42 THE METABOLISM OF ALGAE 



confirmation from studies with radioactive carbon as 

 tracer. 



Since the first recognizable product of photosynthesis is 

 identical with an intermediate in glycolysis, a process which 

 is known to consist of fully reversible steps, it is reasonable 

 to suppose that carbohydrate formed in photosynthesis is 

 elaborated by simple reversal of the normal glycolytic 

 mechanism. Among the products found to be labelled in algae 

 after periods of photosynthesis in the presence of radio- 

 active carbon dioxide of 15 to 60 seconds duration are triose 

 phosphates, hexose phosphates, hexose diphosphate and 

 sucrose. This rapid appearance of radioactivity in the sugars 

 shows that there is a close connexion between the photo- 

 synthetic mechanism and that responsible for the synthesis 

 of these compounds and the fact that several of the labelled 

 phosphate esters are identical with those involved in glyco- 

 lysis suggests that the pathway of synthesis is the expected 

 one. Furthermore, the distribution of radioactivity in the 

 3 : 4, 2 : 5 and the i : 6 carbons in the hexose (see formula 

 on p. 91) corresponds with that found in the carboxyl and 

 in the 2- and 3-carbons respectively of phosphoglyceric 

 acid as would be expected in hexoses formed by condensa- 

 tion of C3 substances derived from this compound.^- In 

 the synthesis of sugars from phosphoglyceric acid a reduc- 

 tion of the carboxyl to an aldehyde group occurs. The 

 source of the hydrogen necessary for this reduction may 

 be the photochemical reaction itself, but, as we have already 

 seen, it is more probable that it is obtained by purely 

 chemical reactions at the expense of a proportion of the 

 products of photosynthesis. 



The accumulation of radioactivity in pyruvic and malic 

 acids during short periods of photosynthesis by Scenedesmus 

 and Chlorella in the presence of radioactive carbon dioxide 

 has already been noted. Radioactivity appears more slowly 

 in succinic, fumaric and citric acids.^^' ^^ These acids are 

 all concerned in the tricarboxylic acid cycle (Fig. 2) and 

 this evidence indicates that in these algae carbon fixed in 

 photosynthesis may enter the cycle in the expected way 

 through pyruvic acid (Fig. 10). 



