THE MECHANISM OF PHOTOSYNTHESIS 



313 



in which the hydrogen donors and carbon dioxide react are different and, 

 especially, that they have different relations to the energy-transfer sys- 

 tem. Limited availability of the hydrogen donor directly affects fluo- 

 rescence, i.e., energy transfer, whereas limited availability of carbon 

 dioxide affects energy transfer much more indirectly, if at all. The 

 observations quoted thus proved directly the separation of reactions 

 (1) and (3), as given in Sect. 2. Indirectly they furnish evidence 

 for the separation from process (2), the light reaction as such, since 

 limitations in either (1) or (3) have the character of Blackman reactions. 



The following explanation has 

 been suggested (Wassink et al., 

 1942) for the slight influence of 

 withdrawal of carbon dioxide upon 

 fluorescence. It is quoted here in 

 some detail since it illustrates the 

 interaction of the partial processes 

 postulated in Sect. 2 (p. 299). 



|4 



13 



=> 10 



At low light intensities the capacity 

 of the system that transforms the hy- 

 drogen donors [system (1), Sect. 2] is 

 sufficient to prevent empty places in 

 the system of energy transfer (system 

 2) . The number of places in the latter 

 system that are occupied with acti- 

 vated energy acceptor, under station- 

 ary conditions of illumination with 

 light of low intensities, appears to be 

 small even in the absence of carbon 

 dioxide. This means that the time of 

 annihilation of an activated state is 

 small, also in the absence of carbon 



z 



UJ 



(J 



Ld 



cc 

 o 



O WITH CO2 



A WITHOUT COj 



J I L 



I 2 3X10' 



INCIDENT INTENSITY, ergs /cmZ sec 



Fig. 5-11. Fluorescence of Chromatium, 

 strain D, as influenced by the presence 

 or absence of CO2 (N2 -t- 15 per cent 

 Ho ± 5 per cent CO2, phosphate buffer 

 pH 6.3, 29°C). {From Wassink et al, 

 1942.) 



dioxide, as compared with the intervals 

 at which light quanta lead to activation at a certain spot. The energy-trans- 

 fer process, under these conditions, is Umited only by the number of absorbed 

 quanta. 



At medium light intensities this situation gradually changes. Obviously, in 

 the absence of carbon dioxide, the system of energy transfer is not capable of get- 

 ting rid of all activated molecules before the next quantum hits the same area. 

 This leads to an increased concentration of excited bacteriochlorophyll, which 

 cannot transfer its excitation energy to an acceptor molecule rapidly enough 

 because an activated molecule still occupies the place. The result is an increased 

 fluorescence, which is actuallj' found at medium incident light intensities. The 

 process of energy transfer now is hmited by the number of places ready to accept 

 energy. Since the absence of carbon dioxide emphasizes the situation outlined, 

 an indirect coupling may be accepted between the transfer of energy and the 

 consumption of activated energy acceptor in process (3) of Sect. 2. 



