112 RESPIRATION 



Anaerobes 



We now come to the exceptions mentioned in the first Hne 

 of this chapter. When I was a student we used to be taught 

 that all plants and animals took in oxygen and breathed out 

 carbon dioxide. Further investigation, however, has shown 

 that this statement is not true of all plants or animals. Many 

 of the bacteria, like the vast maj ority of other living organisms, 

 can only live in the presence of free oxygen. These organisms 

 are termed aerobic. Many others, and such organisms as the 

 yeast plant, can live without free oxygen for a considerable 

 time, attaining the energy for their growth by breaking up 

 sugar into alcohol and carbon dioxide. But after a time they 

 require oxygen. Others can live entirely without oxygen, 

 whose presence is often fatal to them, obtaining their energy 

 by breaking down complex compounds. They are called 

 anaerobic. 



After Lavoisier's classical work on oxidation the idea of 

 the impossibility of life without oxygen dominated the minds 

 of all scientific men for many years. 



Spallanzani, contemporary with Lavoisier, proved however, 

 experimentally, that small infusorial animalcules and snails 

 live and produce carbon dioxide when the supply of oxygen 

 is entirely cut off. This very important investigation of 

 Spallanzani remained unnoticed until the year 1861, when 

 Pasteur published his first monumental note on "Animal- 

 cules infusoires vivant sans gaz oxygene libre et determinant 

 des fermentations." In this note Pasteur introduced for the 

 first time the terms aerobic and anaerobic. Since Pasteur much 

 work has been done on this subject and we know now that 

 while some micro-organisms can live temporarily without 

 oxygen, for others which are permanently anaerobic even a 

 small oxygen tension is a deadly poison. 



In 1875 Pfliiger showed that frogs can be kept for twenty- 

 four hours or more in pure nitrogen and that during this time 

 they produce a considerable amount of carbon dioxide. 



The presence of free oxygen is poisonous to certain anaerobic 

 organisms: it kills them. The yeast cells on the other hand 

 cannot live for an indefinite time without oxygen, for the 



