COMPARATIVE BIOCHEMISTRY 8l 



from work with isolated chloroplasts which will be discussed 

 in Chapter 7. 



At first sight it might appear that the energy required for 

 the reduction of carbon dioxide by different organisms 

 would differ according to the hydrogen donor utilized. Thus 

 the reduction of carbon dioxide by hydrogen sulphide with 

 the formation of carbohydrate and sulphur requires 17,400 

 cals. per mole, by hydrogen gas only some 2,700 cals., 

 whereas with water 112,000 cals. are necessary. In spite of 

 these differences in the free energy of reaction it has been 

 found experimentally that the number of light quanta ab- 

 sorbed per mole of carbon dioxide reduced is approximately 

 the same in the purple sulphur bacteria (Wassink, Katz, and 

 Dorrestein, 1941), the green sulphur bacteria (Larsen, Yo- 

 cum, and van Niel, 1952), and usually in the green plant (see 

 Chapter 4), namely about 10 quanta. This led van Niel to 

 suggest that in all of these processes the primary photo- 

 chemical reaction is the same and consists in the removal of 

 hydrogen from water with the formation of a hydrogen 

 donor which ultimately donates its hydrogen to carbon di- 

 oxide. As a consequence of the removal of hydrogen from 

 water there is formed a radical, more oxidized than water, 

 which, in the green plant breaks down with the evolution of 

 oxygen and the formation of water, but in the bacteria is 

 reduced by the externally provided hydrogen donor as in the 

 following equations: — 



common primary photochemical reaction 



4H2O + 2A+4X -^ 2AH2+4(XOH) 

 common reaction with carbon dioxide 



CO2+2AH2 -- (CH20)+H20+2A 

 reductive reaction: green plant 4(XOH) + 2H20 -^ 4H2O+4X + O2 

 or bacteria 4(XOH) + 2H2S - 4H2O+4X+2S 



PHOTOREDUCTION AND CHEMOSYNTHESIS IN JHE GREEN PLANT 



Gaffron (1944) showed that under certain conditions the 

 green alga Scenedesmiis shows a photosynthetic reduction of 

 carbon dioxide using hydrogen gas, thus performing the 

 same reaction as do some purple photosynthetic bacteria. 

 This 'photoreduction', as Gaffron called it, is only developed 



