100 RESPIRATION 



set free, wherefore the heat engineer devises the complete 

 oxidation of available fuel, wood, coal, coke, and mineral oil, 

 to carbon dioxide and water. In the living organism, on the 

 other hand, this combustion may not be so complete and may 

 stop at a phase several stages short of the ultimate, a portion 

 of the total potential energy of the material physiologically 

 consumed being sufficient for its needs. For instance, Bacillus 

 coli and B. pyocyaneus in the resting condition obtain sufficient 

 energy in oxidizing succinic acid to fumaric acid, but if these 

 bacteria are proliferating the fumaric acid is also oxidized as 

 soon as it is produced.* 



It is a principle in physical chemistry that the heat of 

 any chemical reaction depends solely upon the initial and final 

 products, and the total heat evolved is the same by whatever 

 method the final products are obtained, i.e. whether in one 

 single process or by a series of intermediate stages, and also 

 whether the reaction proceeds rapidly or so slowly that there 

 is no perceptible rise in temperature. In view of these facts 

 it will be clear that the same laws hold for the low temperature 

 of oxidation of various substances in the living cell as for the 

 combustion of these substances in air or oxygen. 



The heat of combustion of an element is determined by 

 causing it to combine with oxygen in a closed chamber and 

 measuring the heat evolved calorimetrically. The two equa- 

 tions — 



C + 2 = C0 2 + 94-8K 

 H, + O = H a O + 69K 



indicate that by the complete oxidation of 12 grams of carbon 



or 2 grams of hydrogen 94-8 and 69 kg. calories f are evolved. 



These equations may be expressed in another form : — 



1 gram of hydrogen on combustion yields 34 # 5K 

 1 ,, ,, carbon ,, ,, ,, 7-aK 



♦Quastel: " Biochem. Journ.," 1924, 18, 365. Quastel and Whetham : 

 id., 1924, 18, 519; 1925, 19, 519, 645. 



f A kilogram calorie is the amount of heat required to raise the tem- 

 perature of 1 kg. of water through i° C. A statement to the effect that the 

 complete oxidation of glucose, for example, liberates 709 kg. calories there- 

 fore means that the heat energy liberated during the combustion of 180 

 grams of glucose is sufficient to heat 709 kgs. of water through i°C. 



