PLANT METABOLISM 211 



cUrc(ily related lo i)h<)t().synthesi.s can be grouped into two separate 

 processes, llie decomposition ol \valer and the fixation and reduction 

 of carlDon dioxide. Sudi a dixision of tlie reaciions (an f)e adiieved 

 experimentally by illuminating green cells in the absence of carbon 

 dioxide, then admitting carbon dioxide in darkness. In the first stage 

 reducing material is formed pliotochemically from water, and this 

 material reduces carl^on dioxide in the second stage. The overall 

 reaction of photosynthesis may thus t^e Avritten in the form of a paii 

 of consecutive rea( tions, 



2H2O 4- 2A -^ 2AH2 + O., 



COo + 2AH2 -^ (CH2O) + 2A + H2O 



wliere A denotes the hydrogen acceptor system and (CHoO) the 

 1 educed carbon. 



Isotopic tracer experiments indicate tliat tlie oxygen evolved during 

 photosynthesis probably comes from the water rather tlian directly 

 from the carbon dioxide or organic materials. It is generally agreed 

 that there must be more than two reactions in the formation of oxygen, 

 but the sequence has not been worked out. Undoubtedly tlie water is 

 somehow activated by energy absorbed in the pigment system. Excited 

 molecules, free radicals, or perhaps some unknown special inter- 

 mediates are formed and possess sufficient energy to transfer hydrogen 

 to a suitable acceptor. 



The natural hydrogen acceptor in the photolysis of water has not 

 been identified, and perhaps different species employ different com- 

 pounds. HoAvever, in the test tube, chloroplasts promote the transfer 

 of hydrogen from water to any one of a variety of acceptors. The 

 only known requirement in this isolated system concerns the oxidation- 

 reduction potential of the acceptor used. Apparently the size and 

 nature of the molecule are unimportant if the redox requirement is 

 met. Measurements on the actual potentials in cells have been corre- 

 lated with the properties of common biological systems. Of these, 

 only two of the cytochromes (/ and c) and ascorbic acid seem suitable, 

 but proof is still lacking. In any case, the energy stored chemically in 

 the hydrogen acceptor becomes available for the synthetic needs of 

 the cell, including carbon dioxide fixation and reduction. 



The energy requirement for the overall photosynthetic process as 

 written 



CO2 + H2O -^ (CH2O) + O2 AF = 115,000 cal. 

 is much too high for transfer in any single biological reaction. This 



