THE BIOLOGY OF THE CELL SURFACE 



cent, carbon-dioxide together with the rare gases, helium, 

 argon, krypton, in traces. The permeability of the cells 

 of the lung for these gases is an important property for 

 without the entrance of them into the lungs and their 

 subsequent conveyance to all parts of the body where oxy- 

 gen is given up to each living cell, life would cease. The 

 air breathed out contains the same 79 per cent, nitrogen, 

 but only 16 per cent, oxygen and an increase of carbon- 

 dioxide amounting to 4.38 per cent. All of us appreciate 

 that the figures reveal how much oxygen the living cells use 

 and need and how much carbon-dioxide they form and get 

 rid of. 



The passage not only of water into and out of normal cells 

 in normal condition, but also of substances in solution may 

 be compared to the entrance and exit of these gases. Some 

 pass in and out again in equal, others in diminished, and 

 still others in increased amounts. Without the entrance 

 of food-stuiTs and the exit of waste the cells would die. 

 The phenomena of significance are the utilization of incom- 

 ing materials by the cells and the excretion of effete. What 

 has been said concerning water, therefore, may be said of 

 material in solution as salts, sugars, amino-acids. These 

 enter or leave the cells according to the level at which they 

 are present in the cells. They form part of the protoplasmic 

 structure and also take part in reactions going on within the 

 protoplasmic boundaries. If their concentration in the 

 cell is low, the cell retains them as they come in. In order 

 to learn what substances a cell uses, we should not merely 

 inquire what substances get into a cell, but also what of 

 these remain in it. We may imagine that more of a given 

 substance in solution in the surrounding medium passes 

 into cells than remains in them.^ 



^ The mistake is often made of attempting to learn zvhat can get 

 into cells by too severe treatment of them, injuring them by subjecting 

 them to solutions of too great concentration. 



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