50 INTRODUCTION TO GENERAL PHYSIOLOGY 



supposed that the production of optically active compounds is 

 confined to the living organism, as sometimes suggested. All 

 compounds with asymmetrical carbon atoms must be optically 

 active. The point is that in the laboratory the two oppositely 

 rotating isomers are nearly always formed in equal amount, so 

 that the actual rotation is zero. The living organism produces 

 one only, because the formation takes place by means of asymmetric 

 agents, which are themselves already optically active, since they 

 consist of one isomer only. 



The Green Plant 



Since animals cannot do with less complex sources of carbon 

 and nitrogen than glucose and amino-acids, we have next to 

 inquire where the supply comes from. They are only found in 

 nature in the bodies of animals and plants. These bodies, or 

 materials extracted from them, are taken as food by other animals. 

 After being used they are rejected in simpler forms, deprived of 

 energy, the carbon in great part as carbon dioxide (E., p. 182), the 

 nitrogen combined with part of the carbon mostly as urea (E., p. 182), 

 but sometimes in other more complex forms. None of these will 

 serve again as food, until they have been built up by the supply of 

 energy to more complex forms. 



A word of explanation is needed as to urea. The whole of the 

 nitrogen contained in the protein food is not needed for repair 

 purposes, and urea is the way in which the waste ammonia groups 

 are got rid of by combination with carbon dioxide. Urea is 

 obtained from ammonium carbonate by removal of water, and can 

 easily be reconverted by hydrolysis. Thus : 



0-NH 4 .NH 9 



C0< = C0< " + 2 H 



X O NH 4 X NH 2 



Thus urea is the diamide of carbonic acid. 



There is, then, a continuous using up of available carbon by 

 animals, and the same is true for plants, with the exception of 

 certain special structures in the green plants. It is only by the 

 aid of these that the life of both animals and plants on the earth 

 is preserved from final extinction. 



In the oxidation of food, not only are useful carbon compounds 

 used up, but the oxygen of the atmosphere also. We have now to 

 learn something about the wonderful mechanism by which they 

 are both restored in the course of the same reaction. This is 

 probably the most interesting mechanism that exists, as well as 

 being that on which the continued existence of life on the earth 

 depends. 



