1418 INDUCTION PHENOMENA CHAP. 33 



of A -002. The latter should therefore proceed, in strong steady light, 10 

 or 100 times more rapidly than in the weak light used by Emerson and 

 Lewis. Nevertheless, we do not observe a decarboxylation of the acceptor 

 (unless the light is excessively strong and pick-up begins to become notice- 

 able). This indicates that the recarboxylation is rapid enough to replace 

 all the A -002 complexes destroyed — both those utilized for photosynthesis, 

 and those merely decomposed into A and CO2. The pick-up experiments 

 in fact show the recarboxylation time of the acceptor to be of the order of 

 10-20 seconds. In the case of the gush, the recarboxylation seems to be at 

 least 100 times slower. 



One may recall in this connection that, according to figure 8.21, the 

 rate of carboxylation in the dark was found to be unexpectedly slow also 

 in experiments with radioactive carbon dioxide. In this case, however, 

 explanations could be sought in the occurrence of carboxylations not re 

 lated to photosynthesis; furthermore, the observed rates may be at least 

 in part those of the replacement of carbon dioxide in a carboxyl group by 

 C*02, rather than of the uptake of C*02 by a free acceptor. Similar 

 explanations are not possible in the case of the gush. Its occurrence in 

 light indicates close connection to the photosynthetic apparatus; since 

 the gush is revealed by manometric measurements, it is not an exchange 

 phenomenon. 



To sum up: The mechanism of the reversible carbon dioxide gush 

 described by Emerson and Lewis is not clear, and its attribution to a shift 

 of the carboxylation equilibrium of acceptor A, although plausible, remains 

 uncertain. 



Since this section was first written, the question of the hypothetical 

 carbon dioxide acceptor in photosynthesis. A, has been brought closer to 

 direct experimental elucidation by continued studies of the uptake and 

 distribution of radiocarbon in photosynthesis (to be described in chapter 

 36). These studies indicate that one — and perhaps even the only — 

 carboxylation which is directly related to photosynthesis leads to phospho- 

 glyceric acid (PGA). The carboxylation substrate is, however, not yet 

 identified ; it seems likely that it is not a C2 compound (which could give a 

 C3 acid by simple addition), but a longer-chain compound (a pentose) 

 that breaks into two parts (e. g., C3 and C3) upon taking up a CO2 molecule. 

 The reaction that leads to PGA appears to be a carboxylation coupled with a 

 TPN-specific oxidation-reduction. Finally, the compound A, which gives 

 PGA by coupled carboxylation and chain fission, is itself an intermediate 

 product of photosynthesis (sinceit rapidly incorporates radiocarbon) . If this 

 conclusion — which we have tentatively mentioned before — proves to be 

 correct for all (or a large part) of the CO2 acceptor in the cell, this would 

 mean the likelihood of a deficiency of this acceptor at the beginning of a 



