AEROBIC RESPIR.'\TION 509 



Aerobic Respiration. — Respiration of the type described in the preceding 

 paragraphs is termed aerobic respiration, in order to distinguish it from an- 

 other less frequent, but not unimportant kind of respiration called anaerobic 

 respiration, which also occurs in the higher plants. Aerobic respiration, as 

 the term indicates, refers to respiration which proceeds at the expense of 

 atmospheric oxygen. Anaerobic respiration, on the contrary, occurs in the 

 absence of a supply of atmospheric oxygen. In this chapter only the first of 

 these two processes will be considered. 



On the assumption that a hexose is the respiratory substrate the gross 

 chemical aspects of the process are represented by the following equation: 



CeHisOe + 6 O2 -^ 6 CO2 + 6 H2O + 673 kg.-cal. 



The value 673 kg.-cal. is based on the assumption that glucose is the 

 hexose oxidized. However the quantity of energy released by the oxidation 

 of other hexose sugars deviates only slightly from this value. This equation 

 is exactly the reverse of the photosynthetic equation and precisely the same 

 quantity of energy is required in the synthesis of one mol of a hexose sugar 

 as is released when one mol of it is oxidized in respiration. 



The oxidation of a hexose in plant cells does not take place in a single 

 step as indicated in this convenient summary equation. The possible inter- 

 mediate steps in the respiratory process will be considered in the next chapter. 

 This equation merely tells us that for the oxidation of one mol of a hexose, 

 six mols of oxygen are required; that six mols each of carbon dioxide and 

 water result from this oxidation, and that 673 kg.-cal. of energy are released. 

 Since equimolar weights of gases occupy the same volume (Avogadro's hypoth- 

 esis) the volume of oxygen consumed when a hexose is oxidized is equal 

 to the volume of carbon dioxide released. 



The water produced as a result of respiration becomes a part of the gen- 

 eral mass of water present in the respiring cells. Since it is seldom possible 

 to measure experimentally the quantities of water released in respiration, the 

 conclusion that it is an end product of this process is based largely on theo- 

 retical considerations. The water produced in respiration is often termed 

 metabolic water. 



The gaseous exchanges accompanying respiration w^ere discovered and ex- 

 tensively studied before any especial significance was attached to them. This 

 was true both for plants and animals in both of which oxygen is usually 

 consumed, and carbon dioxide is usually released during respiration. The 

 term respiration has therefore long been used to refer to these externally 

 apparent gaseous exchanges, and is still employed in this sense by most animal 



