ENTRY INTO THE BIOSPHERE 359 



molecules of ATP and four reduction equivalents (4 electrons) are required. 

 In nature, according to Calvin, they are provided by the conversion of 

 electromagnetic energy. We require nothing more. The dynamo of photo- 

 synthesis (see p. 146) is therefore a means of providing ATP and TPNH. 

 As for the reductive cycle of photosynthesis, according to this scheme it 

 consists of a multienzyme system of which the hexosemonophosphate 

 shunt is another variant. 



It is only necessary to compare the pentose cycle of photosynthesis 

 (Fig. 94) with the hexosemonophosphate shunt in Fig. 38 for their relation- 

 ship to become apparent. Since the two mechanisms are still partly 

 hypothetical, it is preferable to await further developments in our know- 

 ledge before attempting to establish a definite relation between respiration 

 involving the hexosemonophosphate shunt and photosynthesis. The mode 

 of formation of a triosephosphate molecule from three CO2 molecules is 

 shown graphically in Fig. 95. 



According to the above views, photosynthesis appears as one aspect 

 of intracellular regulation. A new stationary state is established when 

 particles containing the electromagnetic dynamo are illuminated and a 

 high reduction potential is produced due to a high concentration of TPNH, 

 and there is an accompanying increase in the amount of ATP. In this 

 manner photosynthesis is the opposite to respiration which continues 

 again when illumination is stopped. Thioctic acid, inserted by Calvin into 

 his description of the electromagnetic dynamo, appears to exert an addi- 

 tional regulatory action. Thioctic acid is one of the coenzymes for the 

 oxidative decarboxylation of pyruvic acid and it functions thus in the 

 disulphide form. When the photosynthetic cycle is functioning it is con- 

 verted to its sulphhydryl form thus blocking the way to the tricarboxylic 

 acid cycle, to the corresponding respiratory mechanism, and diverting the 

 flow of carbon to photosynthesis. Inversely, in the dark, the disulphide 

 form is re-established and the way is again open for the flow of carbon to 

 the tricarboxylic acid cycle. 



Thus from the above view-point photosynthesis can be regarded as 

 being simply a special form of the priming reactions set out in Fig. 62. 

 The pentose phosphate cycle is partly reversed and coupled to a reductive 

 stage catalysed by triosephosphate dehydrogenase, thus replacing the 

 oxidation catalysed by glucose-6-phosphate dehydrogenase. In the plant 

 during photosynthesis, the decarbox}4ation of 6-phosphogluconic acid is 

 replaced by the carboxylation of Ru-PP. However many of the ideas 

 expounded in this chapter are hypothetical in nature and other theories 

 of photosynthesis exist. No one theory can explain all the known facts 

 and we must wait until the future for agreement to be reached upon the 

 process which is the principal entry of inorganic carbon and energy into 

 the biosphere. 



