Osmosis vs Diffusion – Definition and Examples


Diffusion is the movement of molecules from higher to lower concentration. In osmosis, only the solvent (water) is free to move across a semipermeable membrane from higher to lower concentration.
Diffusion is the movement of molecules from higher to lower concentration. In osmosis, only the solvent (water) is free to move across a semipermeable membrane from higher to lower concentration. (images: JrPol, Llywelyn2000)

Osmosis and diffusion are two important types of mass transport. Here are the definitions of osmosis and diffusion, examples of each process, and a look at the differences between them.

Osmosis and Diffusion Definitions

Osmosis – Osmosis is the movement of solvent particles (usually water) across a semipermeable membrane from a dilute solution to a concentrated solution. The solvent dilutes the concentrated solution until concentration is equalized on both sides of the membrane.

Diffusion – Diffusion is the movement of solvent and solute particles from an area of higher concentration to lower concentration. At equilibrium, the net effect is a homogeneous concentration throughout the medium.

Osmosis and Diffusion Examples

Osmosis Examples: Soaking gummy bear candies is an easy osmosis demonstration. The gelatin in the candies acts as a semipermeable membrane. A good biology example is the swelling of red blood cells when they are placed in fresh water and their shrinkage (crenation) when they are placed in salt water. Plant root hairs uptaking water is another example of osmosis.

Diffusion Examples: A good example of diffusion is the way perfume fills an entire room. Another example is the movement of small molecules and ions across the cell membrane. Dropping food coloring into water is an example of diffusion. Other processes occur, but diffusion is the main transport method.

Comparing Similarities

There are similarities between osmosis and diffusion:

  • Both osmosis and diffusion are passive transport processes. In other words, they are spontaneous and there is no energy required for them to occur.
  • Both processes equalize the concentration of the solution.

Osmosis vs Diffusion

There are key differences between osmosis and diffusion:

  • Osmosis only occurs across a semipermeable membrane, while diffusion can occur in any mixture (including across a semipermeable membrane).
  • In biology, osmosis only refers to the movement of water. In chemistry, solvents other than water may move. Diffusion has the same meaning in both disciplines.
  • In osmosis, only the solvent is free to move across the membrane. In diffusion, both solvent and solute particles are free to move.

Key Points

  • Both osmosis and diffusion are passive transport processes that equalize concentration. In other words, no energy needs to be supplied to the system for them to occur.
  • In diffusion, particles move from higher concentration to lower concentration until equilibrium is reached.
  • In osmosis, there is a semipermeable membrane that only allows the solvent particles to move. Solvent (usually water) moves until equilibrium is reached.
  • Don’t be confused by concentration in osmosis. Remember, the only way to equalize concentration on both sides of the barrier is for water to move. The solvent molecules move from lower solvent concentration to higher solvent concentration, but they move from higher solute concentration to lower solute concentration. The concentration of the solution is what’s important.
  • Osmosis may be considered a special case of diffusion involving a semipermeable membrane where which only the solvent moves.
DiffusionOsmosis
Solvent and solute move 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 diffused molecules equalize 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.
Comparing and contrasting diffusion and osmosis

Other Types of Transport Processes

There are different types of diffusion. The type of diffusion contrasted with osmosis is simple diffusion. Active diffusion and facilitated transport are other mass transport processes.

  • Simple diffusion – Particles move from higher to lower concentration until concentration is homogeneous.
  • Active transport – Solute particles move from lower to higher concentration. In biology, enzymes (proteins) carry solutes across a membrane and energy (ATP) is required.
  • Facilitated transport – Solutes move from higher to lower concentration across a membrane, assisted by transmembrane proteins. This is a way for large molecules to cross a semipermeable membrane. It is a passive transport process, like simple diffusion.

There are also different types of osmosis, all involving a semipermeable membrane:

  • Regular osmosis – Solvent particles move from higher to lower concentration (low to high solute concentration) until equilibrium is reached.
  • Reverse osmosis (RO) – In a solution of solutes and water, hydraulic pressure forces water across the semipermeable membrane. The result increases solute concentration on one side of the membrane, while adding purified water to the other side. Because solvent moves against the concentration gradient, energy (pressure) is required.
  • Forward osmosis (FO) – Osmotic pressure draws water across a semipermeable membrane, separating it from solutes. Both reverse osmosis and forward osmosis require energy (pressure), but reverses osmosis pushes water, while forward osmosis draws or “pulls” it. Also, reverse osmosis can generate fresh drinking water, while the product of forward osmosis contains solute particles. For example, sugar and salt can be used to draw water from impure water, producing an emergency drink.

Diffusion vs Effusion

Effusion is a type of mass transport process seen in gases. Diffusion allows gas particles to freely disperse through their container. Effusion is transport of gas molecules through pores that are smaller than their average mean free path (distance between particle collisions). Effusion occurs more slowly than diffusion. The rate of effusion is inversely proportional to the square root of the mass of the particles, according to Graham’s law.

References

  • Glater, J. (1998). “The early history of reverse osmosis membrane development.” Desalination. 117 (1–3): 297–309. doi:10.1016/S0011-9164(98)00122-2
  • Haynie, Donald T. (2001). Biological Thermodynamics. Cambridge: Cambridge University Press. pp. 130–136. ISBN 978-0-521-79549-4.
  • Kramer, Eric; David Myers (2012). “Five popular misconceptions of osmosis.” American Journal of Physics. 80 (694): 694–699. doi:10.1119/1.4722325
  • Landau, L.D.; Lifshitz, E.M. (1980). Statistical Physics (3rd ed). Vol. 5. Butterworth-Heinemann. ISBN 978-0-7506-3372-7.
  • Muir, D. C. F. (1966). “Bulk flow and diffusion in the airways of the lung.” British Journal of Diseases of the Chest. 60 (4): 169–176. doi:10.1016/S0007-0971(66)80044-X

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