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 sub- 

 stances 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 reaction 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 tempera- 

 tures. 



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

 complete, with a surprising absence of the side reactions which interfere 

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

 laboratory. 



This second characteristic may however be regarded as a consequence 

 of the first, since an increase in the velocity of any given reaction will deter- 



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