^Enzymes: How Cells 

 Promote Chemical Activities 



IN PROTOPLASM many chemical reac- 

 tions go forward with explosive speed. Yet if 

 the reacting substances are removed from the 

 cell and isolated in a test tube, they become 

 very inert and no longer show much tendency 

 to interact. Such puzzling behavior led a few 

 biologists to postulate a "vital force" that 

 activates the protoplasmic molecules. But 

 others continued to search for missing fac- 

 tors, and during the past 35 years the prob- 

 lem has been greatly clarified. 



A relatively simple protoplasmic reaction 

 will serve to illustrate this problem more spe- 

 cifically. Many plant and animal tissues con- 

 stantly produce hydrogen peroxide (H 2 2 ). 

 This fairly poisonous compound does not 

 accumulate in toxic amounts because it de- 

 composes almost as fast as it is formed. In 

 decomposing, peroxide liberates free oxygen, 

 as is shown in the equation at the top of the 

 next column. 



In protoplasm this peroxide reaction goes 

 forward very rapidly. Yet a pure aqueous 

 solution of H 2 2 , such as is commonly em- 

 ployed as a disinfectant, is relatively stable. 



2H 2 2 «=± Oo + 2H 2 



2 mol 1 mol 2 mol 



hydrogen oxygen water 

 peroxide 



In a stoppered bottle, peroxide keeps for 

 months before it "goes flat," gradually free- 

 ing its content of 2 . Thus one phase of the 

 problem is to ascertain why peroxide shows 

 less reactivity in the test tube than in the 

 protoplasm. 



CATALYSIS 



In many cases, the rate of chemical reac- 

 tion is greatly changed by the presence of a 

 small amount of some specific reagent in the 

 reaction medium. In the case of peroxide, 

 for example, a very faint trace of manganese 

 dioxide (Mn0 2 ), a black powder, will pro- 

 duce a thousandfold acceleration in the lib- 

 eration of oxygen. If just a pinch of the pow- 

 der is added to a test tube of peroxide solu- 

 tion, oxygen begins to come forth so rapidly 



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