92 ORGANIC FUNCTIONING 



IV. THE ENERGETICS OF ORGANISMS 



It is convenient to make the distinction between physico- 

 chemical reactions that occur of themselves, and those that will 

 only occur when they are coupled with other reactions that occur 

 of themselves. 



When a chemical reaction, say 2H2 + 02= 2H2O, occurs of 

 itself free or available energy is generated. Usually heat is 

 produced. Such a reaction is, in cases where heat is generated, 

 called exothermic. When a reaction, chemical or physico- 

 chemical, occurs of itself, energy is dissipated and entropy 

 increases. 



Coal burns in an atmosphere containing oxygen and these 

 reactions happen, C + O2 = CO 2, 2H2 + O2 = 2H2O. But the 

 CO 2 does not, of itself, combine with O to form a compound 

 CO 3. Nor does HoO combine with O, of itself, to form a 

 compound H2O2. 



But the possible reaction, H2O + O = H2O2, can occur 

 when this possibility is coupled with some other reaction in 

 which free, or available, energy is produced. Then hydrogen 

 peroxide is formed and the reaction is called an endothermic one. 



35. ON TYPICAL PLANT METABOLISM 

 The characteristic chemical reactions occurring in the green 

 plant are those leading to carbon- assimilation. Here we find 

 that these chemical reactions occur : 



CO2 + OH2 = H.CHO + O2 



formaldehyde 



6H.CH0 = CeHi20e 



a carbohydrate. 



Now these reactions do not occur of themselves but only when 

 the green plant is exposed to sunlight which contains radiation 

 of high frequency. Usually this high-frequency radiation 

 degrades, as when it falls on the ocean which is then heated, or 

 suffers evaporation, which leads in the long run to the dissipation 

 of heat. In this latter case energy is dissipated and entropy 

 increases. 



But when the high-frequency radiation is intercepted by the 

 chlorophyll of the green plant, some of it does not degrade. 



