Diffusion, Osmosis and Tonicity:
How Osmotic Pressure Effects Plant & Animal Cells Differently

CLASS NOTES from Science Prof Online

Illustration Showing the Diffusion of Molecules

What Is Diffusion?
Diffusion is the passive transport of molecules from an area of higher concentration to an area of lower concentration; and, surprisingly, you are very familiar with this process, whether you realize it or not.

Article Summary: Looking for a simple explanation of the potentially confusing terms 'diffusion', 'osmosis' and 'tonicity'? Well here it is.

Diffusion, Osmosis & Tonicity
Animated graphic of osmosis
Osmosis and Animal Cells
The cells of our body normally exist in an isotonic environment. When cells are placed in a hypertonic environment (higher concentration of solutes than the cell), water leaves the cell and the cell becomes shriveled. 
Diagram of Osmosis in Red Blood Cells
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Endomembrane System of a Eukaryotic Cell, Mariana Ruiz
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Instructor's Corner
When you put on perfume or aftershave, you can't just do it once and stay smelling sweet for the rest of your life (much to the relief of perfumers). The molecules of scent slowly diffuse from the area where you applied them, until so many have departed that you can no longer detect your signature scent.

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See Page 2 for homework assignment and other free teaching materials on Diffusion & Osmosis!
Illustration Depicting Diffusion
A less pleasant example is the very recognizable smell of skunk stink. If you are driving down the road towards a dead skunk, the unpleasant smell gets stronger and stronger as your car approaches the skunk. 
This is because the stinky molecules are more concentrated closer to their skunky source.

What Is Osmosis?
Osmosis is a special kind of diffusion; the diffusion of water molecules across a membrane, typically the membrane of a living cell. The environment surrounding each of our cells may contain small amounts of dissolved substances (solutes) that are equal to, less than, or greater than those found within the cell. The relationship between the concentrations of solutes on either side of the membrane is referred to as tonicity.

How Does Tonicity Relate to Osmosis?
If a cell is in a surrounding environment that's:
  • isotonic: There is no net movement of water between cell and environment. The concentration of solutes is the same on either side of the membrane.
  • hypertonic: This term refers to the side of the membrane with a higher concentration of solute.
  • hypotonic: This term refers to the side of the membrane with a lower concentration of solute.

These terms describing tonicity are dependent on the relationship between the environments on either side of the membrane, and can apply to the environment inside the cell or the environment outside the cell. The key to understanding osmosis and tonicity is to remember that water will always move toward a hypertonic environment!

Conversely, when animal cells are placed in a hypotonic solution (lower concentration of solutes than the cell), water moves into the cell; it swells and may explode.