SECTION IV 



THE MECHANISM OF CHEMICAL CHANGES IN 

 LIVING MATTER. FERMENTS 



ALL the events which make up the life of plants and animals are 

 accompanied and conditioned by chemical changes of the most varied 

 character. In a previous chapter we have endeavoured to form an 

 idea of the ways in which some of the synthetic processes that occur 

 in the living body may be effected. We saw that, although it was 

 possible to imitate in many respects the vital syntheses by ordinary 

 laboratory methods, the imitation fell far short of the process as it 

 actually occurs in the living cell, both in completeness of the reaction 

 and in the ease with which it could be effected. We can, for instance, 

 by passing carbon dioxide over red-hot charcoal, convert it into carbon 

 monoxide, and this gas, acting on dry potassium hydrate, forms 

 potassium formate. Formate of lime, on dry distillation, gives a small 

 proportion of formaldehyde which, under the influence of dilute 

 alkalies, will condense to the mixture of sugars known as acrose. The 

 green leaf in sunlight absorbs the minimal quantities of carbon dioxide 

 present in the atmosphere and converts it almost quantitatively into 

 starch within a few minutes, and this change is effected in the absence 

 of any concentrated reagents and at the ordinary temperature of the 

 atmosphere. Many of the chemical transformations effected by living 

 cells we have so far been quite unable to imitate. The problem of the 

 synthesis of camphor, of the terpenes, of starch, of cellulose, is still 

 unsolved, and even in the case of those substances which we can 

 manufacture outside the living cell our methods involve the use of 

 powerful reagents and of high temperatures, and result in most cases 

 in the production of many side reactions, besides that which it is our 

 special object to imitate. The distinguishing characteristics of the 

 chemical changes wrought by the living cell are : 



(1) The rapidity with which they are effected at ordinary tem- 

 peratures. 



(2) The specific direction of the process, which is therefore almost 

 complete, with a surprising absence of the side reactions which inter- 

 fere to such an extent with the yield of the methods employed in a 

 chemical laboratory. 



This second characteristic may, however, be regarded as a con- 

 sequence of the first, since an increase in the velocity of any given 



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