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Calculate ψ = ψs + ψp, solute potential with ψs = −iCRT, predict water movement, and analyse practical calibration curves — step-by-step for GCSE & A-Level Biology.
Solve for any variable in the water potential equation
Quick Examples
Total = solute + pressure potential
i=ionization, C=molarity, R=8.314, T=Kelvin
Always convert to Kelvin first
For practical data analysis
Water potential (ψ) measures the tendency of water to move from one area to another. It is measured in pressure units, typically kilopascals (kPa). The key principle is simple: water always moves from higher ψ (less negative) to lower ψ (more negative) by osmosis across partially permeable membranes.
Pure water has a water potential of 0 kPa — this is the highest possible value. Adding any solute makes ψ negative. The more concentrated the solution, the more negative the water potential becomes.
Water potential has two components: ψ = ψs + ψp, where ψs is the solute potential (always ≤ 0) and ψp is the pressure potential (positive in turgid plant cells due to the cell wall pushing back).
ψ = ψs + ψp
Total = solute potential + pressure potential
ψs = −iCRT
i=ionization, C=molarity, R=8.314, T=Kelvin
T(K) = T(°C) + 273
Always convert to Kelvin before calculating
% = (mf − mi) / mi × 100
For practical calibration curve data
The cell has absorbed water by osmosis. The cell wall pushes back, creating positive pressure potential. The cell is firm and swollen. ψ is less negative than ψs because ψp adds a positive value. Important for plant support — turgor pressure keeps plants upright.
No turgor pressure. The cell has lost water until ψ = ψs. This is incipient plasmolysis — the point where the cell membrane just starts to pull away from the cell wall. Exam shortcut: at incipient plasmolysis, ψ = ψs (since ψp = 0).
The cell membrane has pulled away from the cell wall due to extreme water loss. The cell is in a very concentrated (hypertonic) solution. This state is rarely tested in GCSE but appears in A-Level questions. The cell can often recover if placed back in pure water.
Water moves by osmosis from regions of higher water potential (less negative) to regions of lower water potential (more negative) across partially permeable membranes.
Higher ψ (less negative) → Lower ψ (more negative)
e.g. −200 kPa → −600 kPa (water moves towards the more negative value)
Common confusion: students often say “water moves from low to high concentration”. While true for solute concentration, it is the opposite for water potential — water moves from HIGH ψ to LOW ψ. Remember: “more negative” means LOWER, not higher!
ψs = −iCRT
i
Ionization constant
i = 1 for sucrose/glucose (don't ionize). i = 2 for NaCl (splits into Na⁺ + Cl⁻). Doubles the osmotic effect.
C
Molar concentration
In mol/L (moles per litre). Higher concentration = more negative ψs.
R
Gas constant
R = 8.314 kPa·L/(mol·K). If working in bar, use R = 0.0831.
T
Temperature in Kelvin
T(K) = T(°C) + 273. Always convert before calculating!
The negative sign ensures ψs is always ≤ 0. Example: 0.4M sucrose at 25°C → ψs = −(1)(0.4)(8.314)(298) = −990.5 kPa.
AQA Required Practical 3 asks you to use a calibration curve to determine the water potential of plant tissue (usually potato). Here's the method:
A plant cell has ψs = −450 kPa and ψp = 200 kPa.
ψ = ψs + ψp
ψ = (−450) + (200)
ψ = −250 kPa
Cell state: Turgid (ψp > 0)
Calculate ψs for 0.4M sucrose at 25°C.
T = 25 + 273 = 298 K
ψs = −iCRT = −(1)(0.4)(8.314)(298)
ψs = −990.5 kPa
i = 1 for sucrose (does not ionize)
Cell A: ψ = −200 kPa. Cell B: ψ = −600 kPa.
−200 > −600
Cell A has higher ψ (less negative)
Water moves A → B
Higher ψ → Lower ψ by osmosis
Potato data shows % change crosses zero at 0.4M.
Isotonic: 0.4M at 20°C
T = 293 K
ψ = −(1)(0.4)(8.314)(293)
ψ = −974.0 kPa
Forgetting the negative sign
ψs = −iCRT. The minus is essential! Without it, you get a positive ψs, which is impossible.
Using °C instead of Kelvin
Always convert to Kelvin (°C + 273) before substituting into ψs = −iCRT.
Water moves low → high
Water moves from HIGHER ψ to LOWER ψ. Less negative = higher. This is the opposite of solute concentration.
Wrong i value for NaCl
NaCl dissociates into Na⁺ and Cl⁻, so i = 2 (not 1). This doubles the osmotic effect.
Confusing "more negative" with "higher"
−600 kPa is MORE negative but LOWER water potential than −200 kPa. −200 is the higher value.
Not converting units
kPa, MPa, and bar are different units. 1 MPa = 1000 kPa. 1 bar ≈ 100 kPa. Check your exam board.
Water potential (ψ) measures the tendency of water to move from one area to another. Measured in kPa. Pure water = 0 kPa. Adding solute makes it negative. Water moves from higher ψ (less negative) to lower ψ (more negative).
ψ = ψs + ψp, where ψs is solute potential (always ≤ 0) and ψp is pressure potential. To calculate ψs from concentration: ψs = −iCRT.
Dissolved solutes reduce the free energy of water molecules. The formula ψs = −iCRT always produces a negative result when concentration > 0. Pure water has ψs = 0.
ψp = 0, so ψ = ψs. The cell membrane begins to pull away from the cell wall. This is a key exam shortcut.
ψs = −iCRT. Where i = ionization constant (1 for sucrose, 2 for NaCl), C = mol/L, R = 8.314, T = Kelvin.
From HIGHER ψ (less negative) to LOWER ψ (more negative) by osmosis. e.g. −200 → −600.
A graph of % mass change vs concentration. Where the line crosses zero = isotonic point. Use that concentration in ψs = −iCRT to find tissue water potential.
1 MPa = 1000 kPa. 1 bar ≈ 100 kPa. Use R = 8.314 for kPa, R = 0.0831 for bar.
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