320 LIFE : OUTLINES OF GENERAL BIOLOGY 



LIFE WITHOUT OXYGEN.— Pasteur discovered that some 

 moulds and bacteria could live without free oxygen, and he called 

 this mode of life anaerobic. In the presence of oxygen, the yeast 

 plant feeds on glucose and multiplies rapidly, forming carbon 

 dioxide and water. In the absence of oxygen the yeast plant ceases 

 to grow or multiply, but it ferments the sugar into alcohol and 

 carbon dioxide, producing heat in the process. At some stage, 

 oxygen must be formed which unites with part of the carbon of the 

 glucose to make carbon dioxide, some energy being released for the 

 plant's use, but in a somewhat wasteful way. Some bacteria are 

 killed, as Pasteur showed, by oxygen; and it is noteworthy that 

 the chemical results brought about by putrefactive organisms differ 

 greatly according to the presence or absence of oxygen. In general 

 it may be said that the anaerobic respiration does not carry the 

 breaking down of the proteins so far as to the usual carbon dioxide 

 and water, and does not liberate enough energy to admit of growth. 



It is convenient to refer here to cases of "anaerobiosis" (life 

 without oxygen) in the animal kingdom. In experimental conditions 

 some animals, e.g. leeches, remain alive for over a week without any 

 oxygen ; and in natural conditions they can live with a very scanty 

 allowance. Thus there is very little oxygen in the depths of many 

 sluggish lakes, especially when they are frozen at the surface; yet 

 freshwater mussels survive these conditions, passing into a lethargic 

 state at the unfavourable season. Spallan?ani showed that snails 

 can survive for some time in an atmosphere of nitrogen or hydrogen ; 

 and it is said that they continue to give off carbon dioxide in these 

 conditions. 



Experiments on muscle are very instructive in this connection, 

 for when stimulated in the absence of oxygen it gives off (for a 

 short time) the usual lactic acid, which remains as such, while in 

 the presence of oxygen the lactic acid is partly oxidised, with the 

 production of carbon dioxide. Thus the end-product is different 

 according as the process is anaerobic or aerobic. 



Many Nematode parasites spend the greater part of their life 

 within the intestine of higher animals, surrounded by the partly 

 digested food — from which they exact a toll — and the other contents 

 of the alimentary canal. Now this medium, as Bunge showed long 

 ago, contains almost no oxygen. In some cases no trace of oxygen 

 can be detected, yet the worms produce carbon dioxide. In solution 

 of this puzzle, it has been suggested that the worms bring about in 

 their body a fermentative process, which supplies the energy needed 

 for movement, growth, and other activities. It may be that the 

 fermentation of glycogen, abundantly stored in the worms, yields 

 in the absence of oxygen valerianic acid, carbon dioxide, and water, 

 and releases energy as the rather abnormal yeast plant does in its 

 fermentation of sugar into alcohol. 



