Water Movement in Plant Tissue (HL) (HL IB Biology)

• Water potential measures the potential energy of water compared to pure water
  • we use the symbol ψ or ψw to represent water potential 
  • Pure water has a water potential of zero at standard temperature and pressure
• Pressure potential is the hydrostatic pressure to which water in a liquid phase is subjected
  • it is also referred to as turgor potential or turgor pressure and represented by ψp

  • Pressure potentials are generally positive inside cells, although negative pressure potentials occur in xylem vessels where sap is being transported under tension

• Solute potential, also called osmotic potential, is a component of water potential and is represented by the symbol  Ψ_w 
  • Solutes reduce water potential by consuming some of the potential energy available in the water, this results in a negative solute potential
  • Solute molecules can bind to water molecules using hydrogen bonds, this allows them to dissolve in water
  • The energy in the hydrogen bonds between solute molecules and water is no longer available elsewhere which is why the water potential is reduced, in other words the potential energy that was available in the water is transferred to the hydrogen bonds
  • Therefore solute potentials can range from zero downwards as the concentration of solutes increases
• The equation ψw = ψs + ψp can be used to summarise the interactions between solute potential, water potential and pressure potential
  • Water potential is directly proportional to the solute concentration and pressure potential

Water potential diagram

The water potential is affected by the solute potential and the pressure potential

• When plant tissue is bathed in either a hypotonic or hypertonic solution some changes occur
  • In a hypotonic solution plant cells will gain water and will have a higher water potential (closer to zero)
  • Plant cells will lose water in a hypertonic solution and will have a lower water potential (more negative)
• The reason for these changes is 
  • In a hypotonic solution
    • Pressure potentials increase because there are a greater number of water molecules present in the cell and therefore more molecules to exert pressure
      • With more water molecules comes greater potential energy meaning more energy for molecules to move and and exert pressure on the cell membrane (the cell becomes turgid)
    • Solute potentials decrease (be more negative) because the number of solute molecules relative the number of water molecules is less
  • In a hypertonic solution
    • As water molecules move out of the cell the pressure potential is decreased
      • There are fewer water molecules in the plant cell and so decreased potential energy of water to move and exert pressure on the cell membrane (the cell becomes flaccid)
    • Solute potential increase because there are greater numbers of solute molecules relative to water molecules