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Osmosis and diffusion are two related concepts whose differences and similarities can sometimes cause confusion. The cause of this confusion is the fact that both processes, osmosis and diffusion, are transport mechanisms that involve the movement of chemical substances down their concentration gradient .
To clearly understand the difference between osmosis and diffusion, let’s start with the definition of each of these concepts:
Osmosis
Osmosis is the movement of solvent molecules across a semipermeable membrane from a dilute solution to a more concentrated solution . The osmosis process dilutes the more concentrated solution (since it receives more solvent) and concentrates the more dilute solution (since it loses solvent), so it stops when both concentrations become equal.
Osmosis occurs because the solvent is actually more concentrated in the dilute solution (less concentration of solute means higher concentration of solvent), so it moves towards the concentrated solution where the solvent is actually more dilute. . That is, it follows its concentration gradient.
Diffusion
Diffusion is the movement of any type of particle from an area of high concentration to an area of lower concentration . The final effect of diffusion is to equalize the concentrations in all the spaces available to the particles.
In diffusion, the particles that diffuse can be from the small droplets of an aerosol, as when we spread a perfume with an atomizer, to molecules of solute or even solvent in a solution.
Similarities Between Osmosis and Diffusion
Before we start listing the differences between osmosis and diffusion, let’s start by seeing how they are similar:
In both cases the substances follow their concentration gradient
Both in diffusion and in osmosis, the phenomenon of particle transport occurs due to the difference in concentrations in different areas of the container that contains them, and it is always in the direction of higher to lower concentration.
Both types of transport are passive
Both diffusion and osmosis are spontaneous processes, so they do not require a supply of energy for them to occur.
The effect in both cases is to equalize the concentration
As the particles move down their concentration gradient, the concentration difference decreases, so both diffusion and osmosis stop when the concentration is the same everywhere.
This is where the similarities end. Now, let’s look at the differences between these two transport mechanisms.
Differences Between Osmosis and Diffusion
| Osmosis | Diffusion |
| It only involves the transport of solvent molecules, never solutes. | Involves the transport of any type of particle, including both solutes and solvents |
| It only occurs between two liquid solutions. | It can occur in any medium, regardless of whether it is solid, liquid or gas. |
| It involves the existence of two physically separate compartments or media. | It can occur either between two separate media, as long as the disseminating species can cross the barrier that separates them, or it can occur within a single compartment. For example, within a solution, or in the air contained in a closed room. |
| There must necessarily be a semipermeable membrane (which only allows the passage of the solvent) that separates both compartments so that it can occur. | It occurs without the need for membranes or barriers of any kind. |
| Due to phenomena such as the increase in hydrostatic pressure in one of the compartments (as within a cell, for example), osmosis usually stops before the concentration on both sides of the membrane is equalized. | The substance that diffuses always ends up equalizing its concentration in all the available space. |
| Turgor and hydrostatic pressure oppose osmosis. | Hydrostatic pressure and turgor do not generally affect diffusion. |
| Osmosis depends on the total concentration of all solutes that are not capable of crossing the semipermeable membrane (osmotically active solutes). | Diffusion depends solely on the concentration of the solute or the particles that are diffusing. |
| In biology, we only talk about osmosis in terms of the movement of water across the cell membrane. Other solvents are not considered. | In biology, we only talk about diffusion in terms of the movement of solutes across the cell membrane, through the intracellular fluid, or through the extracellular fluid. The movement of water across the membrane is not considered diffusion, although osmosis is a particular type of diffusion. |
