Question

Describe osmotic potential, pressure potential and water potential and explain their mutual relationship.

Answer 1

The hypothesis of water potential was proposed by Slatyer and Taylor (1960). According to laws of thermodynamics, each component of any system has some free energy which allows it to work. 

• During osmosis water molecules move across the semipermeable membrane and for this free energy is required. 
• When a cell is kept in pure water, there exists a difference of free energy between molecules of pure water and the free energy of water molecules present in the solution.
• This difference of free energy between the pure water and water molecules of the solution is called water potential.
• In other words, the value of water potential of a solution is equal to the difference in the value of free energy of solvent molecules of the solution and free energy of molecules of the pure solvent. 
• It is expressed by Greek symbol ψ(psi) and its unit is Bar or atm.
• It is not possible to determine the absolute value of water potential and hence its value is taken as zero. 
• Free energy of pure water (solvent) is maximum and when some solute is added to it, the free energy of solvent (water) decreases.
• Hence free energy of any solution is less than that of pure water meaning by always negative.

According to laws of thermodynamics:

• DPD of a solution is equal to the water potential but the value of DPD is positive and that of water potential is negative. 
• OP is called solute potential or osmotic potential and it is expressed as (ψ_s)
• The value of osmotic pressure and the osmotic potential is equal but the only difference is that the value of osmotic pressure is positive and that of osmotic potential is negative. 
• Its value is also negative
• TP is called pressure potential and it is expressed as  (ψ_p) .
• Value of pressure potential is positive.

Concept of an Osmotic System: 

• The concentrated sugar solution is filled in a bag made up of the semipermeable membrane. 
• It is kept in a beaker containing dilute sugar solution. 
• Water molecules from beaker begin to move to the concentrated solution in the bag through the semipermeable membrane. 
• After some time, the bag stretches due to endosmosis. 
• When the bag becomes fully stretched it creates pressure on the solution to prevent further entry of water. 
• This pressure is called turgor pressure.
• The effect of turgor pressure on water potential is called pressure potential. In a plant cell, the cell wall is referred to as a matrix.
• In a fully turgid plant cell, the pressure developed by the cell wall is called matrix potential. It is represented as .
• Although in the soil the matrix potential is important it is negligible in the cellular osmotic system.