PHOTOCHEMICAL REACTIONS 443 



organic compounds, but also a supplementary source of 

 energy, namely light. 



In the first period of the existence of life, while there were 

 plenty of organic compounds, which had arisen primarily, 

 in the external medium, light, as a source of energy, cannot 

 have been of decisive significance for the organisms. How- 

 ever, as the ready-made organic materials disappeared and 

 the deficiency of them in the external medium became more 

 marked, so a greater and greater advantage in the struggle 

 for existence accrued to those organisms which were in a 

 position to use the porphyrins present in them not only as 

 catalysts of reactions occurring in the dark, but also as photo- 

 sensitisers. 



In this way they were able to use light as a supplementary 

 source of energy. The most important result of this was to 

 enable the first coloured organisms, without undertaking 

 any considerable reconstruction of their already existing 

 organisation, to rationalise their heterotrophic metabolism 

 fundamentally by using exogenous organic substances far 

 more economically. 



Ordinary heterotrophs have to transform a considerable 

 proportion of the organic substances which they obtain from 

 the external medium into waste products which cannot be 

 used further. Only thus can they mobilise the energy bound 

 up in these substances ^vhich is indispensable for synthesising 

 the components of protoplasm. By contrast, the first coloured 

 organisms used the * extra ' energy of light for this purpose 

 and were thus freed from the need to waste exogenous organic 

 substances. 



This may be understood more clearly by reference to a 

 study of the metabolism of the contemporary pigmented 

 bacteria which were discovered quite a long time ago by 

 T. W. Engelmann."- It has been shoAvn by numerous spectro- 

 scopic and chemical studies that these organisms contain 

 considerable amounts of magnesium-porphyrin derivatives, 

 similar in their chemical nature to chlorophyll a/^^ while 

 some of them have also been found to contain free por- 

 phyrins. By virtue of these the bacteria can absorb the 

 visible radiations of sunlight and use their energy for meta- 

 bolism. A particular example of such organisms is provided 



