Water diffuses too. When a membrane lets water through but not a dissolved substance (a solute), the water does all the moving. This special case of Diffusion and Concentration Gradients is osmosis, and it decides whether a cell swells, shrinks or stays the same.
Key idea
In osmosis, water moves across a membrane toward the side with more solute (and so less free water). In a hypotonic solution a cell gains water; in a hypertonic one it loses water; in an isotonic one there is no net change.
Why it works
Solute particles hold water molecules around them, so a solution with more solute has fewer free water molecules. Water diffuses from where free water is more concentrated (less solute) to where it is less concentrated (more solute). The words compare the solution outside with the cell: hyper means more, hypo less, iso equal.
| Solution outside | Animal cell | Plant cell |
|---|---|---|
| Hypotonic (less solute): water moves in | Swells, may burst | Firm (turgid), the healthy state |
| Isotonic (equal): no net change | Normal | Limp (flaccid) |
| Hypertonic (more solute): water moves out | Shrivels | Membrane pulls off the wall (plasmolysis) |
Worked example: Name the solution
A red blood cell is about 0.9% salt inside. It is put into 3% salt water. Is the solution hypotonic, isotonic or hypertonic, and what happens?
- Compare outside with inside: $3\% > 0.9\%$, so the solution has more solute.
- More solute outside: hypertonic.
- Water moves out toward the extra solute, and the cell shrivels.
Answer: Hypertonic; the cell loses water and shrivels.
Worked example: Potato data
Potato cubes gain 6% mass in 0.1 M sugar and lose 4% in 0.4 M. Between which concentrations is the solution isotonic to the potato cells, and about where?
- Gaining mass means water went in: 0.1 M is hypotonic. Losing mass means 0.4 M is hypertonic.
- Isotonic is where the change is zero, between them.
- On a straight line from +6% to −4%, zero lies $\frac{6}{10}$ of the way: $0.1 + 0.6 \times 0.3 = 0.28$ M.
Answer: About $0.28$ M
Common mistake
Thinking the solute moves instead of the water, or mixing up hyper and hypo. In osmosis the solute stays put; water moves toward it. To keep the words straight, compare the solution to the cell: hyper means more solute outside, so water leaves the cell. A memory aid: in a hypotonic solution the cell swells up like an "o".