368 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



NH2— CO— NH2 + 2H2O ^ (NH4) 2— CO3 -> 2NH3 + CO2 



If these above reactions are the source of ammonia in the soil, the 

 degradation of amino acids and similar substances is complicated by the 

 occurrence of numerous side-reactions which can inhibit ammonia produc- 

 tion and consequently the nitrogen cycle. 



C^15 



,te 



U* 



Citrulline + NH, 



pco' 



ilv^ 



S^v. 



Arginine 



hemolyticus 

 aureus 



>- Ornithine + 2 NH3 + COj 



o 



Ornithine + urea 



Examples are : the uptake of amino acids resulting from the hydrolysis 

 of dead bodies by the protoplasm of bacteria, or by fungi ; and in the soil, 

 the formation of nitrogen-containing humus, by combination of carbo- 

 hydrates and nitrogenous compounds. 



But the most important side-reaction is bacterial decarboxylation, 

 brought about by organisms of the coli group, by Clostridia, etc. and by 

 fungi. 



R— CH— COOH > R— CH2NH2 + CO2 



NH2 



The amines formed are basic, toxic to animals, and resistant to bacterial 

 decomposition. Their oxidation only occurs in an alkaline medium and 

 they are formed in an acid medium, so that the bacteria which produce 

 them do not oxidize them. 



II. THE EXIT OF CARBON FROM THE BIOSPHERE 

 Carbon disappears from the biosphere chiefly as CO2 formed in the 

 tricarboxylic acid cycle and eliminated by the organism. Equilibrium 

 between this mechanism of loss and entry of carbon due to photosynthesis 

 would not be maintained if there were not other ways in which CO2 is lost. 

 The bacterial mineralization of corpses is partly responsible for filling 

 the gap. 



