98 TEXTBOOK OF BOTANY 



An appreciation of four of these fundamental processes is an invaluable 

 aid to an understanding of many of the material and energy transforma- 

 tions in plants. 



Oxidation and reduction. Originally the term oxidation referred to the 

 combining of oxvgen with other elements and the consequent release of 

 energy. For instance, when coal burns, free oxygen combines with the 

 carbon in the coal, carbon dioxide ( COi- ) is formed, and the chemically 

 bound energy in the coal is liberated as heat and light. The same process 

 occurs when wood or natural gas burns. The burning or oxidation of 

 marsh gas (methane) may be indicated by a simple equation:' 



Methane , Free ^ Carbon ^Vater + ^^^^ 



(bound energy) "' oxygen dioxide energy 



H 



H-C-H + 2 O2 > 0=C=0 + 2 H-O-H + ^^l^^ 



H 



Similar oxidations occur in living plants when the carbon of food sub- 

 stances combines with oxygen and the chemically bound energy in the 

 foods is liberated. 



When sugar is made from CO2 and Hi-O in the green tissues of plants 

 the reverse of the burning process occurs. This reverse process is called 

 reduction. Some of the chemically bound oxygen in the CO 2 or H-O is 

 liberated, and free energy is chemically bound in the sugar that is 

 fonned. In this particular process it is the energy of sunlight that be- 

 comes chemically bound in the sugar. 



This earlv concept of oxidation and reduction is adequate for many 

 of the problems we shall meet in general botany. When we recognize 

 that free oxygen is being chemically bound in the plant and chemically 

 bound energy is being liberated, we may infer that oxidation is occurring 

 in the plant. When the reverse occurs we may infer that reduction is 

 occurring. 



For certain problems, however, we may want to think of these proc- 

 esses in a more fundamental way. When an atom is oxidized it may 

 lose one or more electrons to another atom; it thus gains in positive 

 charges or valence. The atom to which the electron becomes attached 



O 111 



(rains in negative charges or valence; i.e., it is reduced. By this concept, 



2 In the equations in this book in which we wish to indicate energy transformations, the 

 name of the compound containing chemically bound energy will be followed by the words 

 "bound energy" in parentheses. 



