1988 EPILOGUE CHAP. 38 



The nature of these links between photosynthesis and respiration still 

 is only dimly perceived. A blocking of the citric acid cycle in light is 

 postulated by Calvin et al. on the basis of tagging observations, but is not 

 confirmed by Brown's mass-spectroscopic studies, which reveal a more or 

 less steady continuation of respiration in light. If it is true— as it now 

 seems likely— that the reaction sequence converting CO2 to {CH2OI, 

 begins and ends within the chloroplasts, cytoplasmic respiration could have 

 a less close relation to photosynthesis than chloroplastic respiration; and 

 the results of Calvin's experiments may perhaps indicate, not that the 

 citric acid cycle in the whole cell is stopped by light, but that the tagged 

 products of photosynthesis are prevented from leaving the chloroplasts, 

 and joining the reservoir from which the respiration in the cytoplasm is 

 sustained. Another facet of this interpretation is the conclusion that in 

 cells showing in Brown's experiments, a practically unchanged respiration 

 in light, by far the most important part of respiration occurs outside the 

 chloroplasts. 



A very important conclusion, derived from tagging experiments, is that 

 in synthesis, as in the breaking down of carbohydrates, a cyclic mechanism 

 operates, with the carbon dioxide acceptor (such as ribulose diphosphate) 

 being "thrown into the pot" at the beginning of the reaction cycle, and 

 regenerated at its end (together with the hexose which leaves the cycle as 

 its final product). There is something striking— and perhaps significant— 

 in this ''reproductive cycle" of a relatively simple molecule. 



* * * 



It was repeatedly emphasized in this monograph that the biochemical 

 mechanism of the transformation of CO2 to | CH2O ] is only one of the three 

 main components of photosynthesis— the other two being the enzymatic 

 mechanism of oxidation of water to oxygen and the third— and photo- 

 chemically perhaps the most interesting one— the mechanism of photo- 

 chemical hydrogen transfer from the system H2O/O2 to the system j CH2O [ / 

 CO2. Of these three parts of photosynthesis, the first one is much better 

 understood now than the second and the third. 



The enzymatic mechanism of oxygen liberation, for example, remains 

 entirely unknown; nothing can be added, after fifteen years, to the vague 

 general speculations indulged in in Chapter 11. (A similar darkness spreads, 

 incidentally, also over the mechanism of the reverse process — the "mobil- 

 ization of oxygen" in respiration.) Is water oxidized, in photosynthesis, 

 as in certain inorganic systems, directlij, i.e., by remo\'al of an electron 

 from an OH^-ion and transformations of the so obtained OH-radical into 

 O2 (either involving, or not involving, the biradical, H2O2, as intermedi- 

 ate)? Or is the H2O molecule incorporated in an organic molecule, as COo 



