1954 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



water) by reversing one part of the primary photochemical processes and 

 Litihzing the Hberated chemical energy to "promote" the reducing (or oxi- 

 dizing) power of the remaining primary photochemical products. This was 

 called "energy dismutation" in chapters 7 and 9. (Ruben, and van der 

 Veen, cj. p. 1116, suggested more specifically that the chemical energy, 

 obtained by reversal of the primary photochemical processes, is stored for 

 subsequent use in phosphate bonds.) 



New in Warburg and Burk's theory was first, the concept that the 

 energy-storing back reaction is so slow that it can be substantially separated 

 from the photochemical forward reactions by light intermittency with a fre- 

 quency as low as 1 min.~^; and, second, that the back reaction involves 

 molecular oxygen, in other words, that it is a form of complete respiration, 

 and not the reversal of one or several intermediate oxidation-reduction 

 steps in photosynthesis (which would not be detectable by manometry). 



Warburg et al. supported their hypothesis by reference to the inhibition of photo- 

 synthesis by absence of oxygen; however, we have seen (cf. section 2(6) above) that 

 anaerobic conditions prevent photosynthesis only if, during the dark period, fermenta- 

 tion has taken place, and poisonous products have accumulated. 



Warburg, Geleick and Briese (1951) listed the following conditions as im- 

 portant for the successful separation of the photochemical and the dark re- 

 action: (1) precompensation of respiration by background light, "making 

 certain that any decrease of pressure in the dim period must be due to back 

 reactions of photosynthesis," (2) low density of the cell suspension, and (5) 

 intermittency of illumination. Warburg et al. noted that the type of inter- 

 mittency at which photosynthesis appeared most clearly separable into its 

 component reactions was the one known to be least favorable for the growth 

 of algae — 1 minute light-1 minute dark (cf. fig. 34.2) but suggested no ex- 

 planation of this peculiar relation. The situation appears even stranger 

 if one recalls that this type of intermittency has been found (cf. fig. 34.4) 

 to give also the lowest over-all yield of photosynthesis (in moderately strong 

 light). 



The "photosynthetic back reaction" decays in the dark, according to 

 Warburg et al., in about three minutes; a 3 minute "bright" -f 3 minute 

 "dim" cycle was therefore chosen to study this process in more detail. 

 Fig. 33. 6 A represents such a cycle (with points obtained by averaging 

 corresponding on-the-minute readings from ten cycles) . In this particular 

 case, with [CO2] = 11%, and 100 mm.^ of cells in the vessel, the over-all 

 quantum requirement for the whole cycle was 5.1. Warburg et al. con- 

 verted this figure into a "minimum requirement" (to be expected at 5% 

 CO2), by means of the previously determined curve 7 = /[CO2] (of w^hich 

 the beginning is shown in fig. 37D.27), and obtained in this way a value of 

 3.8; with 22 mm.^ cells in the same vessels, the similarly calculated mini- 



