176 LIGHT, VEGETATION AND CHLOROPHYLL 



an environment which is consequently deprived of it or, in 

 correct terms, is reduced. We should not, therefore, speak of 

 oxidation pure and simple, but of oxidation-reduction. If we 

 speak of oxidation, we are considering an isolated substance 

 and all the rest will be the environment in which the com- 

 plementary reduction is effected and with which we are not 

 directly concerned. But obviously a reaction cannot be under- 

 stood and reproduced unless it is seen in its entirety and all 

 the terms in question and all the exchanges involved are 

 included in the equation. 



There are two sorts of valencies that may be called positive 

 and negative. Electronegative atoms lack a few electrons with 

 respect to the complete external layer which characterizes the 

 rare gases. Oxygen is the classic type of these electronegative 

 atoms; it lacks 2 electrons and when it finds a substance 

 capable of supplying them it fixes on and oxidizes it. Electro- 

 positive atoms possess surplus electrons and are capable of 

 giving them up to electronegative atoms. Hydrogen is the 

 classic type of these substances. 



Oxidation corresponds to a Uberation of electrons, while 

 reduction corresponds to a capture of electrons. The more the 

 environment is reduced, the greater the number of electrons 

 in it; the more it is oxidized, the smaller the number of 

 electrons. The arrival of electrons in an environment will 

 therefore be a reduction and, conversely, we should say that 

 iron is oxidized when its third valency is saturated, even if 

 oxygen does not intervene. 



Since electrons have a negative charge of electricity, their 

 presence will necessarily modify the electric potential of the 

 environment and make it negative, while a greatly oxidized 

 environment will be strongly positive. 



Thus there appears the potential of oxidation-reduction 

 which is evaluated in maintaining the pH at 7. A strongly 

 oxidized environment will have a potential of about +0-8 volt, 

 while glucose-6-phosphate will have one of - 0-4 volt. 



The work of photosynthesis consists in maintaining this 

 void which oxygen tends to fill with all the force given to it 



