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.
When you put on perfume or aftershave, you can’t just do it once and be smelling sweet for the rest of your life (much to the relief of perfumiers). 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.
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.
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 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 water on either side of the membrane is referred to as tonicity.
Tonicity and Osmosis
If a cell is in a surrounding environment that is:
- isotonic: no net movement of water between cell and environment
- hypertonic: a higher concentration of solute
- hypotonic: a lower concentration of solute
These terms 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!
Osmosis and Animal Cells
The cells of our body normally exist in an isotonic environment. When living cells are placed in a hypertonic environment (higher concentration of solutes than the cell), water leaves the cell and the cell becomes shriveled. Conversely, when living cells are placed in a hypotonic solution (lower concentration of solutes than the cell), water moves into the cell and it swells and may explode.
This is why a nurse can’t give someone an IV drip of pure water. The patient’s cells would explode. This is also why, if stranded at sea, we are cautioned not to drink the ocean water, no matter how thirsty. Drinking salt water actually robs the body of hydration, because it creates a hypertonic environment in the GI tract, which pulls water out of our cell, dehydrating the body.
Osmosis and Plant Cells
Plant cells are subjected to osmostic pressure, just as animal cells are. However plant cells are surrounded by a strong, rigid cell wall which prevents the cell from taking on too much water and exploding.
When plant cells are exposed to water, the water moves into the cells, making them plump (turgid). This is why house plants look healthy and firm when they are watered sufficiently.
In contrast, plasmolysis, the loss of water by plant cells can occur when plants are not sufficiently watered or are surrounded by a hypertonic environment. When water leaves the plant cell, it is not as plump, although the structure of the cell wall prevents the plasmolyzed cell from losing shape entirely.
Exposure to extremely hypertonic environments can kill a plant cell. Osmosis is how your dog creates spots of dead lawn where he pees. The urine is hypertonic to the interior of the plant cell, and large amounts of water are drawn out of the grass, killing it.
More Information on Osmosis and Diffusion
For more information, see the online tutorial on osmosis and diffusion, or the SPO Virtual Cell Biology PowerPoint Lecture on Cell Theory, Membranes & Organelles.
