66 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



The reaction responsible for the inhibition of photosynthesis by cyanide 

 probably is associated with the entry of carbon dioxide into the photo- 

 synthetic apparatus (cf. Chapter 12). Since carbon dioxide does not 

 participate in Hill's reaction, it appears natural that this reaction is not 

 affected by cyanide. (Less easily understandable is the indifference of 

 this reaction to hydroxylamine, which, according to page 313, is a 

 specific poison for the oxygen-liberating enzymatic system in photo- 

 synthesis.) Urethans, on the other hand, which inhibit photosynthesis 

 in the " light-Hmited " as well as in the "enzyme-limited" state, also 

 inhibit Hill's reaction. Ethylurethane, for example, causes a 50% in- 

 hibition in a 0.6% solution, while phenylurethan produces the same 

 effect in a concentration of only 4 X 10"^%. (The ratio of the efficiencies 

 is 1500, as compared with 450 in true photosynthesis, cf. table 12. VIII). 



The temperature coefficient of hemoglobin oxidation by ferric oxalate 

 in the presence of chloroplasts is 1.3 to 1.4 (while that of methemoglobin 

 reduction is 1.8 to 1.9). This value is smaller than the temperature 

 coefficient of photosynthesis in the light-saturated state ( > 2) (cf. Vol. II, 

 Chapter 31), but figure 6 shows that the conditions of Hill's measurements 

 were not those of complete light saturation. 



These results cannot be fully appreciated in the present chapter, 

 because the corresponding relationships in photosynthesis will first be 

 discussed in chapters 12, 28 and 31. The assumption that Hill's reaction 

 represents "one half of complete photosynthesis" (photochemical oxi- 

 dation of water, with ferric oxalate as a substitute oxidant taking the 

 place of carbon dioxide), seems to be in agreement with most observations 

 (the most notable exception being the insensitivity of Hill's reaction to 

 hydroxylamine). If this interpretation is correct, it means that broken 

 or dried chloroplasts retain an important part of their normal photo- 

 catalytic capacity — they can still produce oxygen from water in light. 

 However, they are not able any more to transfer hydrogen to carbon 

 dioxide as acceptor, and thus cannot synthesize organic matter. 



In chapter 7, we shall discuss several alternative theories of the 

 primary photochemical reaction in photosynthesis — some envisaging a 

 direct participation of carbon dioxide in this reaction, others interpreting 

 it as a photoxidation of water, with the hydrogen being first transferred 

 to an unknown intermediary acceptor. Hill's experiments fit best into 

 this second picture. If, in living cells, the hydrogen atoms find their 

 way from the primary acceptor to carbon dioxide, with the help of a 

 nonphotochemical enzymatic apparatus, it appears plausible that this 

 apparatus may be destroyed by drying or crushing the cells, while the 

 photochemical mechanism remains more or less intact. 



A recent observation of Frenkel (cf. page 204) makes it probable 

 (but by no means certain) that the first transformation of carbon dioxide 



