156 PROBLEMS OF PHOTOSYNTHESIS 



tremely difficult to separate COo fixation upon illumination from that which 

 normally takes place in the dark. Investigations by Miyachi et al. (39) clearly 

 show how slight our knowledge is with respect to CO2 fixation in the dark aero- 

 bically and anaerobically and how careful one has to be in concluding whether 

 CO2 fixation observed in a given experiment is really related to photosynthe- 

 sis or to other metabolic conversions. 



Wood (63) and Utter and Wood (54) point out that it must be far from 

 easy to determine the first product of photosynthesis in which labeled carbon 

 is incorporated. The sequence of the products which occur in the carbon 

 cycle after short illumination is deduced from the measured activities of these 

 products. When, for instance, CO2 is reduced to (CH2O) via the compounds 

 Al, Ao etc. Al must attain the same specific activity as CO2 before Ao does, 

 A2 must reach it before As, etc. This has been correctly observed by Calvin 

 (21). Figure 59 shows a theoretical example by Utter and Wood. CO2 can 

 be reduced to (CH2O) in three ways, according to the sequences Ai, A^ etc., 



-► Bi < ► B2 ■* *■ B3 < 1 



i 



r 



CO2 •*-- Al ■< *■ A2 •« ► A3 •* A4 < ►(CH2O) 



t 

 ► Ci •< *■ C2 < »■ Cs ••--' 



I 



Fig. 59. Different ways of '^C incorporation in photosynthesis (Wood). 



^1, B'l etc. and Ci, d etc. The reaction times are represented by the lengths 

 of the arrows. It is supposed that by decreasing the time of illumination the 

 sequences B and C will become more and more improbable so that finally 

 only sequence A will come into consideration. This, however, does not 

 mean that sequence A is the most important. It may be possible that in se- 

 quence A the reaction A^ -^ A^ proceeds so slowly that the over-all speed of 

 the reaction CO2 -^ (CH2O) in sequence A might be of the same magnitude 

 as that of the sequences B and C. \i Ai represents 3-phosphoglyceric acid, 

 labeled carbon certainly appears very rapidly in the carboxyl group of this 

 acid, but this provides no evidence that 3-phosphoglyceric acid is the primary 

 product on the way to (CHoO) and that all other ways leading to this end 

 product are excluded. As a matter of fact, Badin and Calvin (5) observed, 

 at low light intensities, carboxylation to malic acid as the first reacdon (see 

 § 46). Thus, in any case CO2 fixation can occur in two ways: at high in- 

 tensities via 3-phosphoglyceric acid and at low intensities via carboxylation 

 to a dicarboxylic acid. There is no reason to believe that other reactions 

 may not be involved if the experimental conditions are changed. 



In investigations with radioactive isotopes measurement is made not of the 

 specific activities but of the counts per minute (cpm). However, the rela- 

 tionship between the specific activity of a substance produced and that of 

 ^•*C02 is of much more importance than the cpm values (63). When, for 

 instance, 1 /xmole yfj gives 2000 cpm and 0.1 ^mole A^ 1000 cpm, the latter 



