206 PLANT BIOCHEMISTRY 



PHOTOSYNTHESIS 



The prolonged existence of terrestrial life depends upon utilization 

 of energy from the inanimate environment. It is believed that all 

 species during the early periods of life on earth extracted energy from 

 compounds made by purely geochemical processes during the preced- 

 ing ages. Ultimately the substances containing useful quantities of 

 chemical energy would be used up except for residues buried deeply 

 or restricted to regions of prohibitively high or low temperatures. Life 

 thus limited exclusively to chemical energy must necessarily cease or 

 dwindle to a very low level compatible with the slow geological syn- 

 thesis of substrates. 



However, during the course of evolution another supply of energy 

 came into use. Sunlight of the visible range was converted into chemi- 

 cal energy by new species carrying on a metabolic process called photo- 

 synthesis, named from the light-induced syntheses taking place. Mod- 

 ern photosynthetic plants require only simple inorganic materials and 

 light. With the radiation absorbed they meet the fundamental require- 

 ment for energy needed in the syntheses of the organic compounds on 

 which life depends. 



All species capable of using carbon dioxide or bicarbonate ion as 

 their sole source of carbon are called autotrophs. This group includes 

 the ordinary green plants, algae, photosynthetic bacteria, and certain 

 bacteria using chemical energy stored in inorganic compounds like 

 hydrogen sulfide. Algae and the higher plants appear to employ the 

 same or very similar mechanisms for photosynthesis, and the discus- 

 sion here will be limited to this system. 



All other living species require a continuing supply of from one to 

 many organic substances. This requirement makes such forms (called 

 heterotrophs) dependent upon photosynthesis for the energy stored in 

 these organic compounds. The world is now essentially devoid of the 

 original supply of foodstuffs produced geologically. The term hetero- 

 trophs includes the entire animal kingdom, most bacteria, and 

 saprophytic (non-photosynthetic) plants like the fungi. Hence photo- 

 synthesis occupies a central position in the metabolism of plants and 

 indirectly in that of animals. 



Although photosynthesis leads to many products evolved by many 

 reactions in the plant, the overall process may be indicated by 



6GO2 + 6H2O /'^^' > CeHisOe + 60. AF = +688,000 cal. 



pigments , 

 enzymes glucose 



