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Relationship between osmotic potential & pressure potential with water potential




Explain the relationship between osmotic potential & pressure potential with water potential.

·        Water potential mainly depends on concentration, pressure, and gravity. If the symbols of water potential, the effects of solutes, pressure, & gravity are denoted by Ψw, Ψs, Ψp & Ψg  , then water potential can be expressed as;

Water Potential (Ψw) = Ψs + Ψp+ Ψg 

·        In plants of small height (less than 5 meters), Ψg is negligible. So the equation becomes as;
                        Ψw = Ψs + Ψp
·        Pure water is usually defined as having osmotic potential (Ψs) of zero. As the solute is added solute potential or osmotic potential (Ψs) decreases. So, in this case solute potential can never be positive.
·        The pressure potential (turgor potential) on the other hand in living plant cell is usually positive. In plasmolysed cells & open system , Ψp = 0. Negative pressure potential occurs when water is pulled through an open system such as a plant xylem vessels.

Ø So, in the living cells,
·        If , Ψs = -ve  & Ψp = +ve (or, when pressure potential is less negative than the osmotic potential) then,  Ψw =  -ve.
·        If Ψs =  Ψp i.e., for e.g. Ψs = -1 &  Ψp = +1 ( or, when pressure potential equals to osmotic potential) then, Ψw =   0 (zero).
·        If the value of pressure potential exceeds the value of osmotic potential then, Ψw =  +ve. ( But this is not practically feasible because  it is considered that the value of water potential for pure water is zero).

Give the different parameters involved in the determination of water potential.
OR,
What are the factors involved in affecting the water potential?
·        Basically, there are three parameters involved in the determination of water potential (Ψw). They are:
1.     Solute concentration
2.     Pressure
3.     Gravity
·        Sometimes matrix potential of the system also affects the water potential.


a.     Solute concentration:
 In pure water the value of water potential is maximum i.e., it is zero. Addition of solutes reduces the free energy of water. The term Ψs is used for denoting the concentration of the solute and its effect on the water potential. It is termed solute potential or the osmotic potential.
b.    Pressure:
During osmosis the entry of water results in the development of hydrostatic or turgor pressure which is here called as pressure potential (Ψp). If the pressure potential is positive it will add to the water potential but if it is negative it reduces the value of water potential.
c.      Gravity:
The term Ψg termed gravity potential denotes the effect of gravity on the water potential of a water column in a vertically growing plant. It’s magnitude depends on the height of the plant from the ground level as well as on the density of water and the acceleration due to gravity. In plants of small height (less than 5 meters) the Ψg is negligible.

Ø Water potential is decreased by factors which reduce the relative water vapor viz., by addition of solutes, negative pressure or tensions, reduction in temperature and by matrix forces.
Ø Water potential is increased by factors which increase the negative vapor pressure, mechanical pressure and increase temperature.
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CHAPTER: 6
Concept of Water Potential


Water Potential:
•      The water content in the soil, plants and atmosphere is usually described as water potential (Ψw).
•      By definition, the potential of free pure water at atmospheric pressure and at a temperature of 25°C corresponds to 0 (zero) Mpa or bar.
•      More precisely, the water potential represents all the water pressure in a given system and it is the sum of osmotic potential (Ψs), matrix potential (Ψm), hydrostatic pressure or the turgor potential (Ψt) and the gravitational potential (Ψg).
•      Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as capillary action (which is caused by surface tension).
•      Water potential has proved especially useful in understanding water movement within plants, animals, and soil. Water potential is typically expressed in potential energy per unit volume and very often is represented by the Greek letter Psi (Ψ ) .

Ø  This  water potential depends upon the given five factors;
1) concentration of the substances      2) Pressure
3) Temperature
4) chemical potentiality of the substance in the solution
5) Matrix System
Ø  Water potential can be expressed in equation as,
             Water Potential (Ψw) = Ψs + Ψm + Ψt+ Ψg 
                        where, osmotic/solute potential (Ψs),
                                      matrix potential (Ψm) ,
                                      turgor potential (Ψt) or pressure potential                  
gravitational potential (Ψg)  (significant in tall plant.)

Importance of water potential:
•      It is an importance force which determines the water status in plant cell or organ.
•      The osmotic movement of water into or out of the system is due to Water Potential (Ψw)
•      The cells suffering from water deficit or water system are therefore able to avoid injury by obtaining water from other cell.
•      The air dried seed and spores are able to penetrate and avoid temperature (low & high) injury and drought due to very low water potential.
•      Plants use water potential to transport water to the leaves so that photosynthesis can take place.
•      Water potential is a measure of the potential energy in water as well as the difference between the potential in a given water sample and pure water.



Thermodynamical Concept of Water Potential:
•      According to thermodynamics laws every component of a system possesses free energy capable of doing work under constant temperature condition.
•      Osmotic movement of water involves certain work done and in fact the main driving forces behind this movement is the difference between free energies of water on two sides of the semi-permeable membrane.
•      For non electrolytes, free energy/mol is known as chemical potential(Ψ). With reference to water this is called as water potential(Ψw).  
•      Like other substances the absolute value of water potential cannot be determined and measured ,instead this value for pure water is arbitrarily fixed as zero at one atmosphere and a particular temperature.
•      Water potential is lowered by the addition of solutes. So, due to the zero value of water potential for pure water, all other water potential values will be negative.
•      The movement of water will takes place in osmotic or other system from a region to higher water potential (less -ve) to a region of lower water potential(More -ve).



       1. Explain the relationship between osmotic potential & pressure potential with water potential.

       Water potential mainly depends on concentration, pressure, and gravity. If the symbols of water potential, the effects          of solutes, pressure, & gravity are denoted by Ψw, Ψs, Ψp & Ψg  , then water potential can be expressed as;
       Water Potential (Ψw) = Ψs + Ψp+ Ψg 

·                     In plants of small height (less than 5 meters), Ψg is negligible. So the equation becomes as;
                                                Ψw = Ψs + Ψp
·                     Pure water is usually defined as having osmotic potential (Ψs) of zero. As the solute is added solute potential or osmotic potential (Ψs) decreases. So, in this case solute potential can never be positive.
·                     The pressure potential (turgor potential) on the other hand in living plant cell is usually positive. In plasmolysed cells & open system , Ψp = 0. Negative pressure potential occurs when water is pulled through an open system such as a plant xylem vessels.

Ø     So, in the living cells,
·                     If , Ψs = -ve  & Ψp = +ve (or, when pressure potential is less negative than the osmotic potential) then,  Ψw =  -ve.
·                     If Ψs =  Ψp i.e., for e.g. Ψs = -1 &  Ψp = +1 ( or, when pressure potential equals to osmotic potential) then, Ψw =   0 (zero).
·                     If the value of pressure potential exceeds the value of osmotic potential then, Ψw =  +ve. ( But this is not practically feasible because  it is considered that the value of water potential for pure water is zero).

2.   Give the different parameters involved in the determination of water potential. OR,
What are the factors involved in affecting the water potential?
·                     Basically, there are three parameters involved in the determination of water potential (Ψw). They are:
1.                   Solute concentration           2. Pressure             3. Garvity
·                     Sometimes matrix potential of the system also affects the water potential.
a.                   Solute concentration:
 In pure water the value of water potential is maximum i.e., it is zero. Addition of solutes reduces the free energy of water. The term Ψs is used for denoting the concentration of the solute and its effect on the water potential. It is termed solute potential or the osmotic potential.
b.                   Pressure:
During osmosis the entry of water results in the development of hydrostatic or turgor pressure which is here called as pressure potential (Ψp). If the pressure potential is positive it will add to the water potential but if it is negative it reduces the value of water potential.
c.                    Gravity:
The term Ψg termed gravity potential denotes the effect of gravity on the water potential of a water column in a vertically growing plant. It’s magnitude depends on the height of the plant from the ground level as well as on the density of water and the acceleration due to gravity. In plants of small height (less than 5 meters) the Ψg is negligible.

Ø     Water potential is decreased by factors which reduce the relative water vapor viz., by addition of solutes, negative pressure or tensions, reduction in temperature and by matrix forces.
Ø     Water potential is increased by factors which increase the negative vapor pressure, mechanical pressure and increase temperature.




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CHAPTER: 5
Diffusion & Osmosis

Solutions:
•      Solutions are made of solute and a solvent
•      Solvent - the liquid into which the solute is poured and dissolved. Usually we will use water as solvent.

•      Solute - substance that is dissolved or put into the solvent. Salt, sugar and sucrose are solutes.





Significance of Diffusion:
Importance of Diffusion to plant life in general: 
  1. It is an essential process in exchange of gases (02 and CO2) during respiration and photosynthesis. 
  2. Uptake of minerals is also affected by the process of diffusion. 
  3.  It helps in removal of excess water by the process of transpiration. 
  4. Translocation of organic solutes also takes place by diffusion means. 
  5. Fragrance of flowers or sweet scent emitted by flowers spread in air by diffusion means attracts insects to materialize pollination. 



Factor Affecting the Rate of Diffusion:
Ø  Particle size, temperature, concentration difference and diffusion distance affect the rate of diffusion.
1.Particle Size:
•      The particle size highly influences the rate of diffusion. Since the heat of the environment is the energy source for diffusion, a smaller particle at a given temperature moves faster than a larger particle. The rate of diffusion shares an inversely proportional relationship with the particle size.
2. Temperature:
•      Temperature and the rate of diffusion have a directly proportional relationship with one another. The rate of diffusion increases as the temperature increases. At higher temperatures, particles move faster because more energy is available to diffuse them.
3.Concentration difference:
•      The rate of diffusion increases as the concentration difference increases. A substance diffusing between two areas exhibits a concentration difference as the particles diffuse from one side of the wall to the other side. For example, if a semipermeable bag of plain water is placed in salt water, the rate of diffusion increases because the salt water has a higher concentration of particles than the plain water.
4.Density of Diffusing Substance:
The rate of diffusion is inversely proportional to the square root of their relative density of their diffusing substance. The larger the molecule, slower is the rate of its diffusion. 

Diffusion Pressure (DP):
•      The pressure exerted due to the tendency of the particles of a substance to diffuse is called its diffusion pressure.
•      The DP is directly proportional to the concentration of the number of diffusing particles.
•      The greater is the concentration of particles, the greater is their diffusion pressure.
Diffusion Pressure Deficit (DPD) or Suction Pressure:
•      Diffusion pressure of a solution is always lower than its pure solvent.
•      The difference between the diffusion pressure of the solution and its solvent at a particular temperature and  atmospheric condition called as diffusion pressure deficit (DPD).
•      It is increased by the addition of solutes, lowering temperature and pressure.
•      If the solution is more concentrated, its DPD increases, but it decreases with the dilution of the solution.
•       DPD is directly proportional to the concentration of the solution.
•      The DPD of the cell sap or the cells is a measure of the ability of the cells to absorb water and hence it is often called as suction pressure (SP).
•      It is related with osmotic pressure (OP) and turgor pressure (TP) of cell sap and also the wall pressure (WP) as,
                        DPD (or, SP) = OP – WP
                                    WP = TP
                                    DPD = OP – TP
                        In fully turgid cell (endo-osmosis)
                                    OP = TP , therefore DPD = 0
                        In fully palsmolysed cell  (Exo-osmosis)
                                    TP = 0 , Therefore, DPD = OP



•      Osmosis is the movement of WATER across a semi-permeable membrane
•      At first the concentration of solute is very high on the left.
But over time, the water moves across the semi-permeable membrane and dilutes the particles.
  • Osmosis is a special case of diffusion
  • Osmosis involves the diffusion of water through a membrane
  • The membrane may be artificial and  non-living  e.g. Cellophane
  • In biology, the important membrane is the cell membrane

Osmotic Pressure:
As a result of separation of solution from its solvent or the two solution by the semi-permeable membrane, a pressure is developed in solution due to the presence of dissolved solute in it. This is called as osmotic pressure(O.P.) 
Osmotic pressure is directly proportional to the concentration of dissolved solute in the solution.
* More concentration solution has higher osmotic pressure  



Hypotonic – The solution on one side of a membrane where the solute concentration is less than on the other side. Hypotonic Solutions contain a low concentration of solute relative to another solution.




Hypertonic – The solution on one side of a membrane where the solute concentration is greater than on the other side. Hypertonic Solutions contain a high concentration of solute relative to another solution.






Osmosis system & Plant cell:
•      In plant cell semi-permeable membrane is plasma membrane & cell sap
•      The solvent in case of plant is always water
•      If a living plant cell or tissue is placed in water or hypotonic solution water enters into the cell sap by osmosis/end osmosis
Endosmosis & Exosmosis:
•      If a living plant cell or tissue is placed in water or hypotonicsolution(whose O.P. is lower than that of cell sap) water enter into the cell sap by osmosis. This process is called as endosmosis
•      Entry of water into cell sap – pressure developed and presses the protoplasm  against the cell wall – turgid. The pressure is called turgid pressure.
•      If the plant cell or the tissue placed in hypertonic solution (whose O.P. is higher than that of cell sap) the water comes out of the cell sap into the outer solution and the cell become Flaccid. The process is known as exosomosis.

Plant cell in High water potential:
1.     Cell vacuole has lower water potential compared to solutions outside cell
2.     Water enters cell by osmosis.
3.     Vacuole increases in size, pushes against cell wall
4.     Cell wall exerts opposing pressure (against turgor pressure)
5.     Plant cell expands and become turgid (cell does not bursts) .




Plant cell in Low water potential:
  1. Vacuole has higher water potential compared to solution outside cell.
  2. Water leave cells by osmosis
  3. Vacuole decreases in size
  4.  Cytoplasm shrinks away from cell wall ( Plasmolysis.)
Significant of osmosis in plant:
•      Large quantities of water are absorbed by roots from the soil by osmosis
•      Cell to cell movement of water and other substance dissolved in it involves this process
•      Opening and closing of stomata depend upon the turgid pressure of the guard cell
•      The resistance of plants to drought and frost increases with increases in osmotic pressure of their cell.
•      Turgidity of cells of the young seedling allows them to come out of the soil








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