20 : 3/ Photosynthesis 3^5 



This value may be compared with the energy of photons of red (680 nuz) 

 and violet (400 m/x) light. These are 



AG = 41 kcal/mole (red photons) 

 AG = 65 kcal/mole (blue photons) 



(Often purists object to speaking of a mole of photons and use the unit 

 of 1 einstein which is really the same thing.) If the process were 100 per 

 cent efficient, about 3 moles (einsteins) of red photons would be needed 

 for each mole of C0 2 converted to hexose. 



There is no reason why the energy for this process need come from 

 photons. Indeed, under suitable conditions all living cells fix C0 2 and 

 reduce the product to hexose. In other words, C0 2 conversion to 

 hexose is not a unique property of photosynthetic cells. In most cells 

 and tissues, this conversion takes place at the expense of metabolic 

 energy. Photosynthetic tissues are distinguished by fixing C0 2 and 

 converting the product to hexose, using the energy obtained by the 

 absorption of photons of visible light to drive the reactions. 



B. Photodissociation of Water 



In the over-all process of photosynthesis in green plants, C0 2 is used 

 up and Q 2 appears. In any experiment with whole cells lasting more 

 than a minute or two, the moles of 2 produced are almost equal to the 

 moles of C0 2 fixed. It is therefore natural to guess that the oxygen 

 might come from splitting C0 2 . However, in nonphotosynthetic 

 tissues, C0 2 is fixed without 2 production or the need for water. Thus, 

 if C0 2 fixation is similar in photosynthetic and nonphotosynthetic cells, 

 the oxygen released in photosynthesis might come from the splitting of 

 water rather than C0 2 . 



Experiments using tracer techniques have shown that the photo- 

 synthetic oxygen does ultimately come from splitting water in photo- 

 synthesis. If water of the form H 2 18 + H 2 16 is used, the oxygen 

 formed contains 16 18 . In contrast, if H 2 16 and C0 16 18 are used, 

 the photosynthetically produced oxygen is all O 16 . Thus, C0 2 fixation 

 in photosynthetic and nonphotosynthetic cells is basically similar, in 

 that the C0 2 molecule is not split. As discussed in Section 4 of this 

 chapter, the pathway followed by the carbon is, however, different. 



There is no evidence that water is simply split in photosynthesis to 

 molecular oxygen and hydrogen. Rather, a variety of experiments 

 indicate that the mechanism includes the reduction of oxidized pyridine 

 nucleotide, as 



PN + + H a O i PNH + H + + |O a (5) 



The pyridine nucleotide may be either diphosphopyridine nucleotide, 



