126 PROBLEMS OF PHOTOSYNTHESIS 



assume reactions b and c to be intermediate in the quinone reduction by 

 Chlorella. Two kinds of reactions had to be accepted, one with the hght 

 reaction c and one unknown hght reaction of another type. Such a con- 

 clusion is of course far from satisfactory and very improbable (61, 67). 

 Warburg and Krippahl (63) found that COo is indispensable to the quinone 

 reaction in such small amounts that its necessity had been overlooked in hun- 

 dreds of papers since 1944, but, nevertheless, in amounts still great enough to 

 enable the COo pressures to be measured. However, in his study of the 

 Hill reaction with quinone in 1948, Boyle (16) observed that the presence of 

 very small amounts of CO2 was indispensable. Extracts of young spinach 

 leaves were used. The gas phase contained commercial nitrogen only. 

 In the presence of some KOH in the side-arm of the manometer vessel, traces 

 of COo were removed with the result that Oo production ceased. With- 

 out KOH, the traces of COo in the commercial nitrogen were sufficient to 

 obtain considerable gas pressures due to Oo production. These findings 

 could not be confirmed by either Clendenning and Gorham (19) or Hill (26) 

 so that the necessity for intermediary CO2 in the quinone reaction was re- 

 jected. Today, we know why Boyle's experiments could not be reproduced 

 (63). Spinach grana are very rich in oxalic acid; on illumination this is 

 oxidized to COo by quinone 



light 

 quinone + oxalic acid -^ hydroquinone + 2CO2 



In spite of KOH, illuminated spinach grana normally develop so much CO2 

 that sufficient CO2 still remains in the gas phase. Thus, spinach grana are 

 certainly not an appropriate test object for proving the necessity of COo. 

 It is highly probable that Boyle's extracts had, by chance, a low oxalic acid 

 content. It is obvious that if extracts with a high oxalic acid content are 

 employed, no difference can be expected, irrespective of whether KOH is 

 used or not (63). 



The necessity of CO2 means that CO2 is an intermediate product in the 

 quinone reaction as described in the above equations. It is not paradoxical 

 that with the simultaneous addition of quinone and COo only quinone and 

 not CO2 is used up in the over-all reaction. Quinone inhibits O2 respiration 

 but only because it is replaced by "quinone respiration" which energetically 

 permits reaction b. The above equations show that "quinone respiration" 

 produces as much CO2 as disappears in light, so that the total COo turnover 

 does not appear in the over-all reaction. Thus, reaction c ceases when all 

 the quinone is completely reduced, as no more energy is available for reaction 

 b. 



These conclusions are in good accord with the ferric cyanide experiments. 

 In contrast to quinone, the iron salt does not inhibit respiration of Chlorella. 

 When ferric cyanide and COo are added to Chlorella, the end value of O2 

 produced in light is equivalent to the sum of ferric cyanide and CO2, as, under 



