Water potential
At this point, you know about water's behavior during osmosis and how it impacts tissues. During this, water molecules are in constant motion and when they collide with the plasma membrane this creates a certain pressure on the membrane, called the water potential (ψ). This is officially defined as the potential energy of water per unit volume, measured in Pascals (Pa).
This potential energy refers to the energy required to transport a unit volume of water to a reference pool of pure free water. It can also be thought of as the tendency of water molecules to enter or exit a solution via osmosis. Note that pure water at standard atmospheric pressure (100 kPa) has a water potential of 0 Pa.
The key thing to remember is that water from areas of high water potential to areas of low water potential. To determine which area is which, we need to look at two factors:
- Solute concentration - defined as the solute potential (ψs).
- Water pressure - defined as the pressure potential (ψp).
These are related to one another via the formula:
ψ=ψs+ψp
Solute potential
Solute potential refers to the impact on water potential by the presence of solutes. Solute potential is always negative, which can be explained through various scenarios:
- Pure free water requires no energy to extract. Thus, its water potential is zero.
- A low concentration solution requires a little energy to extract water from the solutes. Thus, its water potential is slightly negative.
- A high concentration solution requires a lot of energy to extract water from the solutes. Thus, its water potential is very negative.
As you can see, the presence of solutes always requires energy to remove, resulting in a negative contribution to water potential. Additionally, osmosis can now be redefined according to water potential. Water moves from a low solute concentration to a high solute concentration can be restated as water moves from a high water potential to a low water potential!