1. Tonicity Isotonic Hypertonic Hypotonic
The above are comparisons and require a reference point
Example: You may be taller than a parent or taller than last year, but you’re never “just taller.”

2. Isotonic
“Iso” - equal
Water molecules move in and out of the cell at an equal rate
An isotonic solution has the same concentration of dissolved particles (ions, sugars, proteins, etc.) as the cell
Cell size remains constant

3. Hypertonic
Starting Point: Water concentration is higher inside cell than outside
As a result, water flows out of cell, causing it to shrivel and possibly die 

4. Hypotonic
Starting Point: Water concentration is higher outside of the cell than inside
As a result, water diffuses into the cell, causing it to expand and possibly burst

5. Summary

9. Active Transport
When a cell uses energy to transport substances that cannot diffuse across a membrane.
Materials move against a concentration gradient (from low concentration to high)
Energy comes from a molecule called ATP
Example: nerve cells need a higher concentration of K ions and a lower concentration of Na ions than the fluid outside the cell