I taught a lab today on osmosis and diffusion. Although I've been teaching for two years now, this is only the second time I've taught this material, and I never really understood it in the first place. So, here I am, trying to keep things straight when I have no clue what I'm talking about, and I'm discovering that my students know more than I do. So, here's what I learned from them, as well as what they learned from me:

Hypotonic is a low concentration of solute. (mnemonic device!)
Alternately, hypotonic solutions POP the cell, but you have to remember that the solution is OUTSIDE the cell.
Plants maintain hypertonicity in the cells (hypotonicity in the environment) in order to maintain the turgor pressure that keeps them standing straight up.
Animals maintain isotonicity between the cells and the environment in order to allow for the fluid movement of their bodies.

The vacuoles of a plant cell respond to iso- or hypertonicity in the environment by releasing the water in them, thus shrinking the vacuole and causing plasmolysis. This event is reversible, to a point, by re-introducing the vacuole into distilled water. (Complete dehydration will kill the cell.)

Red blood cells in a mammal are isotonic with their environment (blood). Introducing hypotonicity into the environment (the cell is now hypertonic) results in a net influx of water into the cell; thus, the cell bursts (or pops, as above). Conversely, introducing hypertonicity into the environment (the cell is now hypotonic) results in a net outflow of water from the cell; thus, the cell shrivels. Returning the solute to isotonicity will reverse the effects.

There, you've just learned the entirety of lab 13 from General Biology 101's summer session class at Rutgers University.

Don't you feel special?

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