RESPIRATION OF PLANTS 105 



Further investigations of Warburg have shown that cell 

 respiration is conditioned by a very definite iron-containing 

 system, which he designated the "respiratory enzyme." This 

 enzyme is a substance of the group of hemins, complex com- 

 binations of iron with a pigment, porphyrin, that contains four 

 pyrrol nuclei. 



Hemins enter into the composition of other cell substances, in 

 particular the red pigment of blood, hemoglobin, but the respira- 

 tory hemin, or hemin enzyme, of Warburg differs in its structure 

 as well as in its greater activity with respect to oxygen. It 

 forms only a negligible part, less than 0.5 per cent, of all the 

 pyrrol compounds of the cell. As it is the primary catalyst of 

 oxidation, Holden calls it "oxygenase," i.e., by the same name 

 that Bach gave to the hypothetical enzyme that leads to the 

 formation of peroxides. 



The theory of respiration elaborated by Warburg, based on 

 the activation of oxygen by the hemin enzyme, was at first 

 thought to be in opposition to Wieland's theory, based on the 

 activation of hydrogen. But in recent years, a series of attempts 

 have been made (Oppenheimer and others^ to unite both theories 

 in a coordinated scheme of respiration, based on the assumption 

 that in the process of respiration there is the activation of both 

 oxygen and hydrogen and that activated hydrogen reacts only 

 with activated oxygen. It is not possible to examine the details 

 of all these theories, which at present are in a state of continuous 

 change and elaboration. During recent years, Warburg has 

 discontinued the assumption that the hemin enzyme is the sole 

 respiratory enzyme and has discovered in yeast an unstable 

 substance of an orange color that he has named the "second 

 respiratory enzyme." This yellow enzyme does not contain any 

 iron and is not hemin. 



25. Energy Relations of Respiration. Oxidation-reduction 

 Potential.— Respiration represents the main process by means 

 of which the cell and the whole plant obtain energy, which is 

 used for a series of vital processes such as movement and growth. 

 At the time Lavoisier conceived the chemical nature of com- 

 bustion, respiration was very often termed "physiological com- 

 bustion," and the substances oxidized in the process of respiration 

 were compared to the consumption of fuel by the furnace of a 

 power plant, thus converting the chemical energy liberated by 



