130 BOTANY: PRINCIPLES AND PROBLEMS 



integration, however, must immediately be assimilated again, for 

 (at least among the higher plants) nitrogenous compounds do not 

 appear among the products of respiration. 



Carbon dioxide is almost invariably a product both of aerobic 

 and of anaerobic respiration. Indeed, the evolution of this gas, 

 even in minute amounts, is regarded as proof that the organism is 

 respiring and therefore alive. The amount of oxygen taken in 

 and the amount of carbon dioxide given off are in the long run 

 equal, though under certain conditions one may temporarily 

 exceed the other. 



Much of the energy liberated in respiration ultimately appears 

 as heat, and a respiring organism will therefore tend to raise the 

 temperature of its surroundings. 



By comparing the chemical equations for photosynthesis and 

 aerobic respiration, it will be seen that one is the precise reverse 

 or reciprocal of the other. Photosynthesis adds carbon dioxide to 

 water and produces sugar and oxygen. Respiration adds oxygen 

 to sugar and produces carbon dioxide and water. These two 

 processes may actually be going on in the same tissue at the 

 same time, a circumstance which has made the study of plant 

 metabohsm pecuharly difficult, since one activity may mask the 

 other. Photosynthesis, however, is confined to the chlorophyll- 

 bearing cells and occurs in them only in the presence of hght. 

 In such cells there is a preponderance of photosynthesis in the 

 daytime and of respiration at night. For a brief period in the 

 morning and again at night, and for longer times when illumina- 

 tion is low or other conditions unfavorable for photosynthesis, 

 the two processes may balance each other exactly, the tissues 

 giving off in photosynthesis just the amount of oxygen which is 

 necessary to carry on their respiratory activity. Respiration, 

 unlike photosynthesis, occurs in every living cell. 



Comparison between Photosynthesis and Aerobic Respiration. — A 

 brief comparison between photosynthesis and respiration is 

 presented in tabular form below : 



PHOTOSYNTHESIS RESPIRATION 



Stores energy Releases energy 



Absorbs carbon dioxide Liberates carbon dioxide 



Liberates oxygen Absorbs oxygen 



Takes place only in green plants Takes place in all plants and animals 



Takes place only in chlorophyll-bear- Takes place in all living cells 



ing cells 



Constructs food Destroys food 



Increases weight Decreases weight. 



