RESPIRATION 121 



Respiration 



The general features of respiration were discussed in connec- 

 tion with seed germination where respiration is not only promi- 

 nent but also must be reckoned with in understanding the ger- 

 minative process. There it was stated that respiration takes 

 place only within the cell and that it is comparable to ordinary 

 combustion in that it is an oxidation process resulting in the 

 breaking down of substances into simpler elements with the re- 

 lease of potential energy. 



It is a well known fact that whenever carbon and oxygen are 

 united energy is released. This is the principle employed in heat- 

 ing plants, steam engines, etc. where energy in the form of heat is 

 obtained through the union of oxygen' with the carbon in the coal, 

 wood, or some other combustible substance. If sugars, starches 

 or other substances containing carbon were used for fuel, the same 

 results would be obtained. In the cell, however, since most of 

 the energy released is used in protoplasmic movements, and in 

 chemical changes involved in enlarging cell walls, making more 

 protoplasm, etc., not much is exhibited as heat, although enough 

 that all living plant parts are generally a little warmer than their 

 surroundings, sometimes 2 or 3 degrees in case of large flowers 

 and often much more in germinating seeds. Again, although 

 the rate of respiration increases with the temperature up to a 

 certain point, respiration proceeds in a lower temperature than 

 does ordinary combustion. In fact, a temperature high enough 

 to start the combustion of most substances is entirely too high 

 for respiration, which in most plants ceases before 60 C. is 

 reached. Also in respiration the process of oxidation is initiated 

 and kept going by enzymes or directly by the protoplasm, while 

 there are no such agents involved in combustion. Thus, although 

 similar in results, in operation respiration is very different from 

 combustion. 



In combustion there is a constant ratio between the oxygen 

 used and the carbon dioxide produced. Thus in the combustion 

 of Grape sugar, as illustrated by the formula C 6 Hi 2 O 6 + 6 O 2 = 



fi r^o 



6CO 2 + 6H 2 O, the ratio- - is 1. In respiration, however, 



b \)2 



although the ratio is often unity, it varies much, sometimes being 

 greater and sometimes much less than unity. In germinating 



