Water Potential and Osmoregulation

Water Potential Fundamentals
  • Definition: Measures the tendency of water to move by osmosis.
  • Components: Calculated from pressure potential (Ψ<em>p\Psi<em>p) and solute potential (Ψ</em>s\Psi</em>s): Ψ=Ψ<em>p+Ψ</em>s\Psi = \Psi<em>p + \Psi</em>s.
  • Representation: Greek letter psi (Ψ\Psi).
  • Units: Bars.
  • Values: Can be positive, zero, or negative.
  • Movement: Water moves from areas of higher water potential (less negative) to areas of lower water potential (more negative).
  • Pure Water: Has a water potential of 00 bars in an open container.
  • Open System: If open to the atmosphere, pressure potential (Ψ<em>p\Psi<em>p) is 00, so Ψ=Ψ</em>s\Psi = \Psi</em>s.
Solute Potential Calculation
  • Formula: Ψs=−ICRT\Psi_s = -ICRT
    • II (Ionization Constant): For sucrose, I=1I=1. For salt, I=2I=2.
    • CC (Molar Concentration): Moles of solute per volume of solution.
    • RR (Pressure Constant): R = 0.0831 \text{ L \cdot bars/mole \cdot K} .
    • TT (Temperature): In Kelvin (T(K)=T(C)+273T(K) = T(C) + 273).
  • Impact of Solutes: The addition of solutes results in a more negative solute potential.
Osmoregulation
  • Purpose: Allows organisms to control their internal solute composition and water potential to maintain water balance.
Example Calculation: Water Potential
  • Problem: Water potential of a 0.50.5 M sucrose solution at 21∘C21^\circ \text{C} in an open system.
  • Step 1: Calculate Solute Potential (Ψs\Psi_s)
    • I=1I=1 (sucrose), C=0.5 MC=0.5 \text{ M}, R=0.0831 \text{ L \cdot bars/mole \cdot K}, T=21∘C+273=294 KT = 21^\circ \text{C} + 273 = 294 \text{ K}.
    • Ψs=−(1)(0.5)(0.0831)(294)=−12.22 bars\Psi_s = -(1)(0.5)(0.0831)(294) = -12.22 \text{ bars}.
  • Step 2: Determine Pressure Potential (Ψp\Psi_p)
    • In an open system, Ψp=0 bars\Psi_p = 0 \text{ bars}.
  • Step 3: Calculate Water Potential (Ψ\Psi)
    • Ψ=Ψ<em>p+Ψ</em>s=0+(−12.22)=−12.22 bars\Psi = \Psi<em>p + \Psi</em>s = 0 + (-12.22) = -12.22 \text{ bars}.