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 Kilo) 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 6 = 72 

 " " H = 1 H 12 = 12 

 " " O = 16 0. = 96 



180 = molecular 

 weight of dextrose (C 6 H 12 6 ) 



3,939 calories x 180 = 709,020 calories = 709. 02 Calories. 

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

 molecule, i. P. 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 " 



