CHAPTER XIV 

 ACTION OF MICRO-ORGANISMS 



WE have traced the process of synthesis by green plants 

 and the using of the products of synthesis by animals. 

 The waste products of animals and the dead parts of 

 plants are converted into the simple substances from which 

 our cycle started. The achlorophyll- plants behave like 

 animals in decomposing the complex substances formed by 

 green plants. 



The final stages of this decomposition are brought about by 

 animal and vegetable micro-organisms. Some of these 

 organisms are saprophytic, i.e., they act on dead organic 

 matter, whilst others are parasitic, i.e., they act injuriously * 

 upon living plants and animals. The micro-organisms are 

 present everywhere, in the alimentary canal, soil, etc., so 

 they are ready to act upon the substrate whenever favourable 

 conditions arise. The subject of bacteriology owes its origin 

 to the researches of Pasteur, a chemist who studied the 

 decompositions brought about by micro-organisms. 



Carbohydrates are decomposed in various ways. As in 

 animal metabolism the chief end products are carbon dioxide 

 and water, but other substances are also found, e.g., hydrogen, 

 marsh gas, acids, etc. The formation of alcohol from sugar 

 by yeast has been the subject of much study. 



Alcoholic fermentation does not depend upon the intact 

 yeast cells, as Buchner showed that a cell-free extract, prepared 

 by grinding the yeast at a low temperature and by subjecting 

 the mass so obtained to hydraulic pressure, is capable of 

 producing alcohol from sugar. He called the active substance 

 Zymase.t 



The various factors in the conversion of glucose into carbon 

 dioxide and alcohol have been carefully studied. The initial 

 rate of reaction depends upon the concentration of sugar. 



* Symbiosis is a partnership of organisms for their mutual benefit ; 

 see, for instance, Plant- Animals, by F. Keeble (Cambridge University 

 Press, 1912). 



j E. Buchner, Ber., 1897, vol. 30, p. 117. 



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