SEVERO OCHOA 



sulfite, selenite, nitrite, elementary sulfur, ammonia, and molecular 

 hydrogen. This process is known as chemosynthesis. 



Both photosynthetic and chemosynthetic organisms are referred 

 to as autotrophic because they can grow in media composed of in- 

 organic substances exclusively. However, a number of bacteria, in- 

 cluding most of the pathogenic species, can live only in media that con- 

 tain one or more organic components, and are known as heterotrophic 

 (24,25,28). 



The importance of photosynthesis and, in general, of carbon 

 dioxide assimilation can hardly be overemphasized, since animals 

 depend for their subsistence on the materials formed through carbon 

 dioxide assimilation by autotrophic organisms. Thus, carbon dioxide 

 assimilation is one of the most fundamental of all life processes. Al- 

 though it was known for some time that heterotrophic bacteria and some 

 animal cells could utilize carbon dioxide to synthesize carbon-to- 

 hydrogen bonds or carbon-to-nitrogen bonds (as is the case in the syn- 

 thesis of formic acid from carbon dioxide and hydrogen by Escherichia 

 coli, or in the synthesis of urea by the liver), the belief was current that 

 such organisms lacked the capacity to utilize carbon dioxide for the 

 synthesis of carbon-to-carbon bonds until the pioneer work of Wood and 

 Werkman demonstrated that heterotrophic bacteria can fix carbon 

 dioxide in this manner (28). The process is now known to occur 

 also in animal cells. 



The synthesis of organic material from carbon dioxide is an 

 endergonic reaction, which results in an increase of the free energy 

 content of the system, and thus requires energy in order to proceed. 

 In other words, such a synthesis must be coupled with exergonic* reac- 

 tions involving a decrease in free energy. For this purpose, photo- and 

 chemosynthetic organisms can use either radiant energy or the energy 

 derived from oxidation of inorganic compounds. Heterotrophs, on 

 the other hand, can assimilate carbon dioxide only at the expense of 

 oxidizing organic foodstuffs, so that no net gain in organic cell constitu- 

 ents can result from carbon dioxide assimilation under these conditions. 

 It is, therefore, difficult to decide whether carbon dioxide fixation is 



* C. D. Coryell, in Science, 92, 380 (1940), introduced the terms "exergonic" 

 and "endergonic" to characterize negative and positive changes in free energy 

 (AF), respectively, and suggested that the use of the terms "exothermic" and 

 "endothermic" be restricted to designate changes in heat (AH). 



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