Water Potential & Pressure Potential | Biology A-Level - A Level PDF Download

WATER POTENTIAL OR ψ_w

The difference between the free energy of molecules of pure water and free energy of the solution is called water potential of the system.

• Now a days according to concept of free energy and thermodynamics DPD of a solution is also represented by water potential. (Given by Taylor and Slatyer.)

• The water potential of pure water is maximum the pure water has greater free energy. The free energy , lower down by addition of solute.

• Water always flows from higher water potential to lower water potential.

• Water potential is represented by Greek word ψ(Psi)/ψ and it is measured in bars or Pascal (Pa). Water potential is equal to DPD, but opposite in sign.  Its value is negative.

• Water potential has following components :                                                                                    1. Osmotic potential  (ψ) :

• Osmotic potential or solute potential represents the concentration of the solutes. Water potential (ψ_w) is negative in the presence of solutes. So that osmotic potential is also negative.

• According to thermodynamics, osmotic pressure is called solute potential or osmotic potential. It is represented by Ys and shown by negative sign (- ve) or it is better to say osmotic potential on the basis of free energy.
  
  

• Osmotic potential or solute potential measured in bars. [1 Bar = 0.987 atmospheric pressure].

• OP = 22.4 atm => osmotic potential = - 22.4 atm. (1 M glucose solution)

2. Pressure potential (ψ_P) : 

• Turgor pressure is known as pressure potential. It is shown by positive sign (+ve).

• 1 Bar = 10⁶ dynes/sq. cm. or 0.987 atm. (1 megapascal = 10 bars)

• According to this concept their relation is as follows.

Water potential = Osmotic potential + pressure potential + matric potential.

• According to the above concept the relation of the three phases of the cell by the water potential will be as follows:-                                                                                                                In case of fully turgid cell –

• There is no flow of water in a turgid cell, because the cell is in equilibrium condition with water which is present out side the cell. So that water potential will be zero at this state, Because osmotic potential and pressure potential are equal in the cell.

• For example - if the value of osmotic potential of a cell is - 10 and pressure potential (Yp) is + 10, then water potential will be zero as -

In case of flaccid cell :-

•  Turgor pressure is zero at this stage. It means pressure potential is zero . If osmotic potential of the cell is - 10 bars then, ψ_W = ψ_S

In Plasmolysed cell :-

• The pressure potential (ψ_P) means turgor pressure is negative in this stage. Therefore water potential (ψ_w) of this cell will be more negative [more - ve].

• If the value of osmotic potential is - 10 bar of a plasmolysed cell and value of pressure potential is - 2 bars then its water potential (ψ_w) will be - 12 bars.

• So , this is the conclusion that water always move from higher water potential towards the lower water potential.

• For example if the water potential of 'A' cell is - 10 bars and water potential of 'B' cell is - 12 in two cells , then water will be flow from'A' cell to the 'B' cell.

TYPES OF SOLUTIONS[i] Isotonic solution : If solution in which a cell is placed, has equal osmotic concentration to that of cell sap, the outer solution is called isotonic solution.

[ii] Hypotonic solution : If the osmotic concentration of outer solution is lesser than that of the cell sap, the outer solution is called hypotonic solution. If a cell is placed in such solution endosmosis takes place, results , cell swells up. e.g.
Swelling of dried grape(resins).

[iii] Hypertonic solution : If the osmotic concentration of a solution is higher than that of the other (cell sap), solution is known as hypertonic solution.

If a cell placed in this type of solution , exosmosis takes place. It means water of the cell sap diffused out into the outer solution, resulting cell become flaccid. e.g. Grapes placed in higher concentration of sugar solution becomes flaccid(contracts).