Water Potential and Osmosis
Use water potential, tonicity, and membrane permeability to predict water movement and its effects on animal, plant, and protist cells.
Water movement across membranes is best predicted using water potential: water moves spontaneously from a region of higher water potential to a region of lower water potential when a pathway is available.
Psi = Psi_s + Psi_pSolute potential (Psi_s) becomes more negative as solute concentration increases. Pressure potential (Psi_p) represents physical pressure on water and is especially important in plant cells.
Osmosis
The net movement of water across a selectively permeable membrane from higher water potential to lower water potential.
Tonicity
Tonicity compares the effective concentration of non-penetrating solutes on two sides of a membrane. A hypertonic solution has greater effective solute concentration than the comparison solution; a hypotonic solution has less; isotonic solutions produce no net osmotic change in cell volume.
| External solution | Animal cell | Plant cell |
|---|---|---|
| Hypotonic | Water enters; the cell swells and may lyse if the imbalance is extreme. | Water enters; the vacuole expands and turgor pressure rises. The wall resists further expansion, producing a turgid cell. |
| Isotonic | No net long-term volume change. | Low turgor; cell is typically flaccid. |
| Hypertonic | Water leaves; the cell shrinks/crenates. | Water leaves; the protoplast can pull away from the wall (plasmolysis). |
Turgor pressure
As water enters a plant cell, the plasma membrane and cytoplasm press against the cell wall. The wall pushes back, increasing pressure potential. At equilibrium the water-potential difference is balanced and there is no net water movement.
Contractile vacuoles
Many freshwater protists continuously gain water by osmosis because their surroundings are very dilute. A contractile vacuole collects excess water and periodically expels it, helping maintain cell volume and osmotic balance.
Tonicity and water potential are related but not identical ideas. Water potential includes both solute effects and pressure, so it is the more complete framework for predicting water movement in plant tissues.