AUTOTROPHIC ASSIMILATION 8l 



above that the synthesis of amides requires energy which 

 can be supplied from high energy phosphate groups and 

 it seems Ukely that the synthesis of amino-acids and proteins 

 must involve the consumption of energy from a similar 

 source. ^^' *^ Upon addition of ammonia to nitrogen-starved 

 cells the synthesis of nitrogenous substances presumably 

 resuhs in the disappearance of high energy phosphate 

 groups and makes more acceptor available so that respiration 

 may proceed more rapidly. In normal cells, in which syn- 

 thesis of organic nitrogenous substances from ammonia 

 does not occur, no high energy phosphate groups are 

 utilized and so the respiration rate is not increased. This 

 mechanism is represented in Fig. 14, in which A stands for 

 a substance such as adenosine diphosphate and A' for a 

 substance such as adenosine triphosphate. In agreement 

 with this hypothesis it has been found that addition of 

 adenylic acid or of dinitrophenol, which promotes break- 

 down of adenosine triphosphate, increases the respiration 

 rate of nitrogen-starved Chlorella, and that the soluble 

 organic phosphorus fraction in the cells decreases after 

 addition of ammonia.^'*^ 



THE ASSIMILATION OF NITROGEN IN ORGANIC 



COMBINATION 



The reactions by which nitrogen is transferred from one 

 carbon residue to another during nitrogen metabolism are 

 evidently reversible in the majority of cases and, just as in 

 the case of carbon metabolism, we are justified in imagining 

 a flexible system into which material may be introduced 

 at several points. The expectation that any substance 

 which is an intermediate in nitrogen metabolism, or readily 

 convertible into such, can serve as a nitrogen source if it 

 is able to penetrate the plasma membrane, appears to be 

 borne out by the observed facts. Thus there are many 

 records of autotrophic algae being able to utilize various 

 organic nitrogenous substances as their only source of 

 nitrogen (e.g. ^^^' ^^'' 3'*'^' ^' '^' ^'i^). Since this form of nitro- 

 gen assimilation is facultative and does not involve any loss 



