610 RESPIRATION 



form these complex substances from inorganic material by permitting 

 carbon dioxid and water to act in the presence of sunlight. Obviously, 

 therefore, animal life depends upon the products of the higher plants, 

 for the reason that the latter contain energy-rich organic material. 



While these general facts are incontestable, much uncertainty 

 still prevails regarding the nature of these reducing processes. In its 

 widest sense, the term oxidation is applied to any chemical reaction 

 which results in an increase of the positive or a decrease of the nega- 

 tive valencies of a compound. Whether or no oxygen or some other 

 agent is the cause of the reduction is not of deciding value. Thus, the 

 evolution of iodin during the action of ferric chlorid upon potassium 

 iodid is essentially an oxidation, as may be gathered from the fol- 

 lowing formula: 



+ + +— - + - ++ — - + 



Fe+36+K+ J = Fe+3&+K+J 



This process has resulted in the passage of a positive charge of elec- 

 tricity from the ferric atom to the iodin atom, or the transfer of a negative 

 charge of electricity from the iodin ion to the ferric ion. It will be seen 

 that a substance which freely yields a negative charge. is a very active 

 reducing agent, while a substance which readily liberates a positive 

 charge is a powerful oxidizing agent. Upon this basis, oxygen may be 

 said to act as an oxidizing body, because it possesses the power of 

 removing a negative charge from other substances and of attaching 

 itself to them as an oxygen ion, or as electronegative oxygen.^ 



At this time, however, we are chiefly concerned with those proc- 

 esses which are consummated in the living tissues with the aid of oxygen. 

 These reductions belong to the class of the slow reactions, and are not 

 simple combustions, because the oxidations are generally initiated 

 by reductions participated in by various ferments, ^.e., the complex 

 molecules are first simplified by catalytic agents before they are actu- 

 ally oxidized. It should also be remembered that these oxidations may 

 result in many cases without any apparent stimulus, while in others the 

 substances must first be activated by some outside agent. Thus, 

 metallic sodium, phosphorus and certain organic bodies bind free oxy- 

 gen even at ordinary temperatures, while the rare metals, wood and coal 

 must first be exposed to a high temperature. The former process 

 takes place slowly and the latter with considerable speed. Quite simi- 

 larly, foodstuffs possess no tendency to take up atmospheric oxygen 

 under ordinary conditions but may be made to unite with this gas 

 by heating them. Their combustion may be incited immediately by 

 exposing them to the temperature of a flame, while at the temperature 

 of the body, the upper limit of which is near 40° C, their oxidation 

 is slow and gives rise to intermediary substances. For this reason, 

 they are classified as dysoxidizable substances. 



^ Barcroft, Ergebn. der Physiol., viii, 1908, and Winterstein, Dissertation, 

 Jena. 1906. 



