THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 37 



pounds. The carbon compounds which make up the living cell are com- 

 bustible, 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 pro- 

 vided 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 2 ) into the molecules of fatty acids, amino-acids may be produced, from 

 which the complex proteins are built up to form the chief constituents of the 

 living protoplasm. 



The synthesis of carbon compounds from the inert carbon dioxide of 

 the atmosphere can be effected only by chlorophyll corpuscles. All animals 

 take in carbon, hydrogen, nitrogen, oxygen, and sulphur in the form of the 

 carbohydrates, fats, and proteins which have been built up in the living 

 plants. In the animal organism these food-stuffs serve as sources of energy. 

 They undergo a gradual oxidation, and finally leave the body in the form of 



