[Chap. XVII RESPIRATION 157 



lactic acid femientation involves only the transfer of oxygen and hy- 

 drogen atoms between the carbon atoms within the molecule of sugar.^ 

 Oxidation-reduction processes of this sort are said to be intramolecular 

 (within the molecule). When respiration occurs in the absence of free 

 oxygen it is said to be anaerobic (without air), in contrast to aerobic 

 respiration. 



Evidently respiration is not always accompanied by a movement of 

 oxygen into the plant, or of carbon dioxide from it. When such move- 

 ments of these gases do occur they are merely the result of oxidation- 

 reduction processes in living cells. Food is either partially or completely 

 oxidized, and chemically bound energy in it is released mainly as free 

 heat energy. These are the most easily detectable facts about respiration. 



Heat energy from respiration in plants is accessory. We may now ask 

 whether the transformation of bound energy in the food to free heat 

 energy during respiration is of any value to the plant. In the human 

 body the release of free heat energy is certainly essential to the mainte- 

 nance of a fairly constant body temperature of about 98° F. But the 

 temperature of plants and of some small animals is primarily dependent 

 upon the temperature of the environment, in comparison with which 

 the amount of heat energy liberated by respiration is insignificant. Since 

 respiration is as essential to these organisms as it is to man, something 

 more generally essential than the liberation of heat energy is involved. 



Accessory features of respiration in plants. As exemplified by this chap- 

 ter, discussions of respiration in plants usually deal mainly with the 

 accessory processes: the liberation of heat energy, and the foraiation 

 of carbon dioxide and water, or of other easily detectable end products 

 such as alcohol and acetic acid. The reasons for this apparently super- 

 ficial treatment of the subject are two. First, the accessory features are 

 valuable means bv which the occurrence and rate of respiration in plants 

 may be measured under various environmental conditions, at different 

 stages of development, and during dormancy. Second, respiration is a 

 very complicated series of chemical processes, and many of the inter- 

 mediate processes and products are unknown or only partially under- 

 stood. 



Essential features of respiration in plants. It is now known that the 

 oxidation of sugar to CO- and H2O in living cells involves a long series 



2 It may further be noted tliat some of the incompletely oxidized products of respiration 

 are less oxidized than the sugar from which they are formed. Alcohol, for instance, is 

 reduced as compared with sugar. The formation of other reduced compounds in plants is 

 also dependent upon the energy transformations in respiration. 



