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

WATER POTENTIAL OR ψw

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.


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Which term is used to represent the difference in free energy between pure water and a solution?
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  Water Potential & Pressure Potential | Biology A-Level - A Level

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

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

  • Osmotic potential or solute potential represents the concentration of the solutes. Water potential (ψw) 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 (ψP) : 

  • 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.

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

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

  • 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 -

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

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, ψW = ψS

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

In Plasmolysed cell :-

  • The pressure potential (ψP) means turgor pressure is negative in this stage. Therefore water potential (ψw) 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 (ψw) will be - 12 bars.

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

  • 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 SOLUTIONSWater Potential & Pressure Potential | Biology A-Level - A Level[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).

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Which component of water potential represents the concentration of solutes?
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[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).

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FAQs on Water Potential & Pressure Potential - Biology A-Level - A Level

1. What is water potential and how is it related to pressure potential?
Ans. Water potential is a measurement of the tendency of water to move from one area to another. It is influenced by various factors such as solute concentration, pressure, and gravity. Pressure potential, specifically, is a component of water potential that is determined by the physical pressure exerted on the water in a system. It can be positive or negative, indicating the presence of pressure or tension, respectively.
2. How is water potential measured?
Ans. Water potential is typically measured using a device called a pressure chamber or pressure bomb. This device applies pressure to a plant leaf or stem, causing water to be forced out. The pressure at which water starts to emerge is then measured, and this value is used to calculate the water potential of the plant tissue.
3. What factors affect water potential?
Ans. Several factors influence water potential, including solute concentration, pressure, and gravity. Solute concentration, or osmotic potential, is determined by the presence of dissolved substances in the water. Higher solute concentration leads to a lower water potential. Pressure potential is influenced by physical pressure exerted on the water, such as turgor pressure in plant cells. Gravity also plays a role, as water potential decreases with increasing height.
4. How does water potential affect plant water uptake?
Ans. Water potential gradient drives the movement of water in plants. Water moves from areas of higher water potential to areas of lower water potential, following the principles of osmosis and diffusion. As a result, plants absorb water from the soil, where water potential is usually higher, and transport it upwards through the roots, stems, and leaves. The water potential of the plant cells also affects turgor pressure, which helps maintain cell shape and rigidity.
5. How does water potential relate to the concept of wilting?
Ans. Wilting occurs when plants do not have sufficient water supply to maintain turgor pressure in their cells. This happens when the water potential of the soil is lower than that of the plant's cells. As a result, water moves out of the plant cells, causing them to lose their rigidity and wilt. By understanding the water potential of the soil and the plant, it is possible to manage irrigation and prevent wilting in agricultural practices.
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