YEAST CELL AND ITS LESSONS 177 



black is due to the oxygen contained in it, which oxidizes 

 a part of the saccharose to one or several organic acids, and 

 thus supplies hydrogen ions to the solution. 



But fermentation is destructive. The ferment of yeast 

 splits up sugar into alcohol and carbon dioxide; pepsin 

 resolves albuminous foodstuffs into substances of, presum- 

 ably, simple molecular composition, and so all through the 

 list we have had occasion to mention. On the other hand, 

 side by side with this incessant destruction, life is charac- 

 terized by incessant construction. These form, indeed, 

 the two most striking and essential phenomena of the life 

 process; the destruction, the analysis, is death; the con- 

 struction, the synthesis, is life. And a constructive fer- 

 ment appears, from our knowledge of enzymes, to be a 

 plain contradiction in terms. However, even this stumb- 

 ling block has apparently been removed, and it was a 

 young Englishman, Croft-Hill, who first showed us that a 

 constructive ferment is not only thinkable, but that it 

 actually exists. And here again the lesson was furnished 

 by a yeast cell. In the month of June, 1899, a paper was 

 presented by Croft-Hill to the Chemical Society, in which 

 it was shown that the action of the maltase of yeast (which 

 is the enzyme charged with the special function of convert- 

 ing the sugar maltose into the simpler, and more readily 

 fermentable, sugar glucose) on maltose is hindered by the 

 presence of glucose, and is incomplete. The effects are 

 more marked the stronger the solution of maltose. If the 

 maltase be allowed to act on a forty per cent, solution of 

 glucose, there is an apparently reversed hydrolytic action 

 resulting in the formation of fifteen per cent, of maltose, 

 at which point equilibrium occurs. The same equilibrium 

 point is reached whether we start with a solution of maltose 

 or glucose, so that the action is clearly a reversible one. It 

 has often struck me that the divergent results various 

 chemists have achieved in the study of the decomposition of 

 starch may, perhaps, in some measure be accounted for by 

 the action of constructive enzymes, or rather, one would 

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