44 THE METABOLISM OF ALGAE 



glycine seems to be closely related to the Cg acceptor. 

 Radioactivity accumulates more slowly in other amino- 

 acids, e.g. threonine, phenylalanine, glutamic acid and 

 tyrosine^^ which are not so readily derived from the im- 

 mediate products of photosynthesis. 



That the first recognizable product of photosynthesis is 

 a phosphorylated compound is evidence of a connexion 

 between this process and the phosphorylation cycles in- 

 volved in respiration and cell synthesis. It seems unlikely 

 on theoretical grounds that the chemical energy produced 

 by the photochemical reaction can first appear in the form 

 of high energy phosphate linkages^'*^ and the phosphoryla- 

 tions involved in photosynthesis are more probably brought 

 about by secondary and purely chemical reactions. 



All the substances that have so far been considered occur 

 in the water-soluble fractions of the algae. Considerable 

 proportions of tracer, however, appear in the benzene- 

 soluble and water-insoluble fractions in the course of quite 

 short periods of photosynthesis (Fig. ii). This demon- 

 strates the speed with which the products of photosynthesis 

 are incorporated in compounds of high molecular weight. 

 The tracer appearing in protein is probably that in the 

 amino-acids mentioned above as being closely related to 

 the immediate products of photosynthesis. 



The radioactivity of the benzene-soluble fraction from 

 cells which have been illuminated for only 40 seconds in 

 the presence of radioactive carbon dioxide has been shown 

 not to be due to contamination with water-soluble sub- 

 stances.^^ The chlorophylls remain free from tracer but 

 tracer carbon is distributed uniformly between the un- 

 saponifiable materials, the saturated and unsaturated fatty 

 acids, and the water-soluble saponification products. ^^ 

 Fatty acids are probably synthesized from Cg units derived 

 from acetic acid, or a derivative of this, most probably 

 arising by oxidative decarboxylation of pyruvic acid. 

 Evidence for this comes from experiments in which it has 

 been shown that in the light, whether carbon dioxide is 

 present or not, acetate is converted by Scenedesmtis to fats 

 as well as to tricarboxylic acid cycle intermediates.^- Algal 



