RESPIRATION OF PLANTS 109 



CH2'SH 



I 

 CH.NHCO.CH2CH2CHXH0COOH 



COOH. 



Alkaline reactions favor the oxidation of glutathione, while 

 the reduction of the oxidized form proceeds more readily in an 

 acid medium. Like all compounds containing the group SH, 

 glutathione possesses a very low redox potential; i.e., it is a 

 strong reducing agent. 



The reverse may be said of systems containing ferric iron. 

 They possess high redox potentials; i.e., they are strong oxidizing 

 agents. Neither ferrous nor ferric ions are found in cells in a 

 free state, but iron is always found in the form of organic com- 

 pounds i.e., ''bound" or ''masked" iron. Among such com- 

 pound may be mentioned Warburg's respiratory enzyme, hemin. 

 In the animal organism, hemoglobin containing iron plays an 

 important part as a carrier of oxygen. Closely allied to hemo- 

 globin is the pigment, cytochrome, discovered by Keilin (1925), 

 which is very widespread in plants, but whose significance is 

 disputed by many authors. In each cell, therefore, there are 

 highly positive and highly negative redox systems, the interrela- 

 tions of which determine the course of the process of respiration. 

 But an understanding of all the details of these interrelations 

 forms the problem of future investigations. 



It is important to note one essential difference of the new 

 viewpoint on respiration. While previously respiration was 

 identified with combustion, the new viewpoint considers respira- 

 tion a complex system of oxidation-reduction processes. Com- 

 bustion represents a typically irreversible process, all the energy 

 of which is released in the form of heat. The identity of respira- 

 tion with combustion would make it quite impossible to under- 

 stand how heat energy might be directed to the accomplishment 

 of other vital processes, such as growth and synthetic processes. 

 Considering respiration as being a complex combination of 

 oxidation-reduction processes, it may easily be imagined that 

 the electrochemical energy liberated in these processes is used in 

 other chemical reactions that require energy. Only after the 

 energy quanta have been transformed into small quanta repre- 



