Water Potential: The "Thirst" of Water
Imagine you have two glasses of water. One is pure water, the other has a spoonful of sugar dissolved in it. If you connect them with a straw, which way does the water move? Your intuition probably says: from the pure water into the sugar water. But why? The water molecules in the pure glass are "freer" — they have more freedom to move around. The sugar molecules get in the way, trapping some water molecules and reducing their ability to escape.
That "freedom" is what water potential measures. It is the potential energy of water per unit volume — essentially, how eager water is to move from one place to another. Water always moves from where it has higher potential (more free energy) to where it has lower potential (less free energy), until equilibrium is reached.
Ψw=Ψs+Ψp
Where:
- Ψw = water potential (measured in pressure units, typically MPa or bars)
- Ψs = solute potential (always negative or zero)
- Ψp = pressure potential (can be positive, negative, or zero)
The Reference Point: Pure Water
By convention, pure water at standard conditions (1 atm pressure, room temperature) has a water potential of zero. This is the maximum possible value. Anything that reduces the free energy of water makes its potential negative.
Think of it like altitude. Sea level is zero. Everything else is either above (positive) or below (negative). Water flows downhill in the energy landscape, from higher potential to lower potential.
The Two Components
Solute Potential (Ψs)
When you dissolve anything in water — sugar, salt, ions — the solute particles surround themselves with water molecules (hydration shells). This "ties up" some water molecules, reducing their freedom to move. The more solute particles, the lower (more negative) the solute potential.
Solute potential is always negative or zero. It can never be positive. Pure water has Ψs=0; any solution has Ψs<0.
The formula for solute potential (also called osmotic potential) is:
Ψs=−iCRT
Where i is the ionization constant (number of particles a solute dissociates into), C is molar concentration, R is the gas constant, and T is absolute temperature. For a non-electrolyte like sucrose, i=1. For NaCl, i≈2.
Pressure Potential (Ψp)
This is the physical pressure exerted on the water. In a plant cell, the cell wall pushes back against the expanding protoplast — this is turgor pressure, and it is positive. In the xylem (water-conducting vessels), tension from transpiration creates negative pressure.
Think of pressure potential as the "squeeze" factor. Positive pressure pushes water out; negative pressure pulls water in.
Putting It Together: The Direction of Flow
Water moves from higher Ψw to lower Ψw. Here is the key insight: pure water has the highest possible water potential (zero). Everything else is lower.
Consider a plant cell:
- Pure water outside: Ψw=0
- Cell interior: Ψs=−0.9 MPa, Ψp=+0.4 MPa, so Ψw=−0.5 MPa
Water moves from outside (0) into the cell (-0.5). As water enters, the cell swells, Ψp increases, and eventually Ψw inside equals Ψw outside — equilibrium. …