120 PHYSIOLOGY 



100 per cent, of the theoretical yield. Even if the above mechanism 

 be regarded as the chief one, it is probable that side reactions take 

 place at the same time, so that we may have the formation of sub- 

 stances such as glyoxylic acid and other derivatives of the fatty acid 

 series. Such by-products might play an important part in the other 

 synthetic activities of the cell, and especially in the formation of fats and 

 proteins. 



THE FORMATION OF PROTEINS 



Our knowledge of the mechanism by which proteins are synthetised 

 in plants is still more incomplete than that of the synthesis of carbo- 

 hydrates, and we are reduced in most cases to a discussion of the possible 

 ways in which, from our knowledge of the chemical behaviour of the 

 constituents of the protein molecule, we might conceive of its forma- 

 tion. We can at any rate state the problems which have to be solved 

 and study the conditions under which the synthesis of protein is 

 possible in plants and in animals. 



We know that plants are independent of any organic food for 

 building up their various constituents, whether carbohydrate, protein, 

 or fat, provided only that they possess chlorophyll corpuscles and so 

 are able to utilise the energy of the sun's rays. Most plants will grow 

 in the dark if supplied with sugar and with combined nitrogen either 

 in the form of ammonia or of nitrates. The higher plants are especially 

 dependent on the presence of nitrogen in the latter form, and it is on 

 this account that the nitrifying bacteria of the soil acquire so great 

 an importance for agriculture. From the carbon dioxide of the atmo- 

 sphere or from the hexose formed by the assimilation of carbon, and 

 from nitrogen, in the form either of ammonia or nitrates, together 

 with inorganic sulphates, the plant cell is able to build up all the 

 various types of protein which are distributed throughout the vegetable 

 kingdom. Our study of the disintegration products of proteins has 

 shown that this class of bodies contains a large number of the most 

 diverse groups, having as a common character the possession of 

 nitrogen in their molecule, generally as an NH 2 or NH group. These 

 disintegration products can be classified as follows : 



(a) Open chain amino-acids. 



(&) Hetero'cyclic compounds; including : 



(1) Pyrrol derivatives. 



(2) Pyrimidine derivatives. 



(3) Tminazol derivatives. 



These two last groups co-exist in all the purine compounds. 



(c) Benzene derivatives. 



(d) Indol derivatives. 



The first step in the synthesis of proteins is probably the formation 



