PHOTOSYNTHETIC AND RESPIRATORY FIXATION OF C*02 1635 



unrelated to photosynthesis. The acceptor responsible for the slow uptake of C(ll) 

 in the dark could be altogether different from that active in photosynthesis. 



Frenkel's observation that the C* uptake in the dark takes place in the cytoplasm, 

 and not in chloroplasts, has been related on page 66 (Vol. I) to the incapacity of isolated 

 chloroplasts to use carbon dioxide as hydrogen acceptor in light. This seems suggestive, 

 but since the early products of carbon dioxide fixation in photosj'nthesis are water- 

 soluble, it is possible for them to be primarily formed in the chloroplasts and yet to dif- 

 fuse (or be eluted) into the cell sap cytoplasm fraction during the maceration and frac- 

 tionation of the cell material. Clendenning and Gorham (1952) found that in multi- 

 cellular algae {Nitella — the organism used by Frenkel — C/iara and Riccia), most of the 

 (C14) taken up in brief periods of photosynthesis —down to 5 seconds— was found in the 

 cell sap; in unicellular algae — Chlorella and Scenedesmus — a larger fraction could be lo- 

 cated in insoluble lipoid and proteinaceous materials from the chloroplast fraction. The 

 question of whether the enzymatic transformations of carbon dioxide in photosynthesis 

 occur inside the chloroplasts, or outside these bodies (in which case they must be medi- 

 ated by a reductant produced photochemically inside the chloroplasts and diffusing out 

 of them), has not been definitely answered by these experiments. The observation of 

 Arnon et. al. (1954), that intact chloroplasts take up C* in light, was described on pp. 

 1537 and 1615. 



Ruben and Kamen's observation that C( 11)02 is taken up by carbo.xylation, with 

 the formation of a water-soluble carboxylic acid, has been confirmed by all the subse- 

 quent investigations with C(14); it is true for photosynthetic as well as for respiratory 

 C* fixation. On the other hand, the conclusion that this acid has a molecular weight 

 of about 1000 appears to be inaccurate for both types of fixation, since the first product 

 of photochemical fixation seems to be phosphoglyceric acid (molecular weight 187), 

 while respiratory fixation is likely to lead to products such as pyruvic or oxalacetic acid, 

 which, too, have low molecular weights. 



The possibility of "respiratory" C* fixation was not fully taken into ac- 

 count also in the early stages of work with the long-lived isotope C(14) by 

 Benson, Calvin and co-workers at Berkeley (1947) . It was first emphasized 

 by Allen, Gest and Kamen (1947), and by Brown, Fager and Gaffron 

 (1948), and subsequently recognized also by the Berkeley group. 



Allen, Gest and Kamen (1947) attempted to distinguish between the two types of 

 C* absorption by recalling an early observation of Ruben, Kamen and Hassid with 

 C(ll) (Vol. I, page 242). The latter had found that in Chlorella the cyanide inhibition 

 of the C* uptake in the dark resembles that of photosynthesis, rather than that of respira- 

 tion (for example 10"^ M KCN reduced the rate of both photosynthesis and dark C* 

 fixation to 0.3% of the normal value, but left respiration almost unchanged). Allen, 

 Gest and Kamen repeated these experiments, using C(14), with both Chlorella and 

 Scenedesmus. (In the second species, respiration is more sensitive to cyanide than 

 photosynthesis, cf. Vol. 1, page 305.) They found that, by the criterion of cyanide sen- 

 sitivity, dark C* fixation in Scenedesmus appears to be related to photosynthesis rather 

 than to respiration, while in Chlorella (in disagreement with earlier observation) the 

 effect of cyanide on dark C* fixation was intermediate between those on photosynthesis 

 and respiration. With Scenedesmus cells starved for 24 hours in darkness, the results 

 were erratic, but with similarly starved Chlorella cells, the effect of cyanide on dark C* 

 fixation was definitely similar to its effect on respiration and quite different from that on 

 photosynthesis. 



Kamen concluded that one part of dark C* fixation is "photosynthetic" and another 



