D2.3.8 Water potential as the potential energy of water per unit volume (HL) Notes

Water Potential as the Potential Energy of Water per Unit Volume

1. Imagine standing at the top of a hill, holding a ball.
2. The ball has potential energy because of its position, and if you let go, it rolls downhill.
3. Now, think of water molecules.
4. They, too, have potential energy, but instead of gravity, their movement is influenced by water potential ($\Psi$).

Measure The Absolute Potential Energy Of Water Is Impossible

1. Potential energy is relative.
2. So, like any other form of potential energy, a reference point is used: pure water at standard atmospheric pressure and 20°C.
3. This is assigned a water potential of zero ($\Psi = 0$ kPa).

Potential Energy Is a Function of Differences

1. Essentially, potential energy is meaningful only when describing differences or gradients, such as:
   1. Water flowing from a high to a low elevation (gravitational potential difference).
   2. Water moving across a semi-permeable membrane (osmotic potential difference).
   3. These gradients, rather than absolute values, drive processes like water movement in plants or cells.

Water Potential Is Determined By Two Main Factors

1. Solute Concentration

1. When solutes are added to water, they reduce its potential energy.
2. This is because water molecules become attracted to the solute particles, restricting their movement.

2. Pressure

1. Pressure can increase or decrease water potential.
2. Positive pressure (e.g., in a turgid plant cell) raises water potential, while negative pressure (e.g., in xylem vessels during transpiration) lowers it.

Calculating Water Potential

The water potential of a system is the sum of its solute potential and pressure potential:

$$\Psi = \Psi_s + \Psi_p$$