ROLE OF ACCESSORY PIGMENTS IN PHOTOSYNTHESIS 557 



The reduction of "oxychlorophyir' by ferrous ions (c/. page 464) 

 points to a rather low potential of the chlorophyll-oxychlorophyll system 

 (< — 0.7 volt) and thus tends to support theory a. 



Some evidence pertinent to the chemical function of chlorophyll in 

 photosynthesis was obtained by Norris, Ruben, and Allen (1942) in the 

 study of photosynthesis of Chlorella in water containing tritium, the 

 radioactive hydrogen isotope, H^ They found no penetration of tritium 

 into chlorophyll after a period of photosynthesis. This result can be 

 interpreted in several ways: 



{1) it may indicate that chlorophyll does not serve as a reversible 

 hydrogen donor and acceptor at all, and may thus be quoted in support 

 of the "physical" sensitization theory; or 



(^) it may mean that chlorophyll donates hydrogen to carbon dioxide 

 by a photochemical reaction, but recovers it from water by a thermal 

 reaction, and that the rate of the latter is much slower for the heavy 

 tritium than for ordinary hydrogen. This interpretation would support 

 scheme a (as against the schemes h and c) because, in the latter theories, 

 hydrogen is acquired by chlorophyll by a photochemical reaction, and 

 no difference between the rates of photochemical transfer of the two 

 isotopes could be expected. 



(3) However, the same result can be reconciled with schemes h and c 

 as well, if one assumes either that an intermediate oxidation-reduction 

 system (e. g., RO-ROH in Franck's scheme 7.VA) serves as a "filter" 

 which "traps" tritium between water and chlorophyll, or that extracted 

 chlorophyll is identical with the oxidized, rather than with the reduced, 

 form of the photocatalyst. Thus, the experiments with tritium, although 

 interesting, do not yet allow of a definitive interpretation. 



Ruben, Hassid, and Kamen (1939) found that, if radioactive carbon is used in 

 photosynthesis, some radioactivity is found afterwards in chlorophyll. The absolute 

 quantity of C* absorbed by chlorophyll is small (e. g., 0.04% C* in chlorophyll as 

 against 24% in carbohydrates after one hour of illumination) but, because of the small 

 concentration of chlorophyll, the probabiUty that a radioactive C* atom will be found 

 in a given chlorophyll molecule is as high as one-fourth of the probabihty of finding it 

 in a given molecule of carbohydrate. This entrance of radioactive carbon into chloro- 

 phyll probably is indicative of a rapid decomposition and resynthesis of the pigment 

 in vivo, and bears no direct relation to the chemical mechanism of photosynthesis. 



4. Role of Accessory Pigments in Photosynthesis 



The function of the carotenoids and phycobilins in photosynthesis is 

 even less well known than that of chlorophyll. Two specific sensitization 

 effects have been ascribed to the carotenoids: phototaxis (cf., for example, 

 the experiments of Voerkel 1934 and Blum 1935, in Vol. II, Chapter 22) 

 and a stimulation of respiration {cf. Fockler 1938 and Emerson and 

 Lewis 1943; these experiments will be discussed in Chapter 20). In 



