The Equation for Water Potential
- Water potential ($\psi_w$) is the key to understanding water movement in plants.
- It's calculated using the equation:
$$\psi_w = \psi_s + \psi_p$$
Where:
- $\psi_w$ = Water potential
- $\psi_s$ = Solute potential
- $\psi_p$ = Pressure potential
Water potential is measured in kilopascals (kPa)or megapascals (MPa), with pure water at standard conditions having a water potential of 0 kPa.
Solute Potential ($\psi_s$): The Effect of Solutes
Solute potential
Solute potential ($\psi_s$) measures how solutes affect water's potential energy.
- Adding solutes lowers water potential because solute molecules bind to water, reducing its ability to move freely.
Solute potential is always negative or zero, with more concentrated solutions having lower(more negative) values.
ExampleIf a plant cell has a solute potential of $-0.8 , \text{MPa}$, it means the solutes inside the cell reduce its water potential by $0.8 , \text{MPa}$ compared to pure water.
Pressure Potential ($\psi_p$): The Role of Pressure
Pressure potential
Pressure potential ($\psi_p$) reflects the physical pressure exerted on water.
- In plant cells, this pressure comes from the cell wall pushing back against the expanding cell membrane.
Pressure potential is usually positive in living cells but can be negative in special cases, such as xylem vessels where water is under tension.
ExampleA turgid plant cell (one that is swollen with water) might have a pressure potential of $+0.5 , \text{MPa}$, meaning the cell wall exerts a positive pressure on the water inside.
