108 PLANT PHYSIOLOGY 



The value of the redox potential can be determmed. It is a 

 function of the hydrogen-ion concentration. 



A very important feature of oxidation-reduction systems is the 

 reversibility of the reaction, which permits the maximum utiliza- 

 tion of the energy developed without its degradation to heat. 

 The presence of reversible oxidation-reduction systems in the 

 plant cells is of great significance, because through this mechan- 

 ism the chemical energy of substances can be transferred to other 

 molecules without passing into heat, as would occur if respiration, 

 for instance, were merely a combustion of the substance. The 

 fact has already been emphasized that the existence of oxidation- 

 reduction systems is assumed in most of the theories of respira- 

 tion. Thus Palladin stresses the significance of respiratory 

 pigments, which are continually oxidized and reduced and which 

 transfer oxygen to intermediate products of fermentation or 

 remove hydrogen from them. In Warburg's theory, great 

 importance is ascribed to iron with transformation from the 

 ferrous Fe++ to the ferric form Fe+++, and the reverse. Recently, 

 in connection with the investigations of Hopkins (1929, 1935), a 

 great significance is attributed to organic compounds of sulphur 

 containing the sulphydryl group — SH. With oxidation, which 

 is actually the subtraction of hydrogen, 2 molecules of such 

 compounds combine by means of their sulphur atoms into a 

 double molecule, and the hydrogen released serves for reduction. 

 Of such sulphur-containing compounds, the greatest significance 

 is ascribed to the amino acid cysteine, SII-CIl2*CHNH2'COOH. 

 Schematically it may be represented by the formula RSH. Its 

 oxidation and reduction may be represented by the following 

 equation : 



2RSH ±^ RSSR + 2H. 



Cysteine Cystine 



Two molecules of cysteine are combined to 1 molecule of 

 cystine; as has been seen in Art. 12, these are among the hydro- 

 lytic products of proteins. 



In animal and plant cells, it is not free cysteine but its com- 

 bination with glutamic acid, glutathione, that is of major impor- 

 tance. Glutathione is a dipeptide and may be represented by 

 the following formula: 



