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CHAPTER I 



INTRODUCTION 



1. BIOLOGICAL SIGNIFICANCE OF THE 

 PYRROLE PIGMENTS 



Life on earth, in its present form at least, depends almost entirely 

 on the process of photosynthesis in green plants. In this process the 

 light energy of the sun is used for the synthesis primarily of carbo- 

 hydrates and secondly of proteins, fats, and other substances which 

 serve as food materials for animals and man, as well as for the syn- 

 thesis of vitamins. Light energy is in this way transformed into 

 chemical energy, which can be stored, and which becomes available 

 for the energy requirements of the plant and animal world. In addi- 

 tion this process liberates oxygen, which all aerobic organisms require 

 for cellular respiration. Chlorophyll, the green pyrrole pigment of the 

 leaves, plays a decisive role in photosynthesis. No photoassimilation 

 has so far been discovered without chlorophyll, although other sub- 

 stances such as carotenoids, and in some algae chromoproteins also 

 act as photosensitizers in conjunction with chlorophyll. 



To deal with chlorophyll and the processes of photosynthesis is 

 beyond the scope of this book. The reader is referred to the recent 

 monograph of Rabinowitch {2198). Only some evolutionary aspects 

 of general importance for the theory of evolution of the pyrrole pig- 

 ments will be discussed in Chapter XIV. There is, in addition, some 

 evidence for the participation of a hematin compound as a catalyst 

 in the photosynthetic process; this will be discussed in Chapter IX. 



The second fundamentally important biological process, that of 

 cellular respiration, depends equally on pyrrole pigments. It has been 



