OXIDATION AND REDUCTION 589 



with the anaerobic production of alcohol in the latter. In the presence of oxygen, 

 the dye does not suffer reduction. 



THE GUAIACUM REACTION OF BLOOD 



Haemoglobin has the power of oxidising guaiaconic acid. This has been shown 

 by Buckmaster (1907) to be due to the iron contained in it; the iron-free 

 derivatives do not give the reaction. 



We have seen reason for regarding iron as a peroxidase, but since a peroxidase 

 requires a peroxide to act upon, it appears that, in the blood reaction, there must 

 be some substance present which is autoxidisable. 



It is interesting to note that lecithin is oxidised in air by ferrous-ammonium 

 sulphate (Thunberg, 1911). It would thus form a peroxide, like benzaldehyde 

 does. Certain other cell constituents, nuclein, albumin, glucose, oleic acid, are not 

 oxidisable by iron alone. 



TYROSINASE 



The production of pigments by the action of an oxidising enzyme on tyrosine 

 has been referred to above (page 359). This enzyme appears to be of frequent 

 occurrence, not only in fungi, where it was first found, but also in animal tissues, 

 amongst others in insect larvae. According to Bach (1914), the system called 

 tyrosinase is a complex one. The effect of one of its constituents is to reduce the 

 tyrosine. The products are then easily oxidised by the oxidase also present. The 

 behaviour is similar to that of the respiratory pigments of plants, described by 

 Palladin (1909), which are alternately oxidised and reduced by the agency of 

 enzymes. 



THE OXIDATION SYSTEM OF THE CELL 



So far as we have arrived, the chemical process of oxidation in the cell seems 

 to be as follows. Some autoxidisable substance in the cell takes up molecular 

 oxygen, with the formation of peroxides and activation of half of the oxygen. 

 The other half of the oxygen serves for complete oxidation of part of the autoxidis- 

 able substance. These peroxides are acted upon by peroxidase, with further 

 increase of active oxygen, which is able to bring about oxidation of substances 

 not autoxidisable and otherwise difficult of oxidation. But when we come to 

 apply the facts learnt by study of extracts or of disintegrated cells to the inter- 

 pretation of phenomena taking place in the living cell, we find that there is some- 

 thing else to be taken account of. This we may call " structure," meaning thereby 

 not merely the coarse structure seen under the microscope, which is probably 

 less important than the ultra-microscopic structure, of colloidal nature, to which 

 attention was called previously (page 19). 



The suggestion made by Warburg (1914), as to the purpose of the energy 

 set free by oxidation in cells which do no external work, has been referred to in 

 an earlier chapter of this book (page 32). 



We have already met with cases in which the importance of structure forces 

 itself upon the attention. The non-disappearance of lactic acid in muscle, after 

 rubbing with sand (Fletcher and Hopkins, 1907), the inability of Harden and 

 Maclean (1911) to obtain press juices from tissues which could continue to consume 

 oxygen, the great effect on oxygen consumption of alkalies which do not enter 

 the cell itself (Warburg, 1910), and other similar actions on the surface, may be 

 mentioned. 



It should be pointed out that it is impossible to draw a hard and fast line 

 between the phenomena to be considered here and those of tissue respiration, to 

 be dealt with in the following chapter, but I will attempt not to repeat statements 

 more than is necessary. 



The observations of Warburg and Meyerhof (1912) serve to illustrate the 

 problem before us. The red blood corpuscles of birds contain nuclei and, in their 

 normal condition, consume oxygen in considerable amount. If a press juice is 



