THE ORIGIN OF LIFE 



a metabolism which was mainly catabolic; that is, it consisted of the 

 breaking down of complex compounds to release the energy stored in 

 them. But, as the environmental supply of high-energy compounds was 

 reduced, the development of a new type of metabolism became necessary. 

 This is anabolic metabolism, whereby complex compounds are built up 

 from simpler ones, and energy thereby stored. Expenditure of energy by 

 the organism is required, and so the evolving bacteria explored possible 

 sources of energy, that is, systems of respiration. Several types of respira- 

 tory reactions, and correlated nutritive systems, have been exploited by 

 bacteria. These may be classified as heterotrophic and autotrophic sys- 

 tems. The heterotrophic bacteria are generally parasitic, deriving their 

 energy by oxidation of the carbohydrates or other organic compounds of 

 their hosts. Or they may be saprophytic, that is, they live by absorption of 

 dissolved organic matter from their environment, much like the primitive 

 ones in the scheme of Oparin. On the other hand, the autotrophic bacteria 

 derive energy from chemical reactions involving simple inorganic com- 

 pounds. They are thus independent of external sources of high-energy 

 compounds. There are three principal groups of such chemotrophic bac- 

 teria, the sulfur bacteria, the nitrifying bacteria, and the iron bacteria. 

 Within each group, there are many different species, and several different 

 respiratory reactions. A few examples may be given: 



2 HoS + O2 — ^ 2 HoO + 2 S + 81,600 calories per mole 



5 + 2 HoO + 3 Oo — > 2 H0SO4 + 214,000 calories 

 NaoSo03 + 2 O2 — > NaoSdi + S + 75,000 calories 

 2 NOo + O2 — >2 NO3 + 46,000 calories 



2 NHo + 3 bo ^ 2 NOo 4- 2 HoO + 156,000 calories 



Iron bacteria oxidize ferrous compounds to ferric compounds with a 

 much lower energy yield than is indicated for the above reactions. But the 

 best of these is vastly inferior to photosynthesis, the autotrophic mecha- 

 nism of all green plants, and perhaps the real basis for the evolution of 

 the higher plants and animals, for this is the mechanism which has made 

 it possible for the world of life to tap the great reservoir of radiant energy 

 from the sun. The photosynthetic reaction may be summarized as follows: 



6 COo + 6 HoO + 677,000 calories — > CcHioOe + 6 Oo 



This large energy reserve is subsequently released by the sugar metabo- 

 lism of the organism. Photosynthesis, then, for which the chlorophyll of 

 green plants is the catalyst, not only vastly increases the energy potentially 

 available to organisms, but it also releases oxygen from its compounds, 

 thus making possible the oxygen respiratory systems of animals and of 

 some bacteria. Photosynthesis is not generally known among the bacteria, 

 but some of the sulfur bacteria contain a green pigment which is capable 

 of absorbing sunlight, and this makes some contribution to the metabolism 

 of these bacteria. 



Oparin was of the opinion that this chemical evolution of life could 

 have occurred only once, because it would require a sterile environment. 



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