7 8 



BIOLOGICAL CHEMISTRY 



equilibrium between the concentration of the substance inside 

 and outside the cell is restored. 



The osmotic activity will cause the phenomena which are 

 described in connection with diffusion either in a homogeneous 

 or heterogeneous system. 



Increase in the active concentration outside a cell will cause 

 similar changes, only the osmotic movement of water will be 

 in the reverse direction. 



A i' B 



FIG. 14. Drawing of plasmolysis. 



In A the cell is distending its cell wall. In B the cell is less distended owing to a stronger 

 salt solution outside. The cellulose wall has contracted, but the cell contents are still in 

 contact with the cellulose framework. In C the cell has commenced to shrink from the 

 cellulose framework and in D the cell has shrunken well away from the framework. 

 (After de Vries. Copied from Algemeine Physiologic Verworn (Fischer, Jena). 



As it is easier to vary the concentration of substances 

 outside of the cell, the simplest experimental procedure is to 

 subject isolated cells to solutions containing different strengths 

 of various substances. 



If the total active osmotic concentration is greater outside 

 the cell water will pass out of the cell and the cell will shrink. 

 In the case of cells surrounded by some rigid framework the 

 shrinkage of the cell will cause the protoplasm to separate from 

 the framework leaving a gap between the two ; this is called 

 plasmolysis (Fig. 14). 



If, on the contrary, the osmotic concentration is less outside 

 water will pass into the cell, causing the cell to increase in 

 size. Naked cells will swell but cells surrounded by a frame- 

 work will entirely fill the spaces allotted to them, thus rendering 

 the structure rigid ; this is called turgor. 



These two changes can be well exemplified by strips of 

 raw potato about five centimetres long and five millimetres 

 square. When placed in five per cent, solution of sodium 

 chloride the strips become soft and limp but in distilled water 



