The below mentioned article will highlight the interrelationship of osmotic quantities of plant cells.
The osmotic pressure, turgour pressure (hydrostatic pressure actually developed on the cell wall) and the suction pressure (diffusion pressure deficit, DPD) are collectively known as osmotic quantities of a plant cell.
It is the turgour pressure which imparts to plant cells, their usual rigid and distended condition, when the supply of water is abundant. This distended condition of the cell is variously termed turgour, turgidity or turgescence. Cells, low or entirely lacking in turgour, are sometimes referred to as flaccid.
The interrelationship of osmotic pressure, turgour pressure and suction pressure of a plant cell could be further clarified by a study of Fig. 666 in which the importance of changes in the volume of the cell has also been taken into consideration; we have disregarded the influence of changes in the volume of the cell on the osmotic quantities in our all previous discussions.
Changes in the volume of the cell certainly bring about changes in the osmotic pressure values due to dilution of the cell sap. An increase in the volume of the cell decreases the osmotic pressure while progressively increasing the turgour pressure on the cell wall due to influx of water.
In all our foregoing discussions, it has been assumed that the membranes are inelastic. In living organisms, however, the plasma membranes of cells are elastic within limits.
In appreciably elastic membranes, a complication actually arises—for-changes in volumes of the cells take place with corresponding increase or decrease in the osmotic pressure values of the internal solution due to entrance or exit of water.
Let us assume that a plasmolysed plant cell with elastic membrane (osmotic pressure 10 atm.) is capable of permitting a fifty per cent increase in the volume of the cell before dynamic equilibrium is attained with the surrounding water medium.
As the cell increases in volume, the internal solution becomes more dilute, i.e., less concentrated. We know that osmotic pressure of a solution decreases with diminution of its concentration.
Now if we assume a proportionate decrease in osmotic pressure with decrease in concentration, the final osmotic pressure of the cell sap would be 6’66 atm. as shown below:
10: X=150: 100
or 150 X = 10 x 100
X=
=6.66 atm.
If the external solution instead of being water is a solution with an osmotic pressure of 5 atm. a hydrostatic pressure (turgour pressure) of 1.66 atm. would be sufficient to bring the cell in equilibrium with the external medium.
If the cell wall were completely inelastic, the turgour pressure volume would certainly have been (10—5) or 5 atm. at equilibrium.
When a plant cell is completely limp and flaccid (i.e., at incipient plasmolysis), the relative cell volume evidently is the smallest and in Fig. 666 taken as 1.0, under such conditions the cell wall is subjected to no turgour pressure while the suction pressure of the cell is, therefore, equal to the osmotic pressure of the solution in the cell sap, and is certainly at its maximum.
As this cell is immersed in pure water, the volume of the cell increases progressively with the progressive increase in turgour pressure; the osmotic pressure also falls due to dilution of the cell sap associated with progressive increase in the volume of the cell.
The suction pressure imposed upon by a progressively increasing turgour pressure, shows a corresponding rapid progressive decrease. When the cell ultimately attains a condition of full turgidity (it has taken up as much water as it could) and the cell volume is at its maximum, 1.5, the turgour pressure, the actual pressure developed within the cell becomes equal to osmotic pressure and the suction pressure, balanced by the imposed turgour pressure falls to zero.
The following fundamental relation among the three osmotic quantities of a plant cell (evident also from our previous discussion with artificial osmotic membranes) can be expressed in simple equation:
S=suction pressure;
P=osmotic pressure;
T=tur- gour pressure;
S=P-T
or T=P—S
or P=T+S
When the osmotic pressure is determined for a plant cell at its incipient plasmolysis, i.e., when turgour pressure is zero and is compared with the values obtained by purely physical methods, e.g., cryoscopic, it is frequently markedly different instead of being identical.
Sometimes positive values, as high as 7 atm., have been observed for some plant tissues. According to the classical view which we have discussed so far, there should have been no difference at all. Generally the osmotic pressure values found by plasmolytic method exceeded those found by purely physical methods.
This excess pressure suggests that the intake of water by plant cells may result from two entirely different types of force, possibly acting independently—one osmotic and, therefore, passive while the other non-osmotic or active.
The non-osmotic uptake of water by plant cells under certain conditions has been examined critically by determining the effects of electrolytes and non-electrolytes on the volume of the cell.
When epidermal cells of onion are plasmolysed in a hypertonic solution of potassium chloride and then transferred immediately into an isotonic (the same strength as the cell sap) solution of sucrose, increase in the volume of the cell is observed.
This observation is certainly against the classical view of osmotic phenomena—according to the classical view, the cell volume should have remained the same. A probable explanation offered is that in sucrose solution, sucrose molecules being non-electrolytes, both the passive osmotic and the non-osmotic active forces were operating freely.
The active forces were drawing the extra water into the cell, thereby giving higher values for osmotic pressure determinations by plasmolytic method. In a solution of an electrolyte such as potassium chloride, the non-osmotic active absorption forces are in some way eliminated and the water is drawn into the cell only osmotically.
The non-osmotic active forces can be of considerable magnitude and are sometimes referred to as secretary forces of the protoplasm and the absorption of water and solutes thus come under the general metabolic control of the tissues.