112 



THE CELL AND PROTOPLASM 



as H2A. The mere fact that there are 

 species of purple bacteria that cL,nnot use 

 formic acid at all indicates that in the 

 metabolism of such species formic acid 

 cannot replace carbon dioxide as the ulti- 

 mate hydrogen acceptor, and consequently 

 that for these strains a reduction of carbon 

 dioxide is not likely to proceed via formic 

 acid. From the point of view of compara- 

 tive biochemistry it is, then, equally pos- 

 sible that the carbon dioxide-reduction 

 mechanism employed by these species may 

 also function in the photosynthetic reac- 

 tions of other organisms. 



"Comparative biochemistry" also shows 

 that the various photosyntheses are not 

 likely to function through a photo-activa- 

 tion of the H2A component. The fact that 

 the decomposition of H2A proceeds also in 

 complete darkness, but with oxygen as 

 acceptor rules this out. The peculiarities 

 of the fatty acid decomposition already 

 mentioned for mixtures, such as acetic and 

 propionic acid, hold good also for the 

 photosynthetic processes. Here again, at 

 suflSciently high light intensity, the rate 

 of decomposition of a propionic and acetic 

 acid mixture may equal the sum total of 

 the rates at which each of the components 

 is utilized. This would lead to the already 

 probable conclusion that the mechanism of 

 the H2A decomposition is independent of 

 the nature of the final acceptor, and, since 

 light is necessary only in case carbon diox- 

 ide acts in this capacity, it follows that 

 this mechanism functions without the co- 

 operation of radiant energy. 



In view of the fact that many biochem- 

 ical processes are now known in which 

 carbon dioxide can be reduced in the dark, 

 it is even doubtful whether this compound 

 plays any direct part in the photochemical 

 reactions involved in photosynthesis; the 

 probability has to be seriously considered 

 that the reduction of this substance takes 

 place only after its incorporation into some 

 organic molecule, and as a result of reduc- 

 ing systems, active in the dark, but gen- 

 erated in the light (Thimann 1938; Gaffron 

 1939). 



It will be clear that these few examples 



have shown how ''comparative biochem- 

 istry" may aid in distinguishing essential 

 and general principles from secondary and 

 fortuitous phenomena, and how a variety 

 of processes which are at first sight entirely 

 unrelated can be regarded from a central 

 point of view. The occurrence of the al- 

 most unlimited variety of metabolic reac- 

 tions among micro-organisms makes it 

 possible to formulate ever more clearly 

 such general principles; but this fact also 

 enables the investigator to select for bio- 

 chemical and physiological problems such 

 organisms as show the phenomenon under 

 investigation in its simplest form, unen- 

 cumbered by the simultaneous occurrence 

 of various side-processes. 



This is clearly brought out by the rela- 

 tionships that have recently been discov- 

 ered between various growth factors for 

 micro-organisms, the vitamin requirements 

 of the higher animals, and the important 

 enzyme systems operative in biochemical 

 reactions. The intensely interesting studies 

 of the Lwoffs and co-workers, as well as 

 those of Knight et al., have made it evident 

 how much can still be learned from studies 

 on microbial metabolism, and how pro- 

 found an influence such studies may have 

 on the problems of the general physiologist 

 and biochemist (Lwoff 1938). 



One of the simplest systems may here be 

 mentioned briefly to demonstrate the pres- 

 ent trends. For many years it has been 

 known that common molds, as well as 

 well as higher plants, require small 

 amounts of copper for normal growth. 

 The recent investigations of Kubowitz 

 (1937), which have proved the existence 

 of copper-proteids as oxidizing enzymes in 

 plants, make it plausible that it is for the 

 elaboration of such enzymes that the cop- 

 per is needed. Bortels (1930, 1936) found 

 that the free-living, nitrogen-fixing organ- 

 ism, Azotohacter, requires small amounts 

 of molybdenum. This fact, amply corrob- 

 orated (Kluyver and van Eeenen 1933; 

 Burk and Horner 1935; Krzemieniewski 

 and Kovats 1936; Kovats 1938), has led to 

 the supposition that some molybdenum- 

 containing bio-catalyst may be needed by 



