MECHANISM OF METABOLISM 1 93 



tinued dialyzing weakens the activity which can be restored only by 

 adding salts. 



Enzymes of Fermentation. — It has been demonstrated in the 

 above paragraph that food is prepared for digestion or fermentation by 

 enzymes. The final decomposition, the process which yields the energy 

 for cell life, must take place within the cell. 



The difference in importance of food preparation and fermenta- 

 tion may be illustrated by the example of Rhizopus dryza. This 

 mold attacks starch, changes it, by means of diastase, to maltose, 

 the maltose to dextrose, dextrose to alcohol and carbon dioxide. The 

 mold grows well in a starch medium, without sugar; it grows equally 

 well in maltose, and equally well, or better, in dextrose; it does not 

 grow at aU with alcohol and carbon dioxide. The last change, dex- 

 trose to alcohol, is absolutely necessary for this organism; it is the 

 source of its life; the others are incidental processes, not absolutely 

 necessary under all circumstances, in fact greatly suppressed if dextrose 

 is given together with starch. The fermentation must take place in 

 the cell; the preparation of food may take place in the cell or outside; 

 it is not essential where it happens. 



The investigations of recent years have demonstrated that fermenta- 

 tions also are caused by enzymes. It has been proved beyond doubt 

 that in the alcoholic, lactic, acetic and urea fermentations the fermen- 

 tation process may continue after the death of the fermenting cells. 

 In the case of alcoholic fermentation, the fermenting agent was 

 separated first by Buchner from the lacerated cells and was 

 filtered through porcelain filters without losing its ability to act. 

 This proves the enzyme-nature of the fermenting agent which, once 

 being formed, remains and acts independent of the cell. These en- 

 zymes are called zymases. They remain within the ceU as long as it 

 is alive. They are much more sensitive to injurious influences than 

 the above-mentioned food-preparing enzymes. Much skill and pa- 

 tience was required to demonstrate their independence of the living 

 cell. After these enzymes were found in microorganisms, similar 

 enzymes were discovered in the cells of higher plants and animals. 

 Many of the biochemical changes taking place in the final dissociation 

 of food within the cell are known to be the result of enzymic 

 action; heretofore these reactions were believed to be a part of the 

 Ufe processes, inseparable from the living cell. Even some of the 



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