What is Water Potential?
Water potential (Ψ) is a measure of the potential energy of water in a system compared to pure water at atmospheric pressure. It determines the direction of water movement in plants - water always moves from higher water potential to lower water potential.
Pure water has a water potential of 0 MPa. Dissolved solutes lower water potential (making it negative), while pressure (turgor) raises it. In most plant cells, water potential is negative because the solute concentration is higher than pure water.
Water Potential Equations
Where i = ionization constant, C = molarity, R = 0.00831 L·MPa/(mol·K), T = temperature in Kelvin
Water Potential Components
| Component | Symbol | Typical Range | Effect |
|---|---|---|---|
| Solute (Osmotic) Potential | Ψs | -0.5 to -3.0 MPa | Always negative; lowers Ψ |
| Pressure (Turgor) Potential | Ψp | 0 to +2.0 MPa | Usually positive; raises Ψ |
| Gravitational Potential | Ψg | ~0 MPa | Negligible except in tall trees |
| Matric Potential | Ψm | -0.01 to -100 MPa | Important in soil, usually ignored in cells |
Frequently Asked Questions
What does negative water potential mean?
A negative water potential means the solution has less free energy than pure water. Water will move from a region of higher (less negative) water potential to a region of lower (more negative) water potential. All solutions have negative water potential due to dissolved solutes.
When is gravitational potential important?
Gravitational potential is typically negligible at the cellular level but becomes significant in tall trees. At 10 meters height, Ψg = -0.1 MPa. For a 100-meter redwood, Ψg = -1.0 MPa at the crown, contributing significantly to the driving force for water transport.
How does water potential relate to osmosis?
Water moves by osmosis from high water potential to low water potential. A plant cell in pure water (Ψ = 0) will absorb water because the cell's water potential is negative. Water enters until turgor pressure builds enough to equalize water potentials (equilibrium).