THE MECHANISM OF PHOTOSYNTHESIS 331 



It is clear that, in some way or other, in photosynthesis amounts of 

 energy are being translocated which have their origin in the light energy. 

 Some possible modes of energy transfer will be discussed here. The 

 transport of energy in a stage of the process not far apart from light 

 absorption first became clear from considerations by Gaffron and Wohl 

 (1936) in a combined discussion of the results of quantum measurements 

 by Warburg and Negelein (1923) and of experiments in flashing light by 

 Emerson and Arnold (1932). Gaffron and Wohl (1936) concluded that 

 the high energy yield found in Warburg's experiments is comprehensible 

 only if some 1000 chlorophyll molecules are able to provide energy for a 

 single "reducing centrum." Gaffron and Wohl accepted the existence 

 of "photosynthetic units" containing 2000-3000 chlorophyll molecules. 

 The further history of this concept and its criticism cannot be followed 

 here in detail. The original concept of the unit strongly adhered to the 

 view that a carbon dioxide complex acts as an energy acceptor and has 

 to receive 4 quanta within a short time in order to be reduced to carbo- 

 hydrate. If the view is accepted that a relatively stable reducing agent 

 is generated with each separate quantum — for this the observations on 

 uptake of carbon dioxide after illumination and those on accumulation of 

 TCA-insoluble phosphate compounds furnish evidence — the special diffi- 

 culties for which the original concept was set up no longer exist. As soon 

 as a sufficient concentration of activated energy acceptor (or reducing 

 agent) is built up, each carbon dioxide molecule will be able to meet a 

 sufficient number of acceptor molecules. The experiments leave suf- 

 ficient room for the postulated building of such a concentration, since 

 the rate of photosynthesis is stationary only after some minutes, during 

 which fairly reproducible changes occur [induction phenomena; see van 

 der Veen (1949-1950); Wassink and Katz (1939); Wassink and Kersten 

 (1943-1945)]. 



The transport of a reducing agent, from the viewpoint of the metabo- 

 lizing cell, is an energy transport with a material bearer (an electron, 

 a radical, or a molecule). It would be covered by the "energy transport 

 by diffusion" of MogUch et al. (1942). Moglich also distinguishes vari- 

 ous other types of energy transport, some of which may have a bearing 

 on a more direct energy transport within the chlorophyll-protein complex 

 [chlorophyllin (Wassink, 1948b)] or between the adjacent pigment mole- 

 cules themselves. 



Evidence for such an energy transport is especially furnished by the 

 fluorescence of chlorophyll resulting from light absorption by other pig- 

 ments (e.g., fucoxanthin or phycoerythrin ; cf. Sect. 3). In the case of 

 fucoxanthin the yield of both photosynthesis and chlorophyll a fluores- 

 cence was the same as for light absorbed by chlorophyll a itself. Fuco- 

 xanthin obviously is bound to protein (cf. Sect. 3). The possible modes 

 of energy transport in the chromophyllin are illustrated in Fig. 5-19, upper 



