QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1087 

 or, in the notation used in chapter 28 (pp. 1032, 1036, etc.): 



, . ,TT ^ forward reaction back reaction 



(29.2a) ACOa-Chl-A'HjO > AHCO.Chl-A'OH > 



light 



A'H,0*Chl.ACOf ^'''^'''"''°" A-Chl-A' + H^O + A + CO^ 



Here, asterisks indicate that the compounds formed by back reactions contain consider- 

 able excess energy and therefore tend to decompose into their constituents. These back 

 reactions normally occur only in saturating light (they are, in fact, supposed to be re- 

 sponsible for saturation); but in the first moment of illumination, practically all 

 AHCO2 formed (even the small amounts produced in weak light) undergoes back reac- 

 tion, because during this "induction period," certain catalysts have not yet been "re- 

 activated," and are unable to take care of the products of the first photochemical reac- 

 tion. 



A difficulty of this hypothesis is that, even with a 100% yield of the back reaction, 

 the rate of production of ACO2 in weak light must be small compared with the same 

 rate in strongly oversaturating light. In the latter case, all intermediates formed in ex- 

 cess of the saturating rate are supposed to undergo back reactions; and yet, under 

 appropriate supply conditions, no carbon dioxide limitation is observed, indicating that 

 either the ACO2 complexes formed by back reactions do not dissociate, or the recombina- 

 tion of A and CO2 is so fast as to prevent any exhaustion of ACO2 (in other words, the 

 rate ceiling imposed by the formation of ACO2 must be high compared with the full rate 

 of the primary photochemical process and not only compared with the rate of the finish- 

 ing dark reaction). 



The total volume of the gush — which is about equivalent to the quantity of chloro- 

 phyll present in the cells — is in agreement with Franck's hypothesis; but the slow re- 

 absorption of carbon dioxide in the dark (c/. fig. 29.3B, p. 1092) requires an explana- 

 tion, since the time course of the "pick-up" (c/. Vol. I, fig. 22) indicates that the car- 

 boxylation equilibrium C'Oo + A -^ ACO2 usually is established in a few seconds. It 

 may be noted that a similar difficulty was encountered in the attempt to attribute the 

 uptake of radioactive carbon dioxide in the dark (fig. 21, Vol. I) to the same carboxyla- 

 tion process. Another problem is presented by the necessity of a high carbon dioxide 

 concentration (>o%) for the "saturation" of the gush, since the shape of the carbon 

 dioxide curves of photosynthesis indicates that the acceptor must be saturated with 

 carbon dioxide even below 0.1% CO2. 



These discrepancies suggest that perhaps the gush and its reversal in the dark are 

 manifestations of a carbon dioxide metabolism related to respiration and fermentation 

 rather than to the first step of photosynthesis; but this hypothesis, in turn, fails to ex- 

 plain the apparent close relation of the carbon dioxide liberated in the gush to the 

 chlorophyll complex (without such a relationship, a photochemical liberation of carbon 

 dioxide with a high quantum yield would be difficult to understand). 



The "cross-linking" of respiration and photosynthesis at an intermediate reduction 

 level, between CO2 (L = 0) and carbohydrate (L = 1), e.g., on the level of oxalacetic 

 or malic acids (L = 0.625 and 0.75, respectively), hypothesized by Calvin and co-workers 

 (chapter 36), if confirmed, could explain how respiratory decarboxylations might be af- 

 fected by reactions in the photochemical reaction sequence. 



A carbon dioxide burst of the volume observed by Emerson and Lewis 

 would be largely absorbed in carbonate buffers. As it was, Warburg and 

 Negelein had decided that acid solutions (e. g., water equilibrated with an 



