UNICELLULAR FORMS 79 



Catalysis is also employed in many important commercial processes; 

 for example, in the artificial fixation of nitrogen in the manufac- 

 ture of fertilizers a catalyst is employed to bring about the union 

 of the nitrogen of the air with hydrogen, or in some processes, 

 with carbon. The exact nature of catalytic action is not fully decided 

 by the investigations that have been made. It is certain that many 

 catalysts are finely divided substances, colloidal in nature, and it also 

 seems certain that the reactions which they initiate or accelerate 

 occur at the minute surfaces of the colloidal system. 



In the cells of all living organisms various types and varieties 

 of enzymes are known to exist which act in speeding up metabolic 

 reactions, or in performing molecular splittings and syntheses. 

 They appear to act like catalysts and must be regarded as such. 

 That a great many of the chemical reactions in protoplasm are 

 catalyzed by enzymes is known; perhaps most of them are so con- 

 trolled. Evidence that one of the most important chemical reac- 

 tions in the human body, respiration, is accelerated by some sort 

 of catalytic system is immediately at hand. If a quantity of grape 

 sugar is placed in the open air at 98.6°F. it is practically unaffected 

 by the oxygen of the air, even when in solution in water. But if 

 this substance is taken into the human body at the same tempera- 

 ture, it is completely oxidized within a few hours, the heat output 

 of its oxidation assisting in maintaining the body temperature. This 

 constitutes strong inferential evidence that the oxidative processes 

 in the human body are controlled, speeded up, by enzymes. More 

 direct evidence confirms this. 



Since much of metabolism is influenced by enzymes, since also 

 the food of all animals is made available for metabolism by chemi- 

 cal processes that are controlled by enzymes, it is most important 

 that the idea of catalysis be understood and made a part of our 

 concept of what is occurring in protoplasm when it carries on 

 respiration, growth, and other metabolic phenomena, and what is 

 occurring when food is digested in the food vacuoles of amoeba 



