40 PHYSIOLOGY 



which was originally the chemistry of substances produced by the 

 agency of living organisms, has come to be synonymous with the 

 chemistry of carbon compounds. The carbon compounds which make 

 up the living cell are combustible, i. e. they can unite with oxygen 

 to form carbon dioxide with the evolution of heat. In the inorganic 

 world practically all the carbon occurs in a completely oxidised form, 

 namely, carbon dioxide. A small amount, 4 parts in 10,000, is present 

 in the atmosphere, while vast quantities are buried in the crust of the 

 earth as carbonates of the alkaline earths, &c., in the form of chalk 

 and limestone. In this condition the carbon dioxide is practically 

 removed from the life cycle, the whole of the carbon contained in the 

 tissue of living beings, whether plant or animal, being derived from 

 the minute proportion of carbon dioxide present in the atmosphere. 

 The energy for the conversion of carbon dioxide into the oxidisable 

 forms with high potential energy, which make up the tissues of plants 

 and animals, is furnished by the sun's rays. The machine for the 

 conversion of the radiant energy into the potential chemical energy of 

 the carbon compounds is represented by the chlorophyll corpuscles in 

 the green parts of plants. In these corpuscles, under the influence of the 

 sun's rays, the carbon dioxide of the atmosphere, together with water, 

 is converted into carbohydrates, viz. starch (C 6 H 10 5 ), and the oxygen 

 liberated in the process is set free into the surrounding atmosphere. 



6C0 2 + 5H 2 = C 6 H 10 5 + 60 2 . 



In this process a large amount of energy is absorbed, an energy 

 which can be set free later by the oxidation of the starch to carbon 

 dioxide. In the oxidation of one gramme of starch about 4500 calories 

 are evolved, and this represents also the measure of the solar energy 

 which must be absorbed by the chlorophyll corpuscle in the process 

 of formation of starch from the carbon dioxide of the atmosphere. 

 By this means the world of life is provided with a source of energy. 

 At the expense of the energy of the starch further synthetic processes 

 are carried out. By the oxidation of a part of the carbohydrates, 

 sufficient energy may be supplied to deoxidise other portions of the 

 carbohydrates with the production of fats. Thus 



3C 6 H 12 6 - 80 2 = C 18 H 36 2 

 (Glucose) (Stearic acid) 



The potential energy of a fat is still greater than that of a carbohydrate, 

 one gramme of fat giving on complete combustion to carbonic acid 

 and water as much as 9000 calories. By the introduction of ammonia 

 groups (NH 8 ) into the molecules of fatty acids, amino-acids may 

 be formed, from which the complex proteins are built up to form the 

 chief constituents of the living protoplasm. 



