SECTION VI 



THE MECHANISM OF ORGANIC SYNTHESIS 

 THE ASSIMILATION OF CARBON 



THE building up of protoplasm from the material which is available 

 at the earth's surface must be an endothermic process. The food 

 presented to the plant contains the necessary elements, but as a 

 rule in a state of complete oxidation. The energy of the living 

 plant, as of animals, is derived almost entirely from the oxidation of 

 its constituents. The building up of unorganised into organised 

 material must therefore be effected at the expense of energy supplied 

 from without. The source of this energy is the sun's rays. The 

 machine for the conversion of solar radiant energy into the chemical 

 potential energy of protoplasm is the green leaf. Here a deoxidation of 

 the carbon dioxide of the atmosphere takes place, with the production of 

 carbohydrates, generally in the form of starch. The formation of starch 

 must be regarded as the first act in the life- cycle, since this substance 

 serves as a source of energy to the already formed protoplasm in its 

 work of building up all the other constituents of the living cell. It is 

 the solar energy captured by the green leaf which is utilised by all 

 plants devoid of chlorophyll, as well as by the whole animal kingdom. 



There are one or two exceptions to this statement. Thus the bacterium 

 nitrosomonas, described by Winogradsky, grows on a medium devoid of all 

 organic constituents, and derives the energy for its constructional activity 

 from that set free in the conversion of ammonia into nitrites. The sulphur 

 bacteria apparently derive their energy from the decomposition of hydrogen 

 sulphide and the liberation of sulphur. 



The fundamental importance of this process of assimilation for 

 the whole of physiology justifies some account of the researches which 

 have been directed to the elucidation of its mechanism. The produc- 

 tion of oxygen by the green plant was first discussed by Priestley in 

 1772, and a few years later Ingenhaus showed that this production 

 occurred only in the light and was effected only by green plants. De 

 Saussure (1804) pointed out that the essential process concerned was a 

 setting free of the oxygen from the carbon dioxide of the atmosphere, 

 and recognised that the co-operation of water was also necessary. 

 Mohl in 1851 observed the formation of starch grains in the chloro- 



121 



