Chapter 15 



PLANT METABOLISM 



by PROFESSOR R. H. BVRRIS 



Department of Biochemistry, University of Wisconsin 



PHOTOSYNTHESIS 



Man's existence on this planet is directly dependent upon plants, for 

 they furnish him nearly all his food, fuel, and fiber. About a billion 

 tons of organic matter are oxidized on the earth each day. It is obvious 

 that such oxidation would exhaust the world's supply of organic matter 

 in a few years if there were no process to balance the oxidation. Photo- 

 synthesis provides such a balance by reducing carbon dioxide, and the 

 energy for the reduction comes from sunlight. The earth intercepts only 

 one-half billionth part of the energy dissipated by the sun, and by photo- 

 synthesis only a fraction of one per cent of the radiant energy reaching 

 the earth is stored as chemical energy, but this is sufficient to maintain 

 a balance in the carbon cycle of nature (Fig. 15-1). One is forced to 

 the conclusion that, from the standpoint of man's welfare, no chemical 

 reactions surpass the photosynthetic reactions in importance. 





Respiration, 

 burning 



Photosynthesis 



Animals 

 Respiration 



CO, 



Green plants 



Microorganisms 



Decay 



XPhotoreduction, 

 chemautotrophic 

 and heterotrophic 

 fixation of COj 



Fossil fuels, 

 wood, etc. 



Burning 



CO2 



CO2 



Fig. 15-1. Diagram of the carbon cycle in nature. 



The chief reaction for fixing atmospheric carbon dioxide is photosyn- 

 thesis, and carbon dioxide is released to the atmosphere mainly through 



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