THE CELL 



37 



From all this it is obvious that the contents of the cell must be 

 isotonic with its surroundings, or else the plasma membrane must 

 be less than freely permeable to water. The osmotic pressure of liv- 

 ing protoplasm varies between difFerent forms, but is usually ap- 

 proximately that of the surroundings of the cell. For example, the 

 osmotic pressure of human body cells is approximately equal to that 

 of a ten-per-cent sugar solution and that of the human blood stream 

 is closely similar. If a blood cell is removed and placed in distilled 

 water which contains no salts, it distends and bursts; if placed in 

 strong salt (hypertonic) solution it shrinks and dies. 



A B C 



Fig. 7. — Effects of different osmotic pressures on the shape of red blood cor- 

 puscles. A, in an isotonic solution the normal shape is retained; B, in a hypotonic 

 solution the corpuscle is distended because of water uptake; C, in a hypertonic 

 solution the corpuscle shrinks because of water loss. Compare with Fig. 6. 



Maintenance of the normal osmotic pressure within the cell is of 

 very considerable importance in the maintenance of structure and 

 function. It determines the turgidity of the cell and thus influences 

 its shape; it in part determines the amount of free water in the cell 

 and thereby in part determines the fluidity of the protoplasm. We 

 have seen how the reactions in protoplasm are closely linked with 

 its physical properties. Any violent disturbance of the normal os- 

 motic pressure relations between the cell and its surroundings tends 

 to disturb the living processes. 



In addition to its properties of osmotic pressure, the cytoplasm of 

 the cell has other equally important functions. It contains stored-up 

 materials; it is a reaction medium, a theatre in which chemical and 

 physical changes are constantly occurring; in tgg cells its organiza- 



