PLANT PHYSIOLOGY 233 



In the animal this liberation of energy is the predominating 

 function of the organism or, as we may say, energy is the net 

 result of the animal physiological reactions. Animals may be 

 characterized therefore as energy-liberating organisms. 



The energy food of animals consists of three classes of com- 

 pounds, viz. carbohydrates, fats and proteins. These three 

 complex compounds, containing carbon, hydrogen, oxygen and 

 nitrogen, on oxidation, are broken down into simpler com- 

 pounds, viz. carbon dioxide and water, which are excreted 

 mostly in the breath, and nitrogen compounds derived from the 

 proteins only, which are excreted in the urine. In this break- 

 ing down by oxidation the potential energy stored or conserved 

 in these complex compounds is liberated as kinetic energy of the 

 living animal. The reaction illustrating such changes is that 

 of the oxidation of glucose as it takes place in the animal cell, 



viz.: 



CeHisOe 4- 6 O2 -> 6 CO2 + 6 H2O -1- energy 

 Glucose 



The energy thus liberated as kinetic is manifested in two 

 forms, (i) as animal heat, (2) as muscular work. As the animals 

 with which we have to do are warm-blooded organisms, a con- 

 siderable amount of energy is required to supply the necessary 

 heat. The expenditure of energy as muscular work is also 

 large because such work is continual, either as voluntary or 

 involuntary, so long as life exists. In man it has been found 

 that the energy so expended for body heat and simply the 

 involuntary muscular work of the body at rest amounts to about 

 1600 to 2000 Calories per day. 



Now all that we have thus reviewed as to the relation between 

 food and energy is true of the plant organism as well as the 

 animal, except as to amount. In animals the expenditure or 

 liberation of energy is the chief or predominating reaction, 

 while in plants it is not the predominating reaction. 



The plant cell or the complex plant organism of many such 

 cells is of exactly the same nature in its living processes as the 

 animal. For the maintenance of life energy must be set free, 



