Nutrition of Green Planf Cells - 163 



Light Energy 

 (Photons) 



Chloroplast 



Electrons 

 Returning 



4^ 



Photolysis \ 



4 (OH 



Photon- 

 Excited 

 Electrons 



-*~e 



Fig. 9-4. In the chloroplasts, light energy is transformed into the chemical potential energy that is stored, 

 mainly, as ATP, TPNH 2 , and FMN-H.,. Note that some of the electrons, after escaping from light-excited chlo- 

 rophyll, finally return to chlorophyll, forming a cycle. Simplified schema, after Arnon. 



excited elections, ejected from chlorophyll, 

 keep returning to the chlorophyll, having 

 discharged their energy in the generating of 

 two new high-energy phosphates (ATP). 



For noncyclic phosphorylation to occur, in 

 contrast, TPN must be added to the medium 

 in which the isolated chloroplasts are operat- 

 ing. This compound, having received elec- 

 trons from chlorophyll (Fig. 9-4), can now act 

 as an acceptor of H+ ion (from water), in 

 which case TPN-H 2 tends to accumulate in 

 the medium. But the electrons that flow back 

 to the chlorophyll via the cytochrome system 

 are derived from OH - ions, as is shown in 

 Figure 9-4. Thus the flow of electrons is not 

 cyclic. However, each electron, as it flows 

 through the cytochrome system, generates 

 energy for "charging up" one more molecule 

 of ATP. 



Certain bacteria, utilizing pigments other 

 than chlorophyll, can draw upon light as the 

 source for building up their reserves of chem- 

 ical potential energy. These more primitive 

 photochemical systems are important in rela- 



tion to the evolution of plant life and they 

 will be considered briefly later (p. 187) in that 

 connection. 



Dark Reactions: Assimilation COo. As cur- 

 rently conceived, the net effect of light 

 energy, operating through mechanisms within 

 the chloroplast, is to build up the cellular 

 reserves of certain energy-rich molecules, par- 

 ticularly ATP and TPN-H,. Moreover, it is 

 now known that even isolated chloroplasts 

 can continue to utilize C0 2 for the synthesis 

 of glucose and other compounds in total 

 darkness, if adequate amounts of ATP and 

 TPN-H 2 are provided. Arnon in fact; has 

 obtained similar results with chloroplasts 

 from which all the chlorophyll had been re- 

 moved. 



The problem of tracing carbon from the 

 time when it enters the green plant cell as 

 C0 2 until it is built into the structure of 

 glucose and other organic molecules repre- 

 sents a herculean project. The solution of 

 this problem, which now appears to be al- 

 most complete, required the cooperation of 



