BIOCHEMICALLY IMPORTANT COMPOUNDS 189 



The origin of carbohydrates, lipids, porphyrins, 

 amino acids, nucleotides, polynucleotides 

 and protein-like polypeptides. 



We shall confine ourselves to an attempt to draw a possible 

 picture of the formation of only some isolated groups of 

 organic substances of the greatest biological significance : 

 carbohydrates, some lipids, organic acids, porphyrins, nucleo- 

 tides and, finally, protein-like substances. 



However, before turning to this stibject we must discuss 

 briefly a phenomenon ^\ hich is characteristic of many organic 

 substances of biogenic origin, namely their dissymmetry^*^ 

 and the possible ways in \vhich this could have arisen on the 

 Earth before the appearance of life. 



The gradual increase in the complexity of organic sub- 

 stances which occurred during their evolution led, at a par- 

 ticular stage in their development, to the emergence of a 

 new property, the dissymmetry of molecules. This property 

 appears whenever an increase in complexity of the molecule 

 leads to at least one of its carbon atoms being united through 

 each of its four valencies to different groups of atoms. For 

 example neither methane, nor carbon monoxide, nor the acet- 

 aldehyde which was formed from them, nor even acetic acid 

 possessed this property, in that three of the valencies in their 

 methyl groups were satisfied in the same way, with hydrogen. 

 Neither does dissymmetry arise when glycine is formed by 

 substituting an amino group for one of the hydrogen atoms 

 in acetic acid. However, when another hydrogen atom is 

 replaced by a methyl group with the formation of alanine, 

 dissymmetry arises. This property of molecules is expressed in 

 the existence of two very similar forms of the given organic 

 substance ; their molecules contain exactly the same atoms 

 and even exactly the same groups, but these groups are differ- 

 ently disposed in space. If a particular radical is on the right 

 in one of the forms it will be on the left in the other and 

 vice versa. Our two hands serve as a simple model of this 

 dissymmetry. If we lay them side by side with the palms 

 down we shall see that, for all their similarity, the right and 

 left hands are radicallv different in the arrangement of their 

 separate parts. If the thumb is on the left of the right hand, 



