METABOLISM AND TRANSPORT IN ANIMALS 



511 



Plant and 



animal 



protein 



NITROGEN (Nj) 80% Fr/ee Nj in olr 



Independent 



N2 - fixing 



bacteria 



Figure 377. The nitrogen cycle in nature. 



sists of 78 per cent free nitrogen) with at- 

 mospheric oxygen to form potassium nitrite 

 (KNO2) and sodium nitrite (NaNOs). 

 From these nitrates, minute plants (bac- 

 teria) are able to form other nitrogenous 

 substances such as nitrate (NO3) and 

 ammonia (NH3). By linking these with 

 the carbohydrates formed by photosynthesis, 

 amino acids are produced, which the plant 

 then constructs into proteins. These plant 

 proteins constitute one of the principal food 

 elements of animals. The nitrogen cycle is 

 illustrated in Fig. 377. 



Energy 



During photosynthesis, light energy from 

 the sun is transformed into chemical energy. 

 In all nutritive processes, energy transfor- 

 mations occur. Energy may be defined as 

 "the capacity to do work." It may appear to 

 us in the form of motion, heat, chemical 

 changes, light, electric currents, etc. Two 



types of energy are recognized: kinetic and 

 potential. Kinetic energy is the energy of 

 movement and potential energy that of posi- 

 tion. For example, a brick that is held 6 feet 

 above the ground possesses potential energy 

 equal to the kinetic energy required to raise 

 it to that height against the force of gravity. 

 Dropping the brick brings about the trans- 

 formation of this potential energy into the 

 kinetic energy of motion, and into that of 

 heat when the brick strikes the ground. Po- 

 tential energy is stored in food. When one 

 type of energy is transformed into another, 

 according to the law of conservation of en- 

 ergy, the quantity of energy remains the 

 same. "Conservation of energy" means that 

 the total energ}' of the universe is constant, 

 none being lost and none being gained; all 

 changes are due to transformations. 



Mechanical energy appears in the move- 

 ments of animals and in the streaming of 

 protoplasm; for example, cyclosis in the 

 Paramecium. The energy of heat is mani- 



