Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Osmosis is the passive net movement of water across a selectively permeable membrane. Water moves; the solutes that set up the gradient cannot cross.
- Tonicity compares two solutions. A solution hypotonic to a cell has fewer solute particles, so water enters the cell. Hypertonic: more solute particles, water leaves. Isotonic: no net movement.
- Animal cells burst (lyse) in hypotonic and shrivel in hypertonic solutions. Plant cells become turgid in hypotonic solutions (the wall stops them bursting), flaccid in isotonic and plasmolysed in hypertonic.
- Water potential (Ψ) predicts direction: water moves from higher to lower Ψ.
- Osmoregulation keeps water and solute balance within limits: contractile vacuoles pump out water in freshwater protists; the plant central vacuole keeps cells turgid.
Key relationships
| Equation | Symbols and units |
|---|---|
| Ψ = Ψp + Ψs | all in bars; Ψp = 0 in an open container |
| Ψs = −iCRT | i = particles per unit (sucrose 1, NaCl 2, CaCl₂ 3); C in mol L⁻¹; R = 0.0831 L bar mol⁻¹ K⁻¹; T in K (°C + 273) |
| Solution compared with cell | Solute particles | Water potential | Net water flow |
|---|---|---|---|
| hypotonic | fewer | higher than cell | into cell |
| isotonic | equal | equal | none |
| hypertonic | more | lower than cell | out of cell |
Assumptions behind the numbers
- Salts are assumed to separate completely (i = 2 for NaCl), which is close enough for dilute solutions.
- In an open container Ψp = 0, so Ψ = Ψs.
- An isotonic point from mass-change data assumes the solute does not enter the cells and equilibrium was reached.
Mistakes to avoid
- Using tonicity words without saying what is being compared.
- Saying water moves toward higher water concentration and then drawing the arrow the wrong way.
- Forgetting the minus sign, forgetting i, or using °C instead of kelvin.
- Saying plant cells burst in pure water.
- Saying water stops moving in an isotonic solution (only net movement stops).
- Plotting raw mass change instead of percentage change when starting masses differ.
Quick self-check
- Ψs of 0.10 M sucrose at 27 °C? (−(1)(0.10)(0.0831)(300) = −2.49 bar)
- A cell has Ψs = −6.0 bar and Ψp = +2.5 bar. Will it gain water from the solution in question 1? (Cell Ψ = −3.5 bar, lower than −2.49 bar, so yes: water moves into the cell.)
- Why does Paramecium need a contractile vacuole but a plant cell does not? (It has no wall and lives in hypotonic water, so it must expel the water that keeps entering; a plant cell’s wall stops it bursting.)
Next: practice questions.