THE METABOLISM OF THE SULPHUR BACTERLi 37 



2. Reactions whicli place at the disposal of the plant 

 (or animal) a supply of energy to carry on various 

 vital activities. 



It is impossible to select a single feature which will enable 

 us to make a distinction between the fermentative and the 

 katabolic processes. Indeed all katabolic processes associated 

 with vital activities may thus w^ith propriety be named fermen- 

 tative. In actual practice one of two procedures is followed 

 in attempting a distinction : — 



1. The limits are not specifically defined and only those 



processes are named fermentative that bear some 

 resemblance to typical reactions such as, for example, 

 the fermentation of sugar into alcohol by the yeast 

 plant. 



2, The term fermentation is restricted to the disruption 



of easily decomposable carbohydrates and similar 

 substances where the biological significance of the 

 process is the gain of energy to the organism. 



Metabolism, Fermentation, and Respiration in the 

 Sulphur Bacteria. 



The outstanding feature in the physiology of the sulphur 

 bacteria is the large absorption of sulphuretted hydrogen ; 

 and the significance of this absorption is the keynote to an 

 understanding of their physiology. The gain to the bacteria 

 must be in terms of energy or of food. The importance of 

 sulphur as an article of food can be dismissed, for the amount 

 of sulphur which is absorbed is several hundreds of times 

 greater than the amount of that substance required for the 

 building up of the protoplasmic molecule. In this respect the 

 sulphur bacteria do not require any more sulphur than any 

 other bacterial organisms. Again, it has been shown that the 

 energy level of sulphuretted hydrogen in solution is lower 

 than that of the sulphur into which it is transformed. It is, 

 therefore, clear that the gain to the organisms cannot be a 

 direct one, and that the sulphuretted hydrogen must there- 

 fore participate in some larger operation which is beneficial, 



