Turgor Pressure, DPD, Water Potential

TP, DPD, WP, and OP  

Notes:  Turgor Pressure,  Diffusion  Pressure  Deficit,  Water Potential, and Osmotic potential  (solution) Matric Potential 

Turgor Pressure 

It is the pressure  which develops in the confined part of an osmotic system due to osmotic entry of water into it. It  is also called  hydrostatic pressure  or pressure  potential (Ψp). Due to turgo  pressure the protoplast of a plant cell will press the cell to the outside. The cell wall, being elastic, presses the protoplast with an equal and opposite  force. The force exerted by the cell wall over the protoplast is  called  wall pressure ( WP). Walk pressure  is equal and opposite to turgor pressure  except when the cell becomes flaccid. The value of these two opposing forces continue to rise till the cell becomes fully swollen  flaccid. The value of these two opposing forces continue to rise till the cell becomes  fully swollen or turgid. At this time the value  of wall or turgor pressure  becomes  equal  to osmotic potential,  Ψp = Ψs

Turgor  pressure   has number of important  functions, i.e., 

(i) It keeps the cells and their organelles stretched. 

(ii) It provides support to nonwoody tissues like parenchyma. 

(iii) It is essential  for cell enlargement  during growth. 

(iv) It keeps the leaves expanded and oriented to light.  Flowers, young  stems and other  softer organs are able to maintain their form due to turbidity.  In case of loss of turbidity,  the shoots droop down  and the leaves show wilting. In wilting the individual  cells of leaves and other softer parts become flaccid  due to loss of water from their interior. 

(v) Many plant movements  are produced  due to reversible turgor changes in the cells. Sleep movements and shock movements  are produced due to turgor changes in the cells of  their pulvini. Regular turgor  changes  in the leaflet bases cause rhythmic autonomic jerky movements in the lateral leaflets of Indian Telegraph Plant, Desmodium gyrans. 

(vi) The opening and closing of stomata are caused by gain and loss of turgidity by guard  cells. They are hence called " turgor operated values". 

(vii) It keeps a check on the excessive entry of water into cells. 

(viii) Autochory of some fruits. 

Diffusion  Pressure  Deficit ( DPD) 

It is an older  used for water potential.  Pure water has the maximum  diffusion pressure. Its diffusion pressure  gets lowered by the addition  of solute particles in it. The reduction in the diffusion pressure  of water in a solution  over its pure  state is called diffusion pressure  Deficit  or DPD. It is also defined as the pressure  with which water will be drawn in or expelled out of a system. Because  of the  presence of diffusion  pressure  deficit,  a solution  will always  tends to make up the deficit by absorbing  water. It is also  called suction pressure  or suction force or absorption  potential.  Its value is equal  to the  osmotic pressure  or potential  ( positive value taken  in bars or atm) of the solution in a cell or system minus the wall pressure  ( = turgor pressure)  which opposes the entry of water  into it provided the external water is pure. 

                        DPD = OP _ WP  ( = TP) 
If the external fluid is a solution  the value of DPD  will be equal  to the  difference in osmotic  pressure  of cell solution  ( OP1) and external  solution  ( OP2) minus  the wall  pressure. 

                DPD = (OP1_ OP2) _ WP ( = TP) 


Water potential  of pure water  at normal  temperature  and atmospheric  pressure is taken as zero. In solution  the value of water potential  is always  negative. It is represented by Greek  letter Ψ  (psi)  or more ccurately Ψw. The  value of   Ψ  is  measured  in bars or atmospheres. Water  always  moves from the area of high water potential  to the area of low  water potential,  i.e.,  from less  negative  potential  to more negative  potential. Water potential  is the  sum total  of two  potentials__  solute  potential  ( Ψs,  osmotic potential  or osmotic  pressure  as negative ) and pressure  potential  ( Ψp,  hydrostatic pressure  or turgor pressure). 

                            Ψw = Ψs + Ψp
Some  authors write this expression  as :
                           Ψs = Ψs + Ψp + Ψg
Here Ψg denotes gravity and is called the gravity potential. It  depends on the  height of water above  the reference state of water, density of water and acceleration  due to gravity. If  the  vertical  distances are small ( less than 5 meters), the Ψg is negligible , and,  hence  ignored. 

Matric Potential  

(Ψm) is  an important  force ( value  taken as negative  like solute potential) in young  cells, seeds and cells of some desert plants due to presence of hydrophilic colloids. 

          Ψw = (- ) Ψs + (-) Ψm + Ψp 
If a pressure  greater than  atmospheric pressure is applied  to pure  water or a solution, its water potential  increases. It is equivalent  to pumping  water from one  place to another. Pressure  potential is usually positivs, though in plants negative potential  or tension in the  water column in the xylem plays a major role  in water  transport   up a stem. 

Differences between  Diffusion Pressure  Deficit  ( DPO) and Water Potential  (Ψw) 

(A) Diffusion Pressure Deficit (DPO)

(1) : It is an old  term  synonym  with water potential. 

(2):  It has positive value. 

(3):  It is the  reduction in diffusion  pressure of solvent in a system over  its pure  state. 

(4):  Water  is absorbed by a system  having higher DPD from  a system  with lower DPD.

(5):  DPD = OP _ TP where  all the terms are with  positive values.

(6):  Matric potential  is not considered. 

(B) Water Potential  (Ψw) 

1) : It is modern term equivalent  to DPD. 

(2): It has  negative  value. 

(3):  It is reduction in free energy of solvent in a system over  its pure state.

(4)  Water is absorbed by a system  with lower Ψw from a system with  higher Ψw.

(5) Ψw = Ψs + Ψp where Ψs has a negative  value while Ψp has a positive value.

(6) Matric  potential  is considered  wherever  it  is appreciable. 

Osmotic or Solute Potential (Ψs) :

It is the decrease in the  water potential  of pure water  due to the presence of solute particles in it. All solution have  a lower  water  potential than pure water. The more the solute molecules, the lower ( more negative) is the Ψs. For a solution at atmospheric  pressure Ψw= Ψs.  Solution potential  is a colligative property  of solute and is dependent  upon  the number of solute particles and not upon the nature of solute. Solute particles  reduce the free energy  of water by diluting it, increasing entropy, reducing vapour pressure,  raising boiling  point  and lowering  freezing  point. Its value is calculated  by the following  formula.  

                   OP  or SP  = C × R × T
Where C is the concentration  of solute particles  in moles per liter, R  is  gas  constant with a value of  0.083 and T is the temperature  in absolute  degress. 

 Thus  osmotic  potential of a molar solution of a  nonelectrolyte  like sucrose at 0°C or 273A°  and 20°C  or 293A° can be calculated  as below. 
  at 0°C = 1 × 0.083 × 273 = 22.7 bars (as negative) or 22.4 atm 
  at 20°C  =  1× 0.084× 293=24.3 bars ( as negative ) or 24 atm. 

 In  case  of electrolytes, the degress of ionisation  is taken  into consideration  because osmotic or solute potential  depends upon the number of particles  ( here as ions  and molecules) and  not upon  the number  of molecules  alone. In case  of strong electrolytes  the osmotic potential  may be almost double  or triple as compared to nonelectrolytes. For  example, 0.1 N sucrose solution has an osmotic potential  of _ 2.3 bars while 0.1 N sodium  chloride  solution had Ψs values of _ 4.5 bars. 

Osmotic Relations  of Plants Cells : 

 A plant cell is bathed  in a water  medium or lies in contact with other cells having water. It has a permeable  elastic  wall, a semipermeable  membrane  plasma membrane ( alone or along  with cytoplasm  and tonoplast) and an osmetically active solution  called cell sap ( contained in the central vacuole). The  cell sap  has an osmotic potential. It  causes the osmotic entry of water which develops a turgor pressure  or pressure  potential.  

  It is  kept under check by wall pressure. The cell where wall is absent cannot counteract pressure  potential. It will continue  to absorb water due to Ψs till it bursts. It is because  of this fact that the animal cells are not bathed in water but are surrounded  by nearly isotonic tissue fluid. 

 Being  a positive force, Ψp opposes the entry of water into the cell. 

(a) Rise in Turgidity : A flaccid  cell kept in hypotonic solution  will absorb  water. The value of turgor pressure rises. As  the cell becomes fully turgid, the value  of turgor  pressure  becomes equal to that of solute potential ( Ψs) so that  water potential (Ψw) or DPD  becomes either  zero  or equal to that of external  hypotonic solution.  

                        Ψw= Ψs +Ψp = 0 
Though  there is no net  movement  of water between  the cell and its   environment, equilibrium is dynamic . Equal  exchange of water  molecules continues between  the cell and its environment.  

(b) Loss of Turgidity : If plant cell is kept in hypertonic solution , it loses water. The  volume of the central vacuole decreases. The protoplast is reduced in size. This decreases  turgor pressure  or pressure  potential  and corresponding wall  pressure. Solute potential  becomes  slightly  more negative. The cell attains  a minimum  size when turgor  pressure  is zero. If the external solution  does not cause  any further exosmosis, the value of its water potential  will be equal to solute potential  of the cell. 

                Ψw = Ψs + 0Ψp  or Ψw= Ψs 
In case exosmosis  continues, the protoplast shrinks  from the cell wall. The phenomenon  is called  Plasmolysis.  

(c) Water Rotation  between   Adjacent  Cells: Cells  gain or lose water among themselves  on the  basis of their water potential  ( or DPD) and not their  solute or osmotic potentials. A cell having  more negative  osmotic  potential  and high turgor pressure  can lose water to a cell  having  less negative  osmotic potential  provided it has still lower turgor pressure.  

Reverse Osmosis : If  additional  pressure  is applied ( more than the osmotic pressure applied to prevent the flow of water into an osmetically  active solution),  then water can be made  to flow in the  reverse direction i.e., from solution into the water. It is used for removing salts from saline water. 






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