All Formulas for NEET Water Potential Questions
Core Equation:
Ψw = Ψs + Ψp
Van't Hoff Equation (osmotic potential from concentration):
Ψs = -iCRT
Where:
i = van't Hoff factor (ionisation coefficient)
C = molar concentration $\frac{mol}{L}$
R = gas constant (0.0831 L·bar/mol·K or 8.314 J/mol·K)
T = temperature in Kelvin (°C + 273)
Example Calculation: For 0.1 M NaCl (i = 2, T = 25°C = 298 K): Ψs = -2 × 0.1 × 0.0831 × 298 = -4.95 bars ≈ -5 bars
Key Reference States:
| State | Ψp | Ψs | Ψw |
|---|---|---|---|
| Pure water | 0 | 0 | 0 |
| Flaccid cell | 0 | Ψs | Ψs (= incipient plasmolysis) |
| Turgid cell | Positive | Negative | Ψs + Ψp (> Ψs) |
| Fully turgid | Maximum | Ψs | 0 (if in pure water) |
| Wilted/plasmolysed | 0 or negative | Negative | Very negative |
Direction of Water Flow Rule:
Water flows: Higher Ψw → Lower Ψw
(From less negative to more negative)
Pressure Bomb Formula (for measuring xylem tension):
Ψxylem ≈ -$P_{applied}$ (where P is the pressure needed to push sap back to cut surface)