RESPIRATION 23 



heat required to raise 1 gram of water 1° C. in temperature, 

 a great calorie (C) is the heat required to raise 1,000 gr. 

 (1 KUo) of water 1° C.*). But since a gram of dextrose 

 may contain a very different number of molecules from a 

 gram of any other substance, and since the liberation of 

 energy is due to the oxidation of molecules rather than of 

 mere weights of a substance, it is well to convert these 

 figures into a form which will enable one to compare the 

 yield in energy when the substances are similar in quanti- 

 ties. Thus, by using the molecular weights of the com- 

 pounds to indicate the number of grams, we have a com- 

 mon basis for the comparison of the heats liberated, namely, 

 the gram-molecule. Thus the molecular weight of dextrose 

 is 180, obtained thus — 



atomic weight of C = 12 C, = 72 



" " H = 1 H„ = 12 



" = 16 O3 = 96 



180 = molecular 

 weight of dextrose (C, H,^ OJ 



3,939 calories x 180 = 709,020 calories =709.02 Calories. 

 The heat of combustion (complete oxidation) of 1 gram- 

 molecule, i. e. of 180 grams of dextrose, is then 709.00 

 Great Calories ( C. ) 



This reaction and the production of this amount of heat 

 take place only in the presence of sufficient quantities of free 

 oxygen. Molecules more complex than those of carbon- 

 dioxide and water, though simpler than sugar, may be 

 formed from sugar without free oxygen or with free oxygen 

 in smaller proportions than 6 :1. Complete oxidation (nor- 

 mal respiration) yields the largest amount of energy, less 

 profound changes yield less energy. Thus the decomposi- 

 tion of sugar by yeasts, according to the following reaction, 



* It may be interesting to compare the amounts of heat liberated by 

 burning 1 gram of different substances, thus : 

 1 gram bread crumbs (contain some proteid) 3984 calories. 

 1 " butter 7264 " 



1 " wood-charcoal (mainly carbon) 8080 " 



1 " hydrogen 33881 " 



