442 FURTHER EVOLUTION 



to suppose that cytochromes arose in organisms which were 

 living under conditions in which the atmosphere was still 

 in its primary, reducing state. In these they might have taken 

 part in anaerobic oxidation-reduction reactions. Only after 

 the appearance of free oxygen did they assume the character 

 of typical aerobic mechanisms. 



All the reactions listed, which are carried out by iron- 

 porphyrin enzymes, take place in the dark and therefore no 

 use is made in them of the important property of porphyrins 

 which is associated with their colours, with their ability to 

 absorb light. 



We can now understand this quite well, for the iron- 

 porphyrins in organisms practically never have a photo- 

 sensitising effect. It is only in the case of cytochrome c that 

 there has recently been found a very weak activity which 

 helps to explain its oxido-reductive transformations.^*^ 



Unlike the iron-porphyrin complexes, porphyrins which 

 are not combined with metals and, especially, magnesium- 

 porphyrin complexes, do not have the properties of ordinary 

 catalysts acting in the dark but are able to carry out photo- 

 sensitising and photocatalytic activities. The mechanism 

 whereby iron-porphyrin complexes participate in biologically 

 important catalytic processes is based on a reversible oxido- 

 reduction of the central atom of iron, which takes place in 

 the dark. The researches of A. A. Krasnovskii and his col- 

 leagues have shown that magnesium-porphyrin complexes, 

 bacteriochlorophyll and the chlorophyll of higher plants, and 

 even porphyrins without metals (e.g. haematoporphyrin) can 

 be reversibly reduced (accepting an electron or hydrogen) 

 only when they absorb a corresponding quantum of light. ^•'° 

 When this happens the photocatalytic transfer of an electron 

 or of hydrogen, unlike ordinary catalytic processes occurring 

 in the dark, leads to a raising of the energy level of the 

 products of the photoreaction ; it, so to speak, ' puts into 

 store ' a part of the absorbed energy of the light in a very 

 easily mobilised form.^^^ 



Thus, the mere presence of these porphyrin pigments in 

 the primaeval organisms enabled them to use in their vital 

 processes not only the readily available energy of exogenous 



