38 THE METABOLISM OF ALGAE I 



evidence has established that this substance or its imme- 

 diate precursor is an intermediate in photosynthesis. ^2, 114 

 Evidence that substances such as pyruvic acid and maHc 

 acid are directly involved in photosynthetic carbon dioxide 

 fixation is less satisfactorv. 



Examination of degradation products of the phospho- 

 glyceric acid produced in short periods (5 to 10 seconds) of 

 photosynthesis with radioactive carbon dioxide shows radio- 

 activity to be almost entirely confined to the carboxyl group. 

 This suggests that the phosphoglyceric acid is formed by 

 the carboxylation of some two carbon (C2) compound. As 

 the period of exposure to the labelled carbon dioxide is 

 lengthened radioactive carbon appears to increasing ex- 

 tents in the 2 and 3 positions in phosphoglyceric acid and 

 after about two minutes' exposure the molecule is uniformly 

 labelled. The Cg acceptor thus appears to be itself derived 

 from phosphoglyceric acid.^^' ^^'* 



These well-established facts point to the existence of a 

 self-multiplying fixation cycle which, each time it is com- 

 pleted, results in the incorporation of three molecules of 

 carbon dioxide and the formation of another molecule of 

 phosphoglyceric acid for each one initially present. One of 

 these phosphoglyceric acid molecules is necessary to main- 

 tain fixation whilst the other may be diverted from the 

 cycle and used for other purposes. 



The nature of the Cg acceptor and the manner of its 

 regeneration from phosphoglyceric acid are as yet unknown. 

 There is no evidence that the acceptor is formed by con- 

 densation of two Ci molecules, but there are indications 

 that it is derived from C3 or C4 compounds. Thus, if plants 

 are illuminated in the presence of radioactive carbon dioxide 

 for short periods so that labelled C3 and C4 substances are 

 formed, a further period of illumination in the absence of 

 carbon dioxide causes the disappearance of such substances 

 and the appearance of radioactive glycollic acid and glycine, 

 both C2 compounds. This suggests a close relationship 

 between the two latter compounds and the C2 acceptor. ^^ 

 As an example of the type of fixation cycle which may 

 operate, a scheme may be given in which it is supposed 



