196 ORGANIC AGRICULTURAL CHEMISTRY 



Respiratory Quotient. — Another proof of the metaboUc con- 

 version of carbohydrates into fat is furnished by a study of 

 what is termed the respiratory quotient. The two classes of 

 compounds concerned, viz. carbohydrates and fats, are alike 

 in containing the elements carbon, hydrogen and oxygen. 

 They differ, however, in the relative proportions of these ele- 

 ments. On oxidizing, the products of oxidation are alike in 

 both cases, being simply carbon dioxide and water. As the 

 relative amount of oxygen in each compound is different, the 

 amount of added oxygen necessary for complete oxidation is 

 different also. This will be made clear if we write the reactions 

 for the oxidation of the two substances. Glucose, as we have 

 just discussed, is the final form in which carbohydrate food is 

 oxidized. Taking this as our example, we have the reaction of 

 oxidation as follows: 



CeHiaOe + 6 O2 ^ 6 CO2 +6 H2O 



I mol. 6 mol. 6 mol. 6 mol. 



6 vol. 6 vol. 



This means that to oxidize one molecule of glucose 6 molecules 

 of oxygen are required and 6 molecules of carbon dioxide are 

 produced. Now, by a fundamental hypothesis of chemistry 

 (Avogadro's Hypothesis) equal molecules of gases represent 

 equal volumes. Therefore, in the oxidation of glucose the 

 volume of oxygen gas used is exactly equal to the volume of 

 carbon dioxide gas produced. 



If now we write the corresponding reaction for the oxidation 

 of a fat, we have, taking glyceryl tri-stearate as an example : 



2 C57H110O6 +163 O2— > 114 CO2 +110 H2O 



163 mol. 114 mol. 

 163 vol. 114 vol. 



This means that in the oxidation of fats, which contain less 

 oxygen relatively than do carbohydrates, the volume of oxygen 

 used is greater than that of the carbon dioxide produced. 



