CHLOROPHYLL AND CARBON DIOXIDE 455 



the two sorption phenomena do not interfere with each other. If we assume, with 

 Hanson, that hydration is associated with an enohzation in position 9, hydrated mole- 

 cules should be incapable of absorbing carbon dioxide in position 10. Perhaps the water 

 molecules are attracted by the cyclopentanone ring, while the carbon dioxide molecules 

 are bound by the magnesium in chlorophyll or by the imino groups in pheophytin. 



It must be realized that, even if the existence of a stoichiometric 

 ratio between chlorophyll and carbon dioxide (in the saturated state) 

 were definitely established (so far, it is only made plausible by extrapo- 

 lation), this would not prove that the binding of carbon dioxide is due 

 to a true chemical reaction (e. g., of one of the reactions 16.3, 16.5, or 

 16.6). When gases are taken up by crystals, they enter channels and 

 holes in the regular lattice, each of w^hich has room for one or a small 

 number of sorbate molecules. Thus, stoichiometric ratios may arise 

 even if the sorption is due to "nonchemical" attraction forces. Several 

 observations indicate that the sorption of carbon dioxide (and water) by 

 chlorophyll is "zeolitic" in nature. In the first place, the sorption 

 isotherms are smooth (Fig. 50) and similar to those of the zeolites. 

 (However, it may be argued that smooth isotherms can occur also in a 

 chemical equilibrium, if the binding of a gas does not destroy the crystal 

 lattice of the solid, that is, does not create a new phase.) In the second 

 place, one may quote observations on the binding by solid chlorophyll 

 of gases other than carbon dioxide and water: Rabinowitch (1938) found, 

 for example, that nitrous oxide, N2O (whose volatility is similar to that 

 of carbon dioxide), is absorbed by ethyl chlorophyllide in roughly the 

 same quantity as carbon dioxide. Similarly, Smith has observed the 

 uptake of small quantities of hydrogen and nitrogen by sohd chlorophyll 

 (3-4% of the quantity of carbon dioxide taken up under the same 

 pressure). Both observations indicate an indiscriminate ''zeolitic" 

 affinity to gases, in the reverse order of their volatilities, rather than a 

 selective chemical affinity for carbon dioxide. To sum up — the uptake 

 of carbon dioxide by chlorophyll and its derivatives can be explained by 

 two or three different chemical reactions, as well as by an indiscriminate 

 physical absorption. 



Whatever explanation will ultimately prove to be the correct one, it 

 seems doubtful whether the affinity of chlorophyll in vitro to carbon 

 dioxide bears any relation to photosjaithesis at all. It was shown in 

 chapter 8 that leaves possess two mechanisms of reversible carbon 

 dioxide absorption — one of large capacity but weak affinity, and one of 

 small capacity but strong affinity. The first mechanism, which accounts 

 for the bulk of the carbon dioxide absorbed under carbon dioxide pressures 

 of the order of one atmosphere, certainly has no connection with chloro- 

 phyll, since it operates equally well in nonchlorophyllous tissues, and is 

 capable of binding twenty times as much carbon dioxide as there is 



