THE CHEMISTRY OF PHOTOSYNTHESIS 125 



When Chlorella is suspended in an appropriate nitrate medium, and is illu- 

 minated in the absence of CO2 it produces O2 for several hours, according to 

 the following equation 



light 

 NO3- + 2H2O ^ NH3 + OH- + 2 O2 



This reaction, discovered by Warburg and Negelein (66) in 1920, must be 

 considered to be the over-all reaction of a liffht and a dark reaction 



dark 

 dark 

 light 



NO3- + 2 C + 2 HiO -* NH3 + OH- -f 2 CO2 (a) 



2 CO2 -^ 2 CO2* (b) 



2 CO2* ^ 2 C + 2 O2 (c) 



over-all: NO3- + 2 HoO -* NH3 + OH" -f 2 O2 (d) 



In a dark reaction the NO3 ions oxidize C to CO2. After conversion to the 

 loosely bound COo in the photolyte CO2*, this is split in the light. 



It was tentative to explain the reaction with iron salts in the same way. 

 In 1955 Warburg and Krippahl (62) succeeded in distinguishing the following 

 reactions with livina: Chlorella and ferric ions 



overall: 4 Fe3+ + 2 HoO -* 4 Fe2+ + 4 H+ + O2 (d) 



Thus, the Hill reaction can be considered to be an over-all reaction com- 

 posed of dark reactions and a light reaction. The latter is identical with nor- 

 mal photosynthesis. Schwartz (52) and Good (23) came to similar conclu- 

 sions. It is to be noted that the iron reduction with living Chlorella is a rela- 

 tively powerful reaction. In an experiment lasting 45 hours with 20 )ul cells 

 (5 mg dry weight) to which 61 jumole = 20 mg K3Fe(CN)e have been added, 

 61 : 4 = 15 /xmole = 0.66 mg CO2, i.e., 13% of the dry weight of the cells 

 are produced. The addition of the iron salt does practically no damage to 

 the cells, and their normal reproduction is not impaired. 



As already mentioned, Hill used as a reagent ferric oxalate which, after 

 reduction to ferrous oxalate, produces CO2 in the light. Thus, the pres- 

 ence of CO2 was evident so that Hill's original experiments concerned nothing 

 but true photosynthesis. 



In replacing ferric ions by quinone, we would expect the following reactions 



dark: 2 quinone + C + 2 H2O ^^ 2 hydroquinone + CO2 (a) 



dark: CO2 -> CO2* (b) 



light: CO2* ^ C + O2 (c) 



over-all: 2 quinone + 2 H2O — >■ 2 hydroquinone + O2 (d) 



A difficulty arises here insofar as quinone seems to inhibit reactions b and c. 

 The addition of quinone and CO2 stops the production of O2 as soon as the 

 quinone — not the CO2 — is used up. Thus, until now it was im.possible to 



