ENTRY INTO THE BIOSPHERE 355 



linked to an oxido-reduction reaction, and in Van Niel's theory, which is 

 generally accepted today, the hydrogen donor is always HgO. Up to about 

 1935 studies of photosynthesis were concentrated on the effect of various 

 factors on the rate of photosynthesis in the living plant. Much work has 

 been done from this physiological aspect, the main result being only to 

 underline the mystery of photosynthesis. 



With the advent of methods employing radioactive isotopes and bio- 

 chemical methods for the study of isolated systems, it became possible to 

 penetrate into the intimate mechanism of photosynthesis. In 1940 the 

 reaction of Hill (p. 357) was announced and the first experiment using 

 isotopes (by Ruben, Kamen and Hassid, using ^^C, the only available 

 carbon isotope at that time) was carried out. The presence of magnesium 

 in chlorophyll had led, by a process of reasoning on chemical grounds, 

 to the idea that the first step in biosynthesis was a photochemical reaction 

 involving chlorophyll and COg. This idea stemmed from three observa- 

 tions: the presence of magnesium in chlorophyll, the reduction of COg 

 to formaldehyde by mietallic magnesium in acid solution, and the feeble 

 biochemical reactivity of COg. 



Today, we know that on the contrary COg is a metabolite of general 

 importance. Facts obtained since 1940 lead us to beheve that photo- 

 synthesis is a process in which the photochemical reaction is a photolysis 

 of water preparatory to the transfer of hydrogen. 



According to the ideas of Calvin, the priming reaction for photosyn- 

 thesis, that is the transformation of water under the influence of light, 

 furnishes much TPNH and ATP from the electromagnetic energy of the 

 light. Also pyruvic acid is diverted to the photosynthetic cycle as a result 

 of the presence of thioctic acid in its dithiol form. 



By contrast, in the dark, oxidation takes place and the disulphide form 

 is re-established; the carbon then takes a path required by respiration. 



(c) The Initial Stage of Photosynthesis 



The first stage is a process of quantum absorption converting water into 

 a reducing agent and an oxidizing agent. 



H2O -^ [H] + [OH] 



In green plants oxygen is liberated during this first stage. In photo- 

 trophic bacteria, the oxidizing radical [OH] must be reduced by a hydrogen 

 donor which is specific in each case. 



The second stage is a reduction 



CO2 + [H] -> (CH^O)^ 



