Water Potential & Osmoregulation
Calculating Ψ and predicting the direction of water movement - a guaranteed AP math skill.
The two formulas you must know
Water potential is the sum of two terms: Ψ = Ψp + Ψs. Pure water in an open beaker at standard conditions has Ψ = 0 because both terms are zero.
Solute potential is calculated as Ψs = -iCRT, where i is the ionization constant (1 for sucrose, 2 for NaCl), C is molarity in mol/L, R is the pressure constant (0.0831 L·bar/mol·K), and T is temperature in kelvin (°C + 273).
Worked example
A 0.15 M sucrose solution at 25 °C in an open container: i = 1, C = 0.15, R = 0.0831, T = 298 K. Ψs = -(1)(0.15)(0.0831)(298) = -3.71 bars. Because the container is open, Ψp = 0, so Ψ = -3.71 bars.
If a plant cell with Ψ = -2.0 bars is placed in that solution, water moves out of the cell toward the more negative solution potential, and the cell plasmolyzes.
Predicting direction of movement
- Water moves from higher Ψ to lower (more negative) Ψ - never the reverse without energy input.
- Hypotonic environment: water enters. Plant cells become turgid; animal cells may lyse.
- Hypertonic environment: water leaves. Plant cells plasmolyze; animal cells crenate.
- Isotonic: no net movement, though molecules still cross in both directions.
Osmoregulation in living systems
Paramecium in freshwater constantly gains water and pumps it out with contractile vacuoles - an active, ATP-requiring process. Plants rely on the rigid cell wall to build turgor pressure, which supports the stem and drives leaf expansion.
In multicellular animals, kidneys maintain the solute concentration of extracellular fluid, keeping cells near isotonic and preventing lysis or shriveling.
Key terms
4
- Water potential (Ψ)
- The free energy of water per unit volume; determines the direction of water movement.
- Solute potential (Ψs)
- The component of water potential caused by dissolved solutes; always ≤ 0.
- Pressure potential (Ψp)
- The physical pressure component; positive in turgid plant cells, can be negative in xylem tension.
- Turgor pressure
- Outward pressure of the plasma membrane against the cell wall in a hypotonic environment.
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