OXIDATION AND REDUCTION 597 



substances formed in the cell, in many cases the process takes place already 

 inside the cell itself. In other cases the luminosity does not make its appearance 

 until the secretion is extruded. 



The use of the production of light to the organisms themselves is somewhat 

 problematical. It may serve to attract prey and, in some cases, it appears 

 to assist the progression of the organism in the dark, like an ordinary lantern. 



THE COLOURS OF FLOWERS 



In connection with chlorophyll and with oxidation enzymes, the nature of 

 the coloured pigments of flowers is of interest. As Willstatter points out, it is 

 a suggestive fact that the vitally important leaf pigment is the same in all 

 plants, whereas those of the flower are of various chemical constitution. A large 

 number, as shown by Willstatter and Everest (1913), are related to' flavones, 

 but with the oxygen atom marked (A) replaced by hydrogen, while a varying 

 number of the benzene hydrogens are oxidised or replaced by other groups. 

 Flavone is : 



H ,0 H H 



H/V NO- ~>H 



TT J\ H H 



\A / 



H \C^ 

 



This quinonoid structure, as is well known, is associated with coloured 

 substances. 



In the flower, the flavone derivatives are combined with glucose to form 

 glucosides and are capable of oxidation under the influence of an oxidase. Thus 

 Keeble, Armstrong, and Jones (1913) found that the yellow sap pigment of 

 the wallflower is a mixture of hydroxy-flavone glucosides. These are readily 

 hydrolysed by mineral acids, or, more slowly, by emulsin. The hydrolysed 

 product, if reduced and again oxidised, gives a red pigment. 



An interesting fact is the occurrence of the colourless chromogen (reduced 

 pigment) in the flower, together with both an oxidising and a reducing enzyme. 

 The former is active in water, inactive in alcohol. The latter is inactive in 

 water, active in alcohol. Or perhaps this reducing enzyme is overpowered in 

 water by the opposite action of the oxidase. The colour thus disappears in 

 alcohol, returns in water. In our discussion of the synthetic action of enzymes 

 we saw reason to believe that there are mechanisms in the cell capable of 

 reducing the effective concentration of water, and we see here that the removal 

 of water retards oxidation, and thus facilitates the action of reducing enzymes. 



SUMMARY 



There are some substances which are oxidised by the oxygen of the air. Others, 

 and these are the food-stuffs of most physiological importance, require the oxygen 

 to be made "active," as it is often called. In other words, the system which causes 

 the oxidation of such substances as sugar must have a higher oxidation potential 

 than molecular oxygen has. 



It seems to be most in accordance with experimental facts to suppose that it is 

 when in the process of changing its valency, or electric charge, that oxygen is 

 "active." This conception is practically the same as that of the nascent state. In 

 the course of another reaction oxidative effects are frequently obtained, such as 

 oxygen alone is unable to bring about. Chemical energy is thus made use of before 

 it has become degraded to heat. 



When a substance undergoes " autoxidation " by molecular oxygen, there are 

 two oxides formed in equivalent proportion. In the production of the lower oxide, 

 energy is given out, and this energy is utilised to build up the other oxide, which 



