Liquids have diffusion pressure. A pure solvent is supposed to have maximum diffusion pressure. When certain solute particles arc added to the pure solvent, the diffusion pressure of the resulting solution is decreased.
The amount by which the diffusion pressure of a solution is lower than that of its pure solvent at the same temperature and atmospheric pressure is called the diffusion pressure deficit (DPI)). Suppose the diffusion pressure of the solvent is 20 atmospheres and diffusion pressure of the solution is atmospheres then the diffusion pressure deficit of the solution will be atmospheres. Diffusion pressure deficit is increased by the addition of solute, lowering of temperature. And pressure.
The DPD of any cell is the measure of water absorbing capacity of that cell. It is also called suction pressure. Thus suction pressure is a measure of the ability of a cell to absorb water.
Each and every cell has TP, OP, SP and DPD. When a cell is fully turgid, its SP. becomes zero as it cannot absorb more water. But at this stage the cell has OP and DPD because the cell contains dissolved solutes and their (OP, DPD) value cannot be zero. These three quantities i.e. Op, TP and SP are related in a plant cell by the relation:
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DPD (SP) = OP – TP
When a cell is placed in a hypertonic solution, the water from the cell cytoplasm goes out by the process of exosmosis resulting in increase in the OP and decrease in TP of cell cytoplasm.
As a result of ex-osmosis, the cytoplasm contracts and moves away from the cell wall. Such a cell is called plan molasses cell and this stage as flaccid stage. When the cell be-comes completely planmolysed, at that time the TP of the cell is reduced to zero.
DPD (SP) = OP – TP
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DPD (SP) = OP – O
DPD (SP) = OP
This indicates that OP of a plant cell cannot be zero.
When a cell is placed in pure water, it becomes fully turgid; its SP becomes zero, because the cell has no capacity to absorb water due to TP.
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DPD (SP) = OP – TP
0 (zero) = OP – TP or OP = TP
The relation of SP, TP, OP and the volume of the cell is expressed in.
The DPD or SP of plant cell is not directly proportional to the OP or Osmotic concentration of the cell. It depends upon OP and TP. When the OP of the cell sap is more, the DPD of cell will also be more but the TP will be less. Under such conditions the water enters into the cell. Sometimes it happens that the OP and TP of one of the adjacent cells are more than the OP and TP of the other cell, even then the water does not enter into it.
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The reason is that the DPD of that cell is less and there is no effect of high OP on the entry of water indicates movement of water) Plasmolysis
If a living plant cell containing sap is immersed in a hypertonic solution, then exosmosis will take place, i.e. water will come out of the cell sap through the cell membrane into the outer solution. Further loss of water from the cell contents causes contractin of the protoplasm which moves away from the cell wall and forms an irregular mass at the centre or in one corner of the cell.
This phenomenon of shrinkage of the protoplasm which moves away from the cell wall by the influence of certain hypertonic solution is called plasmolysis.
The stage at which the first sign of shrinkage of the cell contents from the cell wall becomes detectable is referred to as incipient plasmolysis or limiting plasmolysis. The space in between the cell wall and the cell membrane contains the hypertonic solution.
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The various stages of plasmolysis can be seen under microscope. Planmolysed is easily observed in cells containing colored plastids (eg. epidermal cells of Rheon discolor) since the presence of colors makes the shrinking plastids easy to observe.
If a planmolysed cell is immersed in pure water or in a hypotonic solution, endosmosis takes place. The protoplasm and the cell as a whole attain their original shape and size. This phenomenon is called deplasmolysis. It is the reverse of plazmolysis.
Importance and application of plasmolytic process
Plasmolysis is of manifold importance and applications.
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(i) It signifies the semi permeable nature of the plasma membrane.
(ii) The appropriate osmotic concentration of cell sap and consequently the osmotic pressure can be determined by plasmolysis.
(iii) It determines the living nature of the cell; since no plasmolysis can occur in dead cells due to absence of semi permeable cytoplasm membrane.
(iv) It is used for preservation of jams, jelly, prickles from the growth of fungi and bacteria by the addition of concentrated sugar solutions.
(v) The Phenomenon is utilised in salting of meet and fishes to keep them unspoiled for some period.
(vi)The phenomenon of deplasmolysis proves the healthy nature of the cell.