Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. If the partition is removed as in the lower illustration, the gases will mix because of the random velocities of their molecules. In time a uniform mixture of A and B molecules will be produced in the container.
The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles.
Rate of Diffusion
Since the average kinetic energy of different types of molecules (different masses) which are at thermal equilibrium is the same, then their average velocities are different. Their average diffusion rate is expected to depend upon that average velocity, which gives a relative diffusion rate
where the constant K depends upon geometric factors including the area across which the diffusion is occuring. The relative diffusion rate for two different molecular species is then given by
If two solutions of different concentration are separated by a semi-permeable membrane which is permeable to the smaller solvent molecules but not to the larger solute molecules, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis.
Osmosis is of great importance in biological processes where the solvent is water. The transport of water and other molecules across biological membranes is essential to many processes in living organisms. The energy which drives the process is usually discussed in terms of osmotic pressure.
Osmotic pressure can be thought of as the pressure that would be required to stop water from diffusing through a barrier by osmosis. In other words, it refers to how hard the water would “push” to get through the barrier in order to diffuse to the other side.
Osmosis and Diffusion Examples
Examples of Diffusion: Examples of diffusion include perfume filling a whole room, a drop of food coloring spreading out to uniformly color a cup of water, and the movement of small molecules across a cell membrane. One of the simplest demonstrations of diffusion is adding a drop of food coloring in water. While other transport processes occur, diffusion is the key player.
Examples of Osmosis: Examples of osmosis include red blood cells swelling up when exposed to fresh water and plant root hairs uptake water via osmosis. To see an easy demonstration of osmosis, soak gummy candies in water.The gel of the candies acts as a semipermeable membrane.
Osmosis and Diffusion Similarities
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.
Osmosis and Diffusion Differences
- Diffusion can occur in any mixture, including one which 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 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.
Table Comparing 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).|
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