Water Potential as the Potential Energy of Water per Unit Volume
- Imagine standing at the top of a hill, holding a ball.
- The ball has potential energy because of its position, and if you let go, it rolls downhill.
- Now, think of water molecules.
- They, too, have potential energy, but instead of gravity, their movement is influenced by water potential ($\Psi$).
Definition
Water potential
Water potential ($\Psi$) is a measure of the potential energy of water per unit volume, determining the direction water will move.
Measure The Absolute Potential Energy Of Water Is Impossible
- Potential energy is relative.
- So, like any other form of potential energy, a reference point is used: pure water at standard atmospheric pressure and 20°C.
- This is assigned a water potential of zero ($\Psi = 0$ kPa).
Water potential is measured in kilopascals (kPa)or megapascals (MPa).
Potential Energy Is a Function of Differences
- Essentially, potential energy is meaningful only when describing differences or gradients, such as:
- Water flowing from a high to a low elevation (gravitational potential difference).
- Water moving across a semi-permeable membrane (osmotic potential difference).
- These gradients, rather than absolute values, drive processes like water movement in plants or cells.
