What Is the Difference Between Osmosis and Diffusion?

You can use gummy candy to demonstrate how osmosis works.
You can use gummy candy to demonstrate how osmosis works. Water travels from an area of high water density through the gelatin to an area of low water density, swelling the candy. Martin Leigh, Getty Images

Question: What is the difference between osmosis and diffusion? Compare and contrast the two forms of transport.

Answer: 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.

Diffusion can occur in any mixture, including one which includes a semipermeable membrane, while osmosis always occurs across a semipermeable membrane.

One big difference between osmosis and diffusion is that both solvent and solute particles are free to move in diffusion, but when we talk about osmosis, only the solvent molecules (water molecules) cross the membrane. This can be confusing to understand 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 (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.

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.

Examples of Diffusion: perfume filling a whole room, a drop of food coloring spreading out to uniformly color a cup of water, movement of small molecules across a cell membrane

Examples of Osmosis: red blood cells swelling up when exposed to fresh water, plant root hairs uptake water via osmosis

Diffusion Versus Osmosis

Any type of substance moves from area of highest energy or concentration to 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 only occurs in a liquid medium.Diffusion does not require a semipermeable membrane.Osmosis requires a semipermeable membrane.Concentration of the diffusion substance equalizes to fill the available space.Concentration of solvent does not become equal on both sides of the membrane.Hydrostatic pressure and turgor pressure to not normally apply to diffusion.Hydrostatic pressure and turgor pressure oppose osmosis.Does not depend on solute potential, pressure potential, or water potential.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 also 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).

 

See Examples of Diffusion