Viii PREFACE 



carbonate) appears as a complication in the interpretation of these observa- 

 tions, particularly if one recalls the complex action of ascorbic acid on the 

 Hill reaction (c/., p. 1568) and its role in the Krasnovsky reaction (cf., pp. 

 1514-1522) . The difference between the C(14)-tagged intermediates identi- 

 fied in Arnon's photosynthesizing chloroplast preparations and those found 

 by Calvin and co-workers in intact photosynthesizing cells also needs clari- 

 fication. 



In any case, the observations of Arnon, and of Thomas, bring nearer to 

 closure the gap between the photochemical activities of chloroplast prepara- 

 tions and whole cells, and between the Hill reaction and true photosynthesis 

 (first narrowed by the coupling of Hill reaction to the malic enzyme system 

 via pyridine nucleotides, described on pp. 1578-1585). 



Vishniac^ reported that the colorless "acetone powder," prepared from 

 chloroplasts, can be restored to photochemical activity (reduction of TPN 

 in light) by addition of chlorophyll solution. Similar activation of macer- 

 ates from white leaves by added chlorophyll was described by Rodrigo^; 

 the material obtained in this way was weakly fluorescent and had an ab- 

 sorption maximum at 678 m/i. 



Kok^ has made new flash light experiments, and suggested a simple two- 

 step enzymatic mechanism, permitting the observations made with instan- 

 taneous flashes (Emerson and Arnold) to be reconciled with those ob- 

 tained by means of flashes lasting several milliseconds (Tamiya). This 

 mechanism, if correct, means that of the two constants previously derived 

 from flash light experiments, one, F™^^ (- 5 X 10 -^^ Chlo), retains its 

 significance as titer (multiplied by a simple fraction, 1/n) of a "stabilizing" 

 enzyme; but that the other, designated in Chapter 34 as fc^A (50 sec."^) 

 loses its significance as rate constant of the same enzyme, and becomes a 

 function of the rate constants of the two enzymes, catalyzing the two reac- 

 tion steps; under certain conditions, it may be practically equal to the rate 

 constant of the second of them. The course of the reaction decay in the 

 dark period is, in this case, more complex than that expected for a simple 

 first order reaction— in agreement with the flash light findings of Gilmour, 

 and Kok, and with the chemiluminescence data of Arnold and Strehler. 



New contribution to the minimum quantum requirement -problem were 

 published by Yuan, Evans, and Daniels,'^ and by Bassham, Shibata, and 

 Calvin'"; the first-named study supported the view that this requirement 



8 Vishniac, W., Gatlinburg Conference on Photosynthesis of the NAS, Oct. 1955 

 (in press). 



7 Rodrigo, T. A., Thesis, Univ. of Utrecht, 1955. 



« Kok, B., Gatlinburg Conference on Photosynthesis of the NAS, Oct. 1955 (in 

 press). 



9 Yuan, E. L., Evans, R. W., and Daniels, F., Biochim. et Biophys. Acta, 17, 185 



(1955). 



"> Bassham, J. A., Shibata, K., and Calvin, M., ibid., 17, 332 (1955). 



