FIRST HETEROTROPHS AND ANAEROBES 403 



synthetic and energy-yielding reactions of metabolism could 

 not occur as a single process following a direct course. 

 It followed different and very divergent paths in different 

 representatives of the living world. As this went on, some 

 organisms were quicker to acquire the power to synthesise 

 complicated organic compounds, while others set up mechan- 

 isms which enabled them to use a greater variety of sources 

 of energy. Owing to this, the heterotrophic nature of con- 

 temporary organisms, their dependence on organic nutri- 

 ment, is very diverse. For example, some representatives of 

 the genus Hydrogenomonas do not require organic nutrients 

 for their energy metabolism and can use co, as the sole source 

 of carbon for the construction of their substance, though they 

 cannot synthesise the prosthetic groups of some of the 

 enzymes which they need. They have to obtain these ready- 

 made from the environment, as vitamins, otherwise they 



cannot exist.^ 



Moulds, on the other hand, have a very highly developed 

 ability to synthesise various very complicated organic com- 

 pounds, vitamins, antibiotics, etc., but they are typical hetero- 

 trophs in the sense that they can only grow on organic sub- 

 strates (e.g. on sugar solutions) which act as non-specific 

 sources of energy and carbon for the construction of the 

 components of their protoplasm.*^ 



In view of this, the concept of heterotrophy itself is far 

 less simple than it might seem at first glance. The classifica- 

 tion of various organisms according to their nutrient require- 

 ments which are current in scientific literature at present 

 (e.g. those of R. HalF and A. Lwoff^) are very complicated 

 and often rather confusing as well. Attempts to form a 

 picture of the progress of the evolution of organisms in this 

 respect are even more contradictory'' ^° because, in some 

 cases, an obligatory requirement for some particular organic 

 substance may also arise secondarily owing to the dropping 

 out of some chemical, metabolic mechanisms which had been 

 elaborated at a preceding evolutionary stage. 



It is, nevertheless, a self-evident and generally accepted 

 fact that the overwhelming majority of biological forms now 

 living on our planet can only exist in the presence of ready- 

 made organic substances. 



