PROVISION OF ENERGY jj 



breaks down the sugar glucose according to the following equation : 



C6Hi206=2C2H60+2C02 



(Glucose) (Alcohol) 



leaving, as an unoxidised residue, ethyl alcohol, which is produced 

 in this way in the brewing of beer and other fermented drinks. 

 Similarly, many animal tissues which are temporarily deprived of 

 oxygen, and perhaps some internal parasites which are perman- 

 ently short of this element, break down the substance glycogen 

 (which is related to starch) so as to form lactic acid, according to 

 the equation : 



(C^'R^fi,)n -{-nll.fi =2nC^Rfi^ 



(Glycogen) (Lactic acid) 



Organisms or tissues which carry out such processes as these 

 are said to be anaerobic. Their mode of obtaining energy, since 

 it leaves a residue containing energy of which they have not 

 availed themselves, is wasteful as compared with that of the 

 majority of living beings, which are aerobic, that is, draw from 

 their surroundings — air or water (see p. 12) — free oxygen, and 

 with it complete the oxidation of the substances from which 

 they obtain their energy. The obtaining of free energy by the 

 disintegration of complex substances is familiar to us in various 

 processes employed by man. Thus the energy imparted to a bullet 

 by an explosive is liberated, like the energy of anaerobic animals 

 and plants, by a decomposition without importing oxygen, while 

 the energy of a petrol or steam engine or the light of a candle 

 is obtained by the use of oxygen from the air in combustion, 

 like the energy of aerobic beings. 



It is not difficult to prove that this disintegration is taking 

 place in the body. The large molecules break down, as we have 

 seen, to produce carbon dioxide and water. Since many of the 

 disintegrated molecules contain nitrogen, there are formed also 

 certain fairly simple nitrogenous compounds, such as urea, 

 CO(NH2)2- The intake of oxygen and loss of carbon dioxide 

 during life are easily demonstrated. Men or animals enclosed in 

 a vessel to which air has not access are unable to live for more 

 than a short time. The animals are stifled, just as a lire or the 

 flame of a candle may be stifled, by want of air, and subsequent 

 examination of the gases in the vessel will show that the oxygen 



