130 



PROBLEMS OF PHOTOSYNTHESIS 



light 



■* 2 ATP + 2 TPNH + 2 H+ + Oo 



2 ADP + 2 ph + 2 TPN+ + 2 H.O 



This reaction, which is the over-all reaction of both the light reactions indi- 

 cated above, shows that only a part of the light energy would be used for 

 ATP production (conversion into phosphate bond energy) and the remaining 

 part would be used for the reduction of TPN^. 



It is to be noted that in Lehninger's reaction of oxidative phosphorylation 

 (§ 50) ATP formation is accompanied by oxidation of reduced phosphopyri- 

 dine nucleotide. In Arnon's reaction ATP formation is accompanied by 

 reduction of oxidized phosphopyridine nucleotide. However, Marre and 

 Forti (38) found that reduction of TPN+ in illuminated chloroplasts is not 

 dependent on the production or presence of ATP. 



Thus, light energy provides ATP and TPNH. Both substances form what 

 Arnon calls the assimilatory power. He considers them to be the true "first 

 products of photosynthesis," a term usually applied to products formed during 

 CO2 fixation (see §53). 



More recently, Arnon (5) succeeded in combining his various findings into 

 an interesting hypothesis of light phosphorylation. Photosynthetic phos- 

 phorylation as shown in reaction a, in which ATP is the sole product of the 

 light action, is designated as cyclic photophosphorylaiion. The process shown in 

 the over-all reaction of a and b, in which light energy is used for the produc- 

 tion of ATP and TPNH is termed non-cyclic photophosphorylation. This process, 

 which provides Oo and assimilatory power for the reduction of COo, must be 

 considered to be the light phase of photosynthesis in green plants. In cyclic 

 photophosphorylation light energy is also converted into chemical energy, 

 but it does not seem to be related to true photosynthesis in green plants as 

 neither Oo production nor CO^ fixation is possible. This type of photophos- 

 phorylation is peculiar to the photosynthetic bacteria. Their chromatophores 

 cannot carry out non-cyclic photophosphorylation. They follow the vitamin 

 K pathway of cyclic photophosphorylation (see Fig. 48). After absorption 



NON-CYCLIC ELECTRON TRANSPORT 



CO3 ASSIMILATION 



CYCLIC ELECTRON TRANSPORT 



REDUCTASE 



VIT.K 



-►TPN* 



I. 



FMN 



i 



► TPNH q: 



' ' LU 



o, 



H^ ^ 



\ DC 



H2O o 



CHL-4 



ML ^ y 



t..X- 



CYT«^ CYT„ V^^ 



OH" 



LIGHT 



ph. 



ADP 



► ATP 



CD 



< 



C02 



-► (CH20) 



Fig. 48. Arnon's scheme for photosynthesis 



