156 ORGANIC AGRICULTURAL CHEMISTRY 



They all contain large amounts of carbon and hydrogen in 

 proportion to the oxygen. This is not so clear from the per- 

 centage composition as when we consider the atomic relations 

 of the elements present. The two elements, carbon and 

 hydrogen, unite readily with oxygen, forming carbon dioxide, 

 CO2, water, H2O. In the carbohydrates only, in which 

 oxygen is in the greatest amount and hydrogen the least, is 

 there enough oxygen to combine with the hydrogen when the 

 compound is decomposed. When therefore these compounds 

 are brought in contact with free oxygen under certain condi- 

 tions, e.g. at raised temperatures or through the agency of 

 enzymes, then the carbon and the excess hydrogen will become 

 oxidized or, as we say, the substance burns. This reaction is 

 the basis for the characterization of these organic constituents 

 of plants and animals as volatile or combustible. Now when 

 this reaction of oxidation takes place and the complex com- 

 pounds are thus decomposed into simpler oxygen compounds, 

 energy is set free. We indicate this in the reaction by writing 

 it as follows, using the carbohydrate glucose as our example : 



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



Glucose 



Each of the other compounds, viz. fats and proteins, under- 

 goes a similar oxidation with the liberation of energy. We shall 

 find later that each of the three compounds yields, per gram 



I or ounce of substance, a different but definite amount of energy. 



^ Oxidation of Food. — Now when the living cell utilizes its 

 food, this reaction of oxidation takes place and energy is set 

 free, and it is this energy that is the energy of the living cell. The 

 reaction is not always as simple as we have written it and it 

 may take place only partially or in several steps, but the result 

 is the liberation of a whole or a part of the energy of such a 

 complete reaction. 



Heat and Work. — In the reaction as it takes place in the 

 air or in oxygen, the energy liberated manifests itself in the form 

 of heat, i.e. the substance burns. Such a manifestation of the 



