The Path of Carbon in Photosynthesis

The radioactive compounds to be observed when algae or green leaves are allowed to photosynthesize in C1402 for short periods are almost all phosphorylated derivatives of sugars. Of these, phosphate esters of trioses, sedoheptulose, and fructose are the first to incorporate Cl4 and are followed closely by ribulose diphosphate, glucose-6-phosphate, and a phosphate of mannose. It has been noted, in earlier papers of this series (l), that on radiograms of the products of photosynthesis a dark area appeared in a position occupied by no known sugar phosphate. Extracts of the area gave glucose on very mild acid hydrolysis or on treatment with a phosphatase preparation. This has hitherto been referred to as an “unknown glucose phosphate.” It was found that this substance was more labile to acid than glucose-l-phosphate, itself a readily hydrolyzable phosphate, and furthermore that other labile glucose derivatives were formed as intermediates during the acid hydrolysis. Accumulation of labeled glucose in this area precedes that of labeled sucrose (1) and suggests a precursor relationship to sucrose synthesis. A closer study of this radioactive area, which becomes appreciable on radiograms only after about 30 seconds of normal photosynthesis by all of the plants studied in this laboratory, has shown that treatment with phosphatase causes liberation of galactose and mannose, as well as glucose. Moreover, at least two weakly radioactive spots with high RF values in phenol were noted, as well as traces of other sugars. When the original spot was extracted from a chromatogram of Scenedesmus (60 seconds in C1402) and rechromatographed, at least eight radioactive areas were found (Figs. 1 and 2) as a result of decomposition. Three of these were due to glucose, galactose, and mannose. Two of the others each gave glucose and galactose, but no mannose, on hydrolysis by phosphatase. This behavior led us to suspect the presence of compounds of the uridine diphosphate