114 life's beginning on the earth 



surrounded by an elastic film. The inside is mostly liquid 

 and in this liquid are various salts and other dissolved sub- 

 stances including the red blood pigment, hemoglobin. 

 These cells float in blood liquid, which contains dissolved 

 salts in the same concentration as that inside the cells. The 

 blood cell may therefore be compared to an artificial cell in 

 which salt is added both to the gelatin and to the tannic 

 acid, with the result that the effects inside and outside 

 equalize. 



What will happen if we remove a number of blood cells 

 from their natural environment and place them in distilled 

 water? We may predict that, the pressure of the salts 

 inside must expand the cell, since the pressure acting on the 

 outside has been removed. An experiment shows that such 

 an expansion actually occurs; indeed more than mere swell- 

 ing occurs : the cell bursts, quite like an over-inflated rubber 

 balloon. This bursting can be best observed under the 

 microscope, since the blood cell is of small size, less than 

 t&o- millimeter. But if a large number of blood cells burst 

 in water, the hemoglobin imparts to the water a distinctive 

 red color and renders it translucent like red wine. (Nor- 

 mally blood is not translucent.) This is known as hiking of 

 blood or hemolysis. 



On the other hand, if we add more than 1% of salt to the 

 solution in which the blood cells are floating, then the blood 

 cells shrink, owing to the excess pressure on the outside. 

 A simple experiment makes this shrinking obvious; if we 

 centrifuge a given volume of red blood cells, the cells fall to 

 the bottom of the centrifuged fluid. After centrifuging we 

 add concentrated salt solution and centrifuge again. The 

 volume of the cells is now considerably smaller. 



This pressure exerted by the salts on thin films is one of 

 the most widely recognized forces acting in the living world. 

 It is termed the osmotic pressure. The swelling or shrin Ic- 

 ing of living or artificial cells under its influence is called 

 osmosis. 



