CHAPTER I 



SOME ASPECTS OF BIOCHEMICAL DIVERSITY 



The chemical processes described in Part Three of this book give an 

 approximate and overall view of the metabolism and biosynthetic mechan- 

 isms in cells. However, numerous variations on these themes are possible 

 and a few examples follow. 



I. TERPENES 



We have described (p. 235) the biosynthesis of isoprene from acetyl-CoA 

 as it usually occurs in cells. In the essential oils of plants we find a large 

 number of compounds which demonstrate the large number of possible 

 compounds which can be formed in a similar manner, starting from acetyl- 

 CoA. They are compounds made up of isopentane units. They contain 5, 

 10, 15, 20 or more carbon atoms and are called respectively, hemiterpenes, 

 mono-, sesqui-, di- or polyterpenes. From the material which is not distill- 

 able in steam, by solvent extraction it is possible to obtain a series of other 

 substances containing 20, 30, 40 carbon atoms or more and belonging to 

 the groups of diterpenes (i.e. the resins), the triterpenes (i.e. the saponins), 

 the tetraterpenes (i.e. the carotenoids) or to the polyterpenes (i.e. rubber). 

 Moreover a whole series of organic compounds synthesized by plants are 

 related to isopentane since they contain such units in their structure. 

 Among these isoprenoids are the irones. There are many monoterpenes in 

 plants and in general, but not always, one can consider their formula as 

 being based on two isopentane units joined in head to tail union. The 

 sesquiterpenes can be considered as formed from three isoprene units in 

 head to tail union. The cyclic monoterpenes and sesquiterpenes can be 

 considered as resulting from the rolling up of the same chains. 



Certain of the diterpenes can be considered as containing four wopentane 

 units in head to tail union. This is the case of phytol and vitamin A. Others 

 have an irregular arrangement. 



Among the tetraterpenes, those related to lycopene and called caroten- 

 oids have been described previously). Plants are able to synthesize 

 carotenoid molecules whilst animals are only able to modify them, for 

 example by oxidation. Astaxanthin a carotenoid usually found in 

 crustaceans, is one such oxidation product. 



In mammals, birds, and certain amphibians, the ingestion of carotenoids in 

 the food results in an absorption of carotene in the intestine, the extent of 



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