvent moves.
Diffusion Versus Osmosis
Diffusion Osmosis
Any type of substance moves from an area of highest energy or concentration to a region of lowest energy or concentration. Only water or another solvent moves from a region of high energy or concentration to a region of lower energy or concentration.
Diffusion can occur in any medium, whether it is liquid, solid, or gas. Osmosis occurs only in a liquid medium.
Diffusion does not require a semipermeable membrane. Osmosis requires a semipermeable membrane.
The concentration of the diffusion substance equalizes to fill the available space. The concentration of the solvent does not become equal on both sides of the membrane.
Hydrostatic pressure and turgor pressure do not normally apply to diffusion. Hydrostatic pressure and turgor pressure oppose osmosis.
Diffusion does not depend on solute potential, pressure potential, or water potential. Osmosis depends on solute potential.
Diffusion mainly depends on the presence of other particles. Osmosis mainly depends on the number of solute particles dissolved in the solvent.
Diffusion is a passive process. Osmosis is a passive process.
The movement in diffusion is to equalize concentration (energy) throughout the system. The movement in osmosis seeks to equalize solvent concentration, although it does not achieve this.

Key Points

Facts to remember about diffusion and osmosis:

  • Diffusion and osmosis are both passive transport processes that act to equalize the concentration of a solution.
  • In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semipermeable membrane is present, so only the solvent molecules are free to move to equalize concentration.
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Your Citation
Helmenstine, Anne Marie, Ph.D. "Differences Between Osmosis and Diffusion." ThoughtCo, Aug. 25, 2020, thoughtco.com/difference-between-osmosis-and-diffusion-609191. Helmenstine, Anne Marie, Ph.D. (2020, August 25). Differences Between Osmosis and Diffusion. Retrieved from https://www.thoughtco.com/difference-between-osmosis-and-diffusion-609191 Helmenstine, Anne Marie, Ph.D. "Differences Between Osmosis and Diffusion." ThoughtCo. https://www.thoughtco.com/difference-between-osmosis-and-diffusion-609191 (accessed May 28, 2023).