7 



Oxygen and 

 Carbon Dioxide 



With this chapter we turn from the more mechanical or physical 

 features of the environment to a consideration of some of the chemical 

 factors. Two fundamental substances taking part in the chemical 

 exchange between the organism and the environment are oxygen and 

 carbon dioxide. These substances enter into the basic processes of 

 photosynthesis and respiration, as may be indicated by the overall 

 reactions in which CgHiaOe is taken to represent the carbohydrate of 

 the organism. 



Light Photosynthesis 



CO2 + H2O ;==^ CeHiaOe + O2 



Respiration 



Oxygen and carbon dioxide thus stand in a reciprocal relation to each 

 other as regards these fundamental reactions of life, and their 

 abundance in the environment is of direct critical concern to the 

 organisms of every habitat. In addition, these materials take part in 

 subsidiary reactions involved in the ecological relations of plants and 

 animals. Of particular ecological importance in this connection are 

 the decomposition and transformation of organic matter, carried out 

 primarily by microorganisms, since in these processes oxygen is con- 

 sumed and carbon dioxide is released. 



OXYGEN 



Oxygen is needed by almost all organisms to make available the 

 energy contained in organic food materials. The great majority of 

 plants and animals use free oxygen from the air or from the water for 

 the oxidation of organic substances; these are aerobic organisms. 



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