62 RESPIRATION 



The heat of combustion of an element is determined by caus- 

 ing it to combine with oxygen in a closed chamber and measuring 

 the heat evolved calorimetrically. The two equations 



C + O 2 = CO 2 + 94-8K 

 H 2 + O = H 2 O + 6gK 



indicate that by the complete oxidation of 12 grams of carbon 



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



Expressing the above two equations in another form 



i gram of hydrogen on combustion yields 34'sK 

 i carbon ,, y-gK 



From this it will be seen that hydrogen on combustion 

 yields relatively much more heat than carbon ; consequently 

 compounds rich in hydrogen have a high calorific value. 

 Moreover, since oxygen itself has no calorific value, it follows 

 that the presence of this element in compounds reduces their 

 calorific value. Thus it comes about that fats which contain 

 only about 1 1 per cent of oxygen have a considerably higher 

 heat of combustion than carbohydrates which contain as much 

 as 5 3 per cent of this element. The actual values of the heat 

 obtainable by the combustion of some of the more important 

 fuel substances of the living cell are as follows : 



i gram of carbohydrate = 4'! calories 

 i alcohol = 7-1 ,, 



i ,, fat = 9-1 



i protein = 5-8 



In the procurement of energy the plant exhibits a wider 

 range than does the animal, and this to a larger extent than is 

 often thought ; thus Ramann and Bauer f have estimated that 

 young saplings of deciduous trees may show a loss of 20 to 

 45 per cent of their dry weight during the burst of activity 

 which follows the winter's sleep. 



The term respiration here is used to include all those 

 processes which involve a liberation of energy employable by 

 the organism in its various activities. Respiration is not 

 merely the absorption of oxygen and the excretion of carbon 



* A kilogram calorie is the amount of heat required to raise the temperature 

 of i kg. of water through i C. A statement to the effect that the complete 

 oxidation of glucose, for example, liberates 709 kg. calories therefore 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. 



f Ramann and Bauer: "Jahrb. Wiss. Bot.," 1911, 50, 67. 



