1678 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



The malic acid tagging curve also appears, in fig. 36.18, to have a finite 

 slope at ^ = 0, indicating the possibility of its formation by uptake of ex- 

 ternal CO2 without sizable accumulation of the label in a "reservoir" (such 

 as oxalacetic acid). However, here, as in the case of pyruvic acid, the con- 

 clusion is not binding. Peculiar in the case of malic acid is the sharp in- 

 crease in the slope of the curve about one minute after the beginning of 

 exposure. The steeper slope is equal to that of PGA, (fig. 36.20) ; it can 

 be taken as indicative of the formation of malic acid, at this stage, by car- 

 boxylation of compounds derived from phosphoglyceric acid. 



TIME (MIN.) 



Fig. 36.21. Distribution of C(14) label between several sugars as function 

 of time of photosynthesis (Scenedesmus, 15° C.) (after Benson et al. 1952). 



The finite initial rate of tagging of MA was still considered by Benson et al. (1952) 

 as evidence of an independent "second carboxylation" leading to malic acid, probably 

 through rapid isotopic equiUbration with a Ct compound in the main photosynthetic 

 sequence (oxalacetic acid?). A tagging curve obtained at 2° C. also indicated a finite 

 initial rate of C* incorporation in malic acid. From the shape of the curve showing the 

 over-all rate of tagging as function of time (which, after a single addition of C*02, must 

 mean a function of decreasing CO2 concentration), it was conjectured that the Ci + C2 

 — »■ C3 carboxjdation required a higher CO2 concentration to maintain its saturation rate 

 than the Ci + C3 ^- C4 carboxylation. 



Among the sugars monophosphates (figs. 36.17 and 36.19), those of 

 fructose and seduheptulose also appeared to be tagged at a finite rate at 

 zero time, while tagged mannose and glucose came up later (cf. also fig. 

 36.21). 



After the single "shot" of labelled C* begins to get exhausted in the 

 medium, the tagged metabolic intermediates are used up by equilibration 

 with the major carbon storage reservoirs in the plant. Figures 36.19 and 

 36.20 show that glucose monophosphate is the first tagged compound to 

 show a decline in activity; PGA seems to be the last one (figs. 36.16 and 

 36.20). The order of disappearance of the label may be taken as indica- 



