TOO BIOLOGICAL CHEMISTRY 



PROCESS OF SYNTHESIS 



The previous facts are comparatively easy of demonstration, 

 but when we turn to the steps of the synthesis we find more 

 difficulty in the interpretation of results. There is some 

 reasonable doubt whether the same intermediate stages occur 

 in photosynthesis and respiration. We are here met by a 

 difficulty found throughout biological chemistry, namely 

 that the intermediate substances are present in extremely 

 minute quantities and are thus difficult to detect. It may also 

 happen that if these substances are present in more than 

 minimal concentration they are toxic; thus their effect on 

 living cells cannot be tested. 



Baeyer suggested that the first stage of synthesis is the 

 production of formaldehyde. * Usher and Priestley have 

 made experiments in which chlorophyll was exposed to light 

 in thin layers in the presence of carbon dioxide, f The chloro- 

 phyll was rapidly bleached, but they got over this difficulty 

 by mixing catalase with the chlorophyll. The bleaching was 

 due to the formation of hydrogen peroxide ; in the presence of 

 catalase it was decomposed with the production of free oxygen, 

 and the chlorophyll was not attacked. Their experiments 

 were made by spreading a mixture of gelatine, chlorophyll 

 and catalase on glass plates. They were able to show the 

 production of something which gave the reactions of an 

 aldehyde. Schryver likewise found that on exposing chloro- 

 phyll to light a substance like formaldehyde is produced. { 



The aldehyde-like substance may be, however, a decom- 

 position product of the chlorophyll or other cell substances 

 under the influence of light. 



Assuming that formaldehyde is produced we can examine 

 the mechanism of this reaction. As already mentioned, a 

 change in a chemical system is accompanied by a tendency 

 to change in the opposite direction. Thus hydrogen and 

 oxygen unite to form water with the evolution of considerable 

 quantities of heat. If the temperature is raised the reverse 

 process occurs, and at high temperatures water is largely 

 decomposed into hydrogen and oxygen. 



On heating carbon dioxide it decomposes into carbon 

 monoxide and oxygen, so that if we heat a mixture of carbon 

 dioxide and water the two are changed, and if the first 



* A. Baeyer, Ber., 1870, vol. 3, p. 63. 



f F. L. Usher and J. H. Priestley, Proc. Roy. Soc. t 1906, B, vol. 

 77. P- 369. 



J S. B. Schryver, Proc. Roy. Soc., 1910, B, vol. 82, p. 226. 



