IB Biology D2.3 Notes

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:

1. Solute concentration - defined as the solute potential (ψ_s).
2. Water pressure - defined as the pressure potential (ψ_p).

These are related to one another via the formula:

$\psi = \psi_{s} + \psi_{p}$