122 CELLS AND TISSUES 



seeds, tubers, and bulbs containing starch and sugars, and in 

 many other plant structures, the volume of oxygen consumed 

 during active respiration is equal to that of the carbon dioxide 

 given off; but in the germination of seeds containing fats and 

 fatty oils, the volume of oxygen consumed is greater than that of 

 the carbon dioxide given off, in which case some of the oxygen 

 is apparently used in changing the fats and fatty oils to other 

 forms of food having a larger proportion of oxygen. 



In higher plants the substances oxidized are organic compounds 

 including the sugars, fats, proteins, organic acids, and probably 

 the protoplasm itself. Some lower forms of organisms oxidize 

 inorganic compounds. Some Bacteria obtain energy by oxidizing 

 the ammonia of ammonia salts to nitrites, while others obtain 

 energy by oxidizing the nitrites to nitrates. Various other sub- 

 stances, such as hydrogen sulphide and iron, are oxidized by 

 certain Bacteria to secure energy. 



There are some forms of respiration which can continue when 

 oxygen is excluded and the one of them best known is fermen- 

 tation, which is prominent in the Yeast Plant and other fer- 

 menting organisms. When proceeding in the absence of oxygen, 

 such forms of respiration are known as anaerobic respiration, that 

 is, respiration in the absence of air. In fact, some micro-organ- 

 isms can not carry on their processes well except in the absence of 

 air. One kind of anaerobic respiration, which is very similar to 

 if not identical with fermentation, can be detected in seeds, fruits, 

 and all living plant parts when oxygen is excluded, so that the 

 process is not obscured by ordinary respiration. This kind of 

 respiration is considered by some to be the initial stage of ordinary 

 respiration, thus being closely related to it. 



The peculiar feature about fermentation in the absence of air 

 is that oxidation of carbon continues with the release of energy 

 and the production of carbon dioxide, although no oxygen is ob- 

 tainable from without. Furthermore, fermentation, whether in 

 the presence or absence of air, differs from combustion and ordi- 

 nary respiration in the completeness with which the substances 

 involved are broken down. This may be illustrated in the case 

 of the fermentation of sugar by Yeast, in which case, as shown by 

 the equation C 6 Hi 2 O 6 = 2 C0 2 + 2 C 2 H 6 O, the molecule of sugar 

 is broken into 2 molecules of carbon dioxide and 2 of alcohol, 

 while in case of combustion and often in respiration the molecule 



