RESPIRATION 291 



In a similar way the stored energy of the food of plants and ani- 

 mals becomes available for work in the katabolic process of respi- 

 ration. 



It is a well-known fact that sugar, oils, meat, etc., can be burned 

 by adding oxygen and that, as a result of this combustion, carbon 

 dioxide and also energy in the form of heat are given off. It 

 should not be surprising that similar energy releases can be ac- 

 complished in the bodies of organisms. Respiration is essentially 

 the same in its broader outlines as the combustion here described, 

 but the details differ widely from those of ordinary burning. In 

 the latter case, the oxygen seems to unite directly with the sub- 

 stance consumed, the union taking place at a high temperature. 

 In the organism, on the contrary, the union must take place at 

 the comparatively low temperature (generally under 35° C.) 

 of the organism, and this requires the aid of enzymes which can 

 make the reaction proceed rapidly enough to supply all the energy 

 needed even though the temperature is low. 



The Gaseous Exchange.— The respiratory equation shows that 

 the food (in this case glucose) is used up with the intake of oxy- 

 gen, resulting in the ultimate formation of carbon dioxide and water 

 with the release of energy : 



C 6 Hi206+602 = 6C02+6H 2 (+3.9 calories of energy per gram). 



Like the photosynthetic equation (Chap. VI), this shows noth- 

 ing of the intermediate steps but only the beginning and the end. 

 That such a gaseous exchange takes place as is indicated by 

 the above equation can be demonstrated easily by placing ger- 

 minating seeds in a confined atmosphere and measuring the amount 

 of carbon dioxide and oxygen at the beginning and at the end of 

 the experiment. It will be found that the air, which at the be- 

 ginning of the experiment was rich in oxygen and able to support 

 combustion, diminishes gradually^ in this component, and at the 

 end of a few hours will be so much less than at the beginning that 

 a glowing splinter is quickly extinguished. The carbon dioxide, 

 on the other hand, increases in amount. This can be tested by the 

 methods of gas analysis or by passing the air through a solution 

 of barium hydroxide (baryta water). The barium carbonate 

 formed is insoluble and forms a white precipitate, which continues 

 to increase as the carbon dioxide is evolved from the respiring 

 seeds. 



