BASIC ASSUMPTIONS 11 



examined is equivalent to a small drop in a very vast ocean. It is 

 pleasing that so much has already been discovered, but there is 

 very little doubt that there is a great deal yet to be discovered about 

 carbohydrate metabolism. It is therefore premature to claim that 

 the " universal " occurrence of the glycolysis and citric cycles 

 is proof of the common origin of life from one source. 



To indicate some of the further biochemical complexities we 

 may briefly mention four points. Firstly, it is often stated that all 

 living systems use the same twenty or so amino-acids. This is 

 a simplification of the known data. At one time it was thought 

 that only the L-amino-acids occurred in natural systems, but 

 since then a few D-amino-acids have been isolated. The number 

 of known natural L-amino-acids has increased with the develop- 

 ment of chromatographic techniques. Meister (1957) quotes some 

 seventy naturally occurring amino-acids and he points out that 

 new ones are being discovered almost every month ! This is a 

 result of the application of new techniques to an extended range 

 of animal and plant material instead of restricting research to 

 mammalian tissues. 



Secondly, there are a large number of bacteria that use aberrant 

 biochemical systems. Outstanding amongst these are the sulphur 

 bacteria which grow quite well on water, carbon dioxide, phosphate 

 and either sulphuretted hydrogen or sulphur. Another bacterium, 

 Thiobacillns ferro-oxidans, can in some cases grow on ferrous iron 

 under acid conditions which prevent the direct aerobic oxidation 

 of ferrous iron. Other bacteria take ammonia and dehydrogenate 

 it, or nitrite and oxidise it. There is some argument whether these 

 systems are primitive or whether they are advanced and overlaid 

 on the basic glycolysis and tricarboxylic cycles (see p. 22). These 

 examples indicate that the metabolic systems in the bacteria are 

 extremely varied. 



Thirdly, even in the higher Metazoa the distribution of hydro- 

 gen acceptor systems such as in the cytochromes, flavoproteins, 

 tocopherols, vitamin K, etc., is no more uniform than the dis- 

 tribution of blood pigments we mentioned previously. 



A fourth generalisation that has been made about protoplasm 

 is that its energetic systems involve the formation and destruction 

 of " high energy " phosphorus compounds and the ubiquity of 

 the phosphorus-containing compounds in living cells has been 



