RESPIRATION OF PLANTS 83 



tions of energy during respiration remain as yet insufficiently 

 understood. The amount of energy liberated in the process of 

 respiration is commonly determined by the heat liberated. 



According to the laws of thermodynamics, the total amount 

 of heat released in an exothermic chemical process does not 

 depend upon the intermediate stages and transformations of the 

 process. That is why it may be assumed that the total energy 

 of the physiological process of oxidation (respiration) of carbo- 

 hydrates and other organic compounds will be equal to the heat 

 obtained in burning these substances in a calorimeter. For 

 the combustion of glucose, the energy liberated as heat amounts 

 to 674 kilogram-calories (Cal.) per gram molecule (180 g.). If 

 the initial substances to be oxidized are not carbohydrates but 

 proteins and fats, considerably larger magnitudes will be 

 obtained. On the average, it may be assumed that in burning 

 1 g. of carbohydrates, 4 cal. are liberated; 1 g. of protein, 5.7 cal. ; 

 and 1 g. of fats, 9.2 cal. The greater liberation of heat by pro- 

 teins and especially by fats as compared with carbohydrates is 

 conditioned by their higher hydrogen and lower oxygen content. 



Respiration is essentially of the same nature in both plant 

 and animal organisms. It consists in obtaining energy for 

 the cells during the combustion of carbohydrates. Usually, 

 the animal organism consists of a massive body, to some 

 parts of which gaseous oxygen cannot easily penetrate; hence, 

 in order to provide all cells with oxygen, a special com- 

 plex mechanism is required, which consists of a blood-circu- 

 lating system with its conveyers of bound oxygen, the red 

 blood corpuscles, and respiratory organs, where the blood can 

 be saturated with oxygen and freed from the accumulated carbon 

 dioxide. The complexity of these essential yet adventitious 

 organs has obscured the true nature of respiration in animals. 

 Even as late as the beginning of the nineteenth century, the 

 opinion still prevailed that respiratory movements and blood 

 circulation were the main features of respiration; and, conse- 

 quently, the existence of respiration in plants was denied. Much 

 effort was necessary to introduce into science the truth that 

 respiration proceeds neither in the lungs nor in the blood, but 

 in each living cell. 



The external conditions of respiration are very much simpler 

 in plants than in animals. Owing to the development of a 



