THE MECHANISM OF PHOTOSYNTHESIS 307 



Eq. (5-1) is tempting. It is, however, a lack in our present-day knowl- 

 edge of photosynthesis that we are largely unfamiliar with the role that 

 the chemistry of the pigment and its association with protein play in 

 photosynthesis. 



Certain reactions of purple sulfur bacteria, e.g., 



CO2 + 2H2 -* CH2O + HA (5-4) 



have hardly any over-all energy requirement. They nevertheless recjuire 

 a considerable amount of light energy (at least 1 quantum per hydrogen 

 atom transferred) for their completion. This strongly suggests that in 

 all types of photosynthesis there is a partial reaction that uses a com- 

 pound stoichiometrically related to the input of light quanta. It has 

 been supposed that this reaction is of the type 



H2O -> H + OH (5-5) 



in both green-plant cells and colored algae and in bacteria. The present 

 writer is not convinced of the correctness of this view, as wall be dis- 

 cussed later (cf. also Wassink, 1947). 



Before the discussion of the photosynthetic mechanism is continued, 

 some further details will be given about the fate of the light energy after 

 absorption in the pigment system of the photosynthetic cell. When a 

 light quantum is absorbed in chlorophyll, it gives rise to an excited state 

 of the chlorophyll molecule. Chlorophyll shows two chief absorption 

 bands in the visible part of the spectrum, namely, in the blue and in 

 the red, with a region of low^er absorption coefficients in the green and 

 yellow. Quanta of various energy contents ultimately yield the same 

 excited state of chlorophyll, namely, the state corresponding to the red 

 absorption maximum (the one of lowest energy shift). This is obvious 

 from the fact that chlorophyll fluorescence shows a relation only to the 

 red absorption band and that the fluorescence spectrum is independent 

 of the wave length of the incident light. Chlorophyll may lose its exci- 

 tation energy in various ways: (1) by transfer to a compound that is able 

 to use the energy in a (photo) chemical process, (2) as heat, or (3) as fluo- 

 rescence. Fluorescence will be the path if an excited state has escaped 

 annihilation by paths 1 and 2. Under physiological conditions, chloro- 

 phyll fluorescence in plant cells is weak, of the order of a few tenths of 

 1 per cent of the incident light. Since no appreciable fraction of the 

 available energy enters into the fluorescence phenomenon, fluorescence 

 can be used as a sensitive indicator for the available concentration of 

 excited chlorophyll and for the changes this concentration undergoes with 

 changes in path 1 or 2. This reasoning has been the basis of numerous 

 studies regarding chlorophyll fluorescence in photosynthesizing cells and 

 the relation of fluorescence to the process of photosynthesis. The general 

 conclusion from this work is that close connections between photosynthe- 



