8 MANUAL OF ANIMAL BIOLOGY 



killed and then prepared for intensive microscopic study by sec- 

 tioning and staining, is not always the same. Accordingly various 

 theories are held regarding the ultimate architectural details. (W. 



C. The Life Processes of Cells 

 1. Metabolism 



In the protoplasm of every living cell, various essential life proc- 

 esses are continually taking place. These processes involve unin- 

 terrupted adjustments and exchanges of materials between a cell 

 and its environment. This continuous interplay is a feature of 

 outstanding importance as indicated by Herbert Spencer's famous 

 definition of life as ' ' the continuous adjustment of internal relations 

 to external relations." These basic intracellular life processes may 

 be grouped under the term metabolism which in turn may be sub- 

 divided into a constructive phase (anabolism) and a destructive 

 phase (katabolism). 



The metabolic processes acquire their paramount importance 

 from the fact that the maintenance of life requires the continuous 

 expenditure of energy. This essential energy is released by intra- 

 cellular, oxidative processes which result in the tearing down of 

 various complex, unstable chemical compounds present in, or form- 

 ing a part of, the protoplasm, together with the formation of new 

 compounds of relatively low energy content, which are given off as 

 excretions. The latter consist of nitrogenous wastes (e.g., urea), 

 carbon dioxide, and water containing various inorganic salts in 

 solution. 



It should be noted at this point that these essential oxidative 

 processes require a continual supply of oxygen from the environ- 

 ment and the continual release of carbon dioxide from the cell into 

 the environment. This constitutes respiration — a function 

 which persists in every cell throughout life and is a true measure 

 of the extent of the life activities. Thus respiration is linked with 

 nutrition in that it supplies a necessary substance — oxygen — to 

 the cell, and it is also linked with excretion in that it removes a 

 metabolic waste — carbon dioxide — from the cell. 



It is clear that these energy-yielding processes are essentially 

 destructive, or katabolic, in their nature and may be crudely 

 compared to the means used to secure energy to run an automobile. 

 In this case gasoline, which is a complex and unstable chemical 



