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.
Combining Solute and Pressure Potentials
- The water potential of a cell is the sum of its solute potential and pressure potential:
$$\psi_w = \psi_s + \psi_p$$
NoteIn a fully turgid cell, the pressure potential ($\psi_p$) balances the solute potential ($\psi_s$), making the water potential ($\psi_w$) equal to zero.
2. Hypertonic Solutions
- In a hypertonic solution (lower water potential than the cell), water moves out of the cell.
- This reduces the pressure potential, and the cell becomes flaccid or even plasmolysed (where the cell membrane pulls away from the cell wall).
Don't confuse solute potential and pressure potential. Solute potential is always negative or zero, while pressure potential is usually positive in living cells.
Negative Pressure Potentials in Xylem
- While pressure potential is generally positive in living cells, it can be negative in xylem vessels.
- This occurs because water in the xylem is under tension as it is pulled upward by transpiration.
- In xylem vessels, the pressure potential might be $-0.5 , \text{MPa}$, reflecting the tension in the water column.
- This negative pressure potential helps draw water up from the roots to the leaves.
Remember: Negative pressure potentials are rare and occur mainly in xylem vessels during water transport.
Theory of Knowledge- How might the concept of water potential connect to broader ideas in biology, such as energy gradients or homeostasis?
- Consider how these principles apply to other systems, like the movement of ions in neurons.
