PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 369 



contribution of light is limited to cyclic photophosphorylation was recently 

 expressed, on the basis of independent evidence, by Stanier et al. 



In certain circumstances, ATP formation may be the sole contribution 

 of the photosynthetic process, not only in bacteria but also in higher plants. 

 We have suggested elsewhere [95] that in green plants cyclic photo- 

 phosphorylation may continue forming ATP when CO2 assimilation is, 

 for one reason or another, reduced or even stopped altogether. This might 

 arise during the well-known midday closure of stomata in leaves of higher 

 plants [126, 127] which restricts the supply of CO.,. The closure of stomata 

 often coincides with an abundance of starch and an incipient water deficit 

 in the photosynthesizing cells. Under these conditions cyclic photo- 

 phosphorylation, which consumes neither COo nor water, would be a 

 useful device for generating ATP to drive the many ATP-dependent 

 reactions, notably the synthesis of polysaccharides, proteins and 

 fats. 



These theoretical deductions for higher plants have recently received 

 experimental support from the work of Maclachlan and Porter [128]. They 

 reported the first known instance of utilization of light energy in leaf tissue 

 for the synthesis of starch from labelled glucose, under conditions when 

 CO2 assimilation was excluded but cyclic photophosphorylation could 

 proceed. 



II. Pyridine nucleotide reduction by hydrogenase in the dark 



In the examples of photosynthesis in which the contribution of light 

 was limited to ATP formation, no reductant was needed in the conversion 

 of glucose to starch in leaves. In the assimilation of acetate by bacteria, 

 hydrogen is released for metabolic purposes and no additional hvdrogen 

 donor is required [121]. But the assimilation of COo requires in addition 

 to ATP, a supply of a reductant, i.e. reduced pyridine nucleotide. It was 

 stated earlier that in photosynthesis of green plants both of these com- 

 ponents of assimilatory power are formed at the expense of light energy. 

 It is necessary, therefore, to trace the transition from a primitive photo- 

 synthesis in which light is used only for the formation of ATP to the 

 "advanced" type of photosynthesis, observed in green plants, in which 

 light energy is used not only for ATP formation but also for the 

 reduction of pyridine nucleotide and the simultaneous evolution of 

 oxygen. 



In the photoassimilation of CO., by Chromatium the added reductant 

 was hydrogen gas [129]. This is the simplest reductant usable by living 

 cells. Cell-free hydrogenases from non-photosynthetic bacteria are known 

 to reduce pyridine nucleotides with molecular hydrogen [130, 131 ; cf. 40]. 



