236 ORGANIC AGRICULTURAL CHEMISTRY 



actions of this kind are those by which the living organism 

 obtains the energy necessary for its life and, as we have shown, 

 take place in both plants and animals. 



Endothermic reactions are in general those in which simpler 

 compounds are synthesized or built up into complex compounds. 

 These may be typified by the reverse of the reaction just men- 

 tioned, viz. 



6 CO2 + 6 H2O + energy -> CeHisOe + 6 O2 



Such a reaction involves the absorption of kinetic energy and 

 the conversion of it into potential energy which is stored or 

 conserved as such in the complex compound formed. At the 

 same time oxygen is set free. These endothermic reactions also 

 occur in living organisms, and in animals may be illustrated by 

 the known conversion of carbohydrates into fats with the 

 storage of energy and the evolution of oxygen. In plants we 

 shall find that the endothermic reaction above given takes 

 place in a wonderful way in the synthesis of glucose from 

 carbon dioxide and water. 



Animals Liberate Energy. — We have mentioned the fact 

 that in animals the reactions which liberate energy, the exother- 

 mic reactions, are predominant. 



Plants Store Energy. — In plants, on the contrary, the 

 predominating reactions are not the exothermic or energy 

 liberating, but are endothermic or energy storing. In other 

 words, the net result of plant physiological reactions is the 

 storage of energy. 



This, then, is the characteristic difference between animals 

 and plants. Both, as living organisms, require complex com- 

 pounds as food material, which by oxidation break down into 

 simpler compounds with the liberation of energy. Both also 

 build up simpler compounds into more complex with the storage 

 of energy and the evolution of oxygen. In animals, however, 

 the liberation of energy is predominant, that is, animals are 

 energy liberators; while in plants the storage or conservation of 

 energy is predominant, that is, plants are energy storers. This 



