Plasmolysis



Plasmolysis and Deplasmolysis, importance of Plasmolysis 

PLASMOLYSIS 

Shrinkage of the protoplast of a cell from its cell wall when placed in a hypertonic solution is called Plasmolysis. Hypertonic solution causes exosmosis. The size of central vacuole and hence protoplast  becomes reduced. The pressure  on the wall is reduced and it contracts causing a reduction in cell size. This is first stage of  plasmolysis called limiting  Plasmolysis. At limiting plasmolysis, the pressure  potential (Ψp) is zero and the osmotic  concentration of cell interior to withdraw water. Central vacuole shrinks further causing  a similar shrinkage of protoplast from  the cell wall. Initially  the protoplast withdraws itself from the corners.  It is called incipient  plasmolysis. The hypertonic solution now enters the cell in between  the protoplast and the cell wall. Due to continued  exosmosis, protoplast shrinks further and withdraws from the cell wall except one  or a few points. It is the final stage known as evident plasmolysis. 




DEPLASMOLYSIS 

The swelling  up of a Plasmolysed protoplast when placed in hypotonic solution  or water is called deplasmolysis. It is due to endosmosis.  Deplasmolysis  is possible  immediately  after plasmolysis   otherwise the cell protoplast becomes permanently damaged.  

Animal cells do not have a well. High turgor pressure  due to endosmosis  causes their bursting  ( e.g., erythrocytes) leaving  behind ghost cells. The animal cells decrease  in size and develop  a number of wrinkles  due to exosmosis. This phenomenon  of development of wrinkled  appearance in animal  cells due to exosmosis  is called crenation.  

Importance of Plasmolysis:

(1) It proves that  the cell membrane  is semipermeable and cell well is elastic and permeable.  

(2) Osmotic pressure  of a  cell can be measured by  plasmolysis. It is equal to the osmotic pressure  of a solution, which will be strong enough  to cause only incipient  plasmolysis. 

(3) Plasmolysis  can be shown only by living  cells, therefore, determine whether a cell is living  or dead. 

(4) By  salting  tennis lawns, the weeds can be killed due to permanent  plasmolysis.

(5)  Plants are not allowed to grow in the cracks of the walls by  the method of salting. 

(6) Salting  of pickles, meat and sweetening  of the jams with sugar, kill the spores of  fungi and bacteria.  

(7)  Excessive concentration  of chemical fertilizers at one place in the soil should be avoided,as it will kill the plants. 

  Dead cells are fully permeable. It can be shown by cutting  beet root  into thin slices and washing  them under water till no more colour diffuses out. The slices are placed in water. No coloured sap comes out. Heat them. A reddish sap beings to come out of the slices. Heating has killed the cell membranes and made them permeable  so  that the sap diffuses out. 


PLASMOLYSIS 

The behaviour  of the plant cells ( or tissues) with regard to water movement  depends on the surrounding  solution. If the external solution  balances the osmotic pressure  of the cytoplasm, it is said to be isotonic.  If the external solution  is more dilute than the cytoplasm, it is hypotonic and if the external solution  is more  concentrated, it is hypertonic. Cells swell in  hypotonic  solution and shrink in hypertonic  ones. 

Plasmolysis occurs when water moves  out of the cell and the cell membrane of a plant cell shrinks away from its cell wall. This occurs when the cell ( or tissue) is placed in a solution  that is hypertonic ( has more solutes) to the protoplasm. Water moves out; it is first  lost from the cytoplasm and then from the vacuole. The water when drawn out of the cell  through diffusion into the extracellular  ( outside cell) fluid causes the protoplast to shrink away from the walls. The cell is said to be plasmolysed. The  movement of water occurred  across the membrane moving  from an area of high water potential (i.e., the cell) to an area of lower water potential  outside the cell .




What occupies the space between the cell wall and the shrunken protoplast  in the plasmolysed cells ?

When the cell ( or tissue) is placed  in an isotonic solution, there  is no net flow of water towards the inside or outside. If the external solution  balances the osmotic pressure  of the cytoplasm it is said to be isotonic. When water flower into the cell and out of the cell and are in equilibrium, the s are said to be flaccid. 

  The process of   plasmolysis  is usually  reversible. When the cells are placed in a hypotonic solution  ( higher water potential  or dilute solution  as compared to the cytoplasm), water diffuses into the cell causing  the cytoplasm to build up a pressure against the wall, that is called turgor pressure. The pressure  exerted  by the protoplasts  due to entry of water against the rigid walls is called pressure  potential  Ψp.  Because  of the rigidity of the cell wall, the  cell does not rupture. This turgor pressure is ultimately responsible  for enlargement and extension growth of cells.

What would be the  Ψp of a flaccid cell? Which organisms other than plants possess  cell wall? 


Differences  between Plasmolysis and Deplasmolysis 

(A) Plasmolysis :

1) It is the contraction  or shrinkage  of protoplast from cell wall. 

(2) It occurs in plant cells when they are placed in hypertonic solution. 

(3) Plasmolysis occurs as a result of exosmosis. 

(B) Deplasmolysis

(1) It is the stage when the plasmolysed  plant cell attains its original  shape and size. 

(2) It occurs in plasmolysed  plant cells when  they are placed in hypotonic solution 

(3) Deplasmolysis  occurs as a  result  of endosmosis. 








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