115. XII. 19521 



M. Calvin and P. Massini: The Path of Carbon in Photosynthesis 



455 



malic acid decreases as well. The rate of labeling of 

 glutarriic acid is increased greatly after a short induc- 

 tion period; citric acid, which contains little activity 

 during the whole light period, shows a sudden increase 

 in the dark, followed by a slow decrease. The labeling 

 of sucrose continues at the same rate as in light for 

 about 1 min, after which it is stopped almost com- 

 pletely. 



Both experiments gave the same picture for most of 

 the compounds, with the two exceptions: In the second 

 experiment the diphosphate area, which in the first 

 contained almost the same number of counts as phos- 

 phoglyceric acid during the light, had only about 15% 

 of it in this second run. This value dropped to 5% in 

 the dark. The phosphoglyceric acid showed a hardly 

 significant rise in the dark during the first 2 min, but 

 again a slow decrease after 5 min. Although we do not 

 know why in this experiment the concentration of 

 ribulose diphosphate was so low in the light, the co- 

 incidence with the lack of increase of phosphoglyceric 

 acid points to a connection between both effects. 



(5) In the light following the dark, the diphosphates, 

 phosphoglyceric and malic acid increase again. 



^'^ "J^f GLUTAMIC 



60sL-60sO^^HHH^M^^^^H 37 



ACID 



l20sL ^^Hi 10 



60(L 



126 



CITRIC 



60sL 

 60sL-60sO I 

 iZOsL 



I 91 



SUCROSE 



1526 



After one-half hour, the aeration bubbler was taken 

 out and a suitable amount of radioactive bicarbonate 

 (sodium) solution added. (The algae, which were grown 

 in slightly acid medium, had enough buffering capacit\- 

 to convert the bicarbonate to carbon dioxide). The 

 vessel was immediately stoppered and shaken in the 

 light. .After 1 min. the suspension was drained into 



C*mOt4V0RATCS 

 NTS 



I-'ig. 13.— Effect of light and dark on the labelingof glutamic and citric 



acid. 0-1% suspension, light intensity 1-6 < 10' ergs/cm'-s (Numbers: 



counts/min x 10~^ on paper per cui' cells). 



The effect of dark on the labeling of glutamic and 

 citric acid was already reported in an earlier paper^ 

 and studied more closely in the following experiments: 

 0-2 cm' wet packed algae (ChloreUa pyrenoidosa) were 

 suspended in 200 cm' distilled water, illuminated in a 

 flat circular vessel of 1 cm thickness by incandescent 

 lights through an infrared filter (intensity 1 -6 x lO^ergs/ 

 cm^-s) and aerated with 008% carbon dioxide in air. 

 The low concentration of cells was chosen to avoid shad- 

 ing of cells in the suspension, so that during the light 

 period all the cells were illuminated continually. 



• .\I. Calvin, J. Chem. Education SC, 639 (1949). 



a darkened flask, and after another minute poured into 

 four times its volume of boiling alcohol. Control samples 

 were treated in the same way, but kept in the light, in 

 contact with radioactive carbon dioxide for 1 and 2 

 min, respectively. The analysis of the fixed radioacti- 

 vity was performed by paper chromatography and 

 radioautography with the technique already described. 

 The results are shown in Figure 13. 



DiuHssioii 



It has already been pointed out that photosynthesis 

 is not a mere reversal of respiration ; this was supported 

 by the observation that the carbon of newly formed 

 photosynthetic intermediates is not available for res- 

 piration while the light is on'. We may thus represent 

 the relationship between photosynthesis and respira- 

 tion by the following scheme (See Figure 14). The 

 labeling of the Krebs cycle intermediates through the 

 storage products (carbohydrates, fats, proteins) of the 

 cells is a slow process, due to the relatively large size 

 of the storage pools. The fact that the photosynthesis 

 intermediates find their way into the tricarboxylic acid 

 cycle very rapidly after the light is switched off means 

 that there is another connection between the two 

 cycles which is blocked as long as the light is on but 

 becomes accessible in the dark. This was interpreted in 

 earlier work^ in terms of the action of the light in 

 maintaining at low concentration the intermediate re- 

 quired for entry into the tricarboxylic acid cycle. A 

 closer specification of how this is accomplished is now 

 possible since the discovery that alpha-lipoic acid is a 



' M. Calvin, J. Chem. Education J«, 639 (1949). - J. W. Weicl, 

 P. iM. Warrington, and M. Calvin, J. Am. Chcm. See. 73, .lO.^ 

 (1951). 



2 M. Calvin, J. Chom. Education J6, 630 (1949). 



89 



