THE STUDY OF ZOOLOGY 9 



of the constituents, both of which are necessary to provide a 

 basis for the orderly interactions that characterize living matter. 



Metabolism. — An organism maintains itself by taking in 

 substances from the environment and at the same time excreting 

 other substances, most of which are of no value to it. The 

 energy generated in an organism and utilized in various ways is, 

 in the case of animals, obtained from food. In the body the food 

 is transformed; some of it is built up into protoplasm and the 

 remainder undergoes oxidation with the release of energy. What 

 are called waste products are the end products of chemical 

 reactions and are of no further use to the body as sources of 

 energy. The organism functions as a transformer of energy and 

 remains alive only as long as it is capable of carrying out this 

 function. Metabolism is a collective term which includes all 

 the chemical and physical changes involved in the transformation 

 of food, the elimination of water, and the functional activity 

 of all the organs of the body. In general it consists of a twofold 

 process of disintegration, or catabolism, and reintegration, or 

 anabolism. In disintegration, complex chemical compounds are 

 broken down, in the course of which energy is released and waste 

 substances, poor in energy content, are produced. This process 

 may take place in certain food substances before they are incor- 

 porated as a part of living protoplasm and it also occurs in proto- 

 plasm itself. The reintegrative phase of metabolism is the 

 constructive process which makes good the losses accompanying 

 disintegration of protoplasmic constituents. The two general 

 processes are taking place continuously in the living organism. 

 When metabolism ceases, death ensues. 



The principal difference in the metabolism of animals and 

 pigmented plants lies in the fact that animals require food of an 

 initial complexity for the support of life while the colored plant is 

 capable of manufacturing food from raw materials of relatively 

 simple composition. Chlorophyll-bearing plants in the presence 

 of sunlight are capable of utilizing water and carbon dioxide to 

 synthesize starch and other energy-containing substances from 

 them. The carbon dioxide comes from the air, the water 

 from the soil. Protein is synthesized from substances in the 

 soil. Nitrogen for the use of protein building comes largely from 

 nitrates in the soil, which also supplies other necessary elements 

 including sulphur and phosphorus. The process of synthesizing 



