402 Mineral Nutrition of Plants 



complicated, many-step process closely integrated with the metabolic 

 activities of the chlorophyllous cell. Of particular interest to the subject 

 of this paper are evidences that phosphorus turnover and nitrate reduc- 

 tion are part of the photosynthetic process. 



Before carbon dioxide enters the reduction phases of photosynthesis, 

 it is fixed in the cytoplasm outside the chloroplast. The fixation is a 

 nonphotochemical, enzymatic carboxylation in which organic acids are 

 formed. This first step, known as carbon dioxide fixation, is not the 

 characteristic feature of photosynthesis nor is it confined to photo- 

 synthesizing cells as it occurs in both autotrophic and heterotrophic 

 organisms. After fixation, the carboxylated product is carried through 

 the various stages of reduction; a hexose derivative usually appears as 

 the end product. 



The most characteristic feature of photosynthesis is the photochemi- 

 cal fission of water with the evolution of oxygen. It has been shown 

 recently that isolated chloroplasts are capable of carrying out this stage 

 of the reaction as a photochemical process. All the steps, from the initial 

 fixation to the final fission of water and reduction of carbon dioxide, 

 involve energy transfers which must require the storage of energy for 

 very short periods in an available form in order to prevent back reac- 

 tions and allow time for the next steps. 



Phosphorus 



The animal biochemists have shown that the energy transformation 

 in the conversion of chemical energy to mechanical energy in muscle 

 and the conversion of chemical energy from one form to another in the 

 processes of respiration occur through the agency of phosphorylated 

 compounds containing high-energy phosphate bonds. Several investiga- 

 tors have presumed that some of the energy storage processes of photo- 

 synthesis may involve similar energy transformations, postulating that 

 inorganic phosphorus would flow into the photosynthesizing system 

 when energy is stored but not used, and be released during energy 

 utilization phases. 



The downgrade flow of energy through respiratory processes has 

 been shown to be carried out in plants, animals, and microorganisms 

 by way of the high-energy phosphate bonds occurring in phosphorylated 



