Water Potential (DP IB Biology): Revision Note

Water Potential

• Water potential (Ψ) can be defined as follows:

The potential energy of water, per unit volume, relative to pure water

• Potential energy is the energy stored in an object due to its position in relation to other objects

• In this instance it is the energy stored in water molecules due to their position in relation to other molecules, e.g. molecules of a solute in a solution

• The unit of water potential is usually kilopascals, or kPa

• Water potential is always stated relative to pure water at atmospheric pressure and 20 °C:

  • The water potential of pure water is given a value of 0 kPa

    • Note that the water molecules in pure water do technically have potential energy, but it is impossible to determine, and this designated value of zero allows for a simple comparison with solutions

  • As solutes are added to a solution, the water potential decreases into negative values; solutions with a high solute concentration have a lower water potential

    • Energy is stored in hydrogen bonds between solute molecules and water molecules in a solution, meaning that less energy is available as potential energy

    • Water molecules in a solution with a higher solute concentration therefore have less potential energy

Water Movement & Water Potential

• Water potential describes the tendency of water molecules to move from a dilute solution to a solution with a high solute concentration

  • Solutions with a high water potential contain water molecules with a greater potential energy for movement, and therefore a greater tendency to move

  • Solutions with a low water potential contain many hydrogen bonds between water molecules and solute molecules, reducing the potential energy for movement of the water molecules, and therefore their tendency to move

• Water molecules move from an area of high water potential to an area of low water potential

  • It can also be said that:

    • Water molecules move from an area of higher potential energy to an area of lower potential energy

    • Water molecules move from an area of low solute concentration to an area of high solute concentration