METABOLISM 129 



Respiration and Life. — With an understanding of the signifi- 

 cance of the plant's energy relationships and the part which 

 respiration plays therein, we may pass to a more detailed study 

 of respiration itself. This process, unlike photosynthesis, is not 

 carried on in particular organs and under particular conditions, 

 but is universal, taking place under all conditions and in every 

 living cell. Respiration, indeed, is believed to be a necessary 

 accompaniment of life itself, as might be inferred from the fact 

 that living protoplasm is continually active and is thus continu- 

 ally expending energy. Even in cells which are dormant and show 

 no external signs of life, respiration, though very feeble, may still 

 be detected. The amount of respiration which takes place is a 

 rough index of the activity of the cell, organ, or organism studied. 



The liberation of energy is the essential feature of respiration 

 and the addition of oxygen is its usual accompaniment, particu- 

 larly in the higher plants; but in certain cases and under certain 

 conditions, notably among some of the lowest members of the 

 plant kingdom, respiration may be carried on in the absence of 

 oxygen. These two types we recognize as aerobic and anaerobic 

 respiration. They are so different as to require separate 

 consideration. 



Aerobic Respiration. — Aerobic respiration is essentially an 

 oxidation process. Free oxygen is added to organic substances 

 (chiefly carbohydrates) with the consequent breaking down of 

 the latter into their original inorganic components, carbon dioxide 

 and water, thus: 



CeHisOs + 6O2 = 6CO2 + 6H2O 



The oxygen is usually taken directly from the atmosphere through 

 stomata, lenticels, or other openings. 



This process of oxidation is one of those chemical changes 

 which are assisted by enzymes, and the oxidizing enzymes or 

 oxidases occur in every living cell. As to whether it is the 

 protoplasm itself or unassimilated food substance within the 

 protoplasm which is oxidized, there is some difference of opinion. 

 It is certain, however, that most of the carbohydrates and fats 

 which are taken into the protoplasm soon furnish, either directly 

 or indirectly, the material which is oxidized in respiration. 

 Proteins, although their chief function is constructive, undoubt- 

 edly also contribute to the supply of oxidizable substances. 

 Whatever nitrogenous waste material is given off in their dis- 



