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(CHg^O)^ + xOg — > XCO2 + xHgO + (Heat)c 



1B.th respect to the gaseous chemical exchange, photosynthesis 

 by green plants was the reverse of combustion and respirationo It 

 was the absorption and utilization or assimilation of carbon dioxide 

 and the liberation of oxygeno But this assimilation of carbon dioxide 

 with the production of organic matter and oxygen differed greatly from 

 the spontaneous combustion and respiration, because it occurred only 

 in the green parts of plants and only in the presence of sunlight-'->'^> 5. 



Once the gaseous exchange phenomena had been elucidated as chemi- 

 cal processes, special efforts were made to elucidate the mechanism of 

 combustion, respiration, fermentation, and photosynthesis as a series 

 of successive chemical reactions. These efforts led to two important 

 lines of investigationo One course of investigation had as its objec- 

 tive the discovery of the chemical steps through which the exchange of 

 gases leads to the formation and degradation of the carbon compounds of 

 organic matterc The other line of investigation, beginning after the 

 concept of phlogiston had deteriorated, had as its objective the use of 

 the concept of energy or work to interpret the reactions. 



The attempts to elucidate the chemical steps of combustion and 

 respiration gradually led to the discovery of many intermediate reac- 

 tions involving oxidationo In respiration, many of these steps are de- 

 pendent upon the enzymatic regulatory systems of the living cells. In 

 some of these steps, oxidation reactions are linked with equivalent re- 

 duction reactions so that reduced substances and oxidized substances 

 are produced simultaneously. Similar, intermolecular, oxidation- reduc- 

 tion or dismutation reactions were found to be an integral part of most 



