CHEMICAL STRUCTURE AND METABOLISM 91 



relatively simple, energy-poor carbon compounds. How could free energy 

 be applied to them to build them into carbohydrates and proteins? Blum 

 (1955) has suggested that the origin of life may have been a very gradual 

 process, so that it might be difficult to draw a precise line between non- 

 living and living. This author suggests the possibility that adenylic acid 

 compounds containing phosphorus may have made possible a first step in 

 this gradual process. We recall that such compounds are essential for the 

 transport of free energy in living cells. These compounds incorporate in 

 their structure "energy-rich phosphate bonds"; when the bonds are bro- 

 ken energy is released to be utilized by the cell. Perhaps such adenylic 

 acid compounds, formed in the days of inanimate chemical evolution, 

 provided the free energy needed for the first step in transforming energy- 

 poor carbon compounds into energy-rich ones. We may imagine that, fol- 

 lowing such a beginning, the process of developing compounds increas- 

 ingly rich in energy progressed step by step until eventually the first 

 proteins appeared. With the appearance of proteins the way would have 

 been open for the gradual development of increased complexities of struc- 

 ture and function. Especially important would be the development of 

 some form of photochemical reaction by which the sun's radiations could 

 be harnessed to provide energy for building energy-rich compounds in 

 quantity needed if life was to progress beyond the first simple beginnings. 

 The photochemical reaction eventually evolved was, as we have seen, the 

 photosynthesis carried on by green plants. It is significant that blue-green 

 algae, single-celled plants capable of this synthesis, are among the first liv- 

 ing things of which we have fossil evidence (p. 138). 



The mystery of the origin of life has such appeal that many scientists 

 have theorized about it. We have touched on the matter only as it in- 

 volved the initial mobilization of free energy in days before the advent 

 of photosynthesis. Readers are referred to Blum (1955), Oparin (1957), 

 and Wald (1954) for further discussion of this fundamental problem and 

 for references to other writings on the origin of life. 



Disposal of Nitrogenous Wastes 



Ammonia is a nitrogenous waste product continually formed in all ani- 

 mals. It is derived not only from the splitting of excess proteins taken in as 

 food but also from the breaking down of proteins from worn-out tissues of 

 the body. The problem of ridding the body of this toxic waste product is 

 faced by all animals. 



It is relatively simple for aquatic animals. Ammonia is highly soluble in 



