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 un- 

 organised 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 carbo- 

 hydrates, 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 nitro- 

 somonas, described by Winogradsky, grows on a medium devoid of all organic con- 

 stituents, 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 production of oxygen by 

 the green plant was discovered 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 chlorophyll corpuscles, and regarded these as the first products of 

 assimilation. The organs of carbon dioxide assimilation are the chloroplasts. 

 These, which are responsible for the green colour of plants, are generally 

 small oval bodies embedded in the cytoplasm, but sometimes, as in spirogyra, 

 may have the form of spiral bands. In a plant which has been kept for some 

 time in the dark, or in an atmosphere free from carbon dioxide, they present 



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