XU UNITY AND DIVERSITY IN BIOCHEMISTRY 



Parts 1, 2 and 3 are devoted to general aspects of the biosphere, i.e. to 

 the biochemical facts common to living beings and which constitute 

 their lowest common denominator, or, their "unity of biochemical plan". 

 The cellular theory, as proposed by Theodor Schwann in 1836, taught 

 that organisms are formed not only of cells, of modified cells and cellular 

 products, but that a multicellular organism has two levels of individuality, 

 one on the cellular level and one at the level of the whole organism. In 

 its final perfected form, the cellular theory recognizes that each cell is 

 derived from a pre-exisiting cell. The results of biochemical research 

 have taught us that the manifestations of an underlying biochemical unity 

 are present in each cell, according to a topochemistry briefly described in 

 Part 4. In this context, the "unity of plan" is simply the cellular theory 

 from a biochemical point of view; in the same way as they are units of 

 structure, cells are units of metabolism. The unity of a structure and 

 cellular metabolism is only another expression of cellular continuity and the 

 persistence in this continuity of a definite collection of genes controlling the 

 synthesis of the collection of enzymes present in each cell. However, no cell 

 is limited by the underlying biochemical unity, for this is only the canvas 

 on to which the cell can embroider the numerous variations constituting 

 its own biological nature, the "unity of plan" remaining an abstraction. 



In Part 5 it is only possible to provide a few very brief examples of bio- 

 chemical variation and biochemical evolution. The few cases quoted 

 will enable the reader to locate some aspects of biochemical variation at 

 the level of cellular diflferentiation and at the taxonomic level in the same 

 organism. These examples show that the biochemical manifestations of 

 variation are founded on the extension of the general processes of cellular 

 biochemistry and constitute atypical expressions of general metabolic 

 systems, variations on each theme being more or less pronounced but 

 fitting in with general ideas of variation of genotype and biochemical 

 adaptation of the organism to its surroundings. 



Part 6 presents the idea of the metabolism of the biosphere as a whole 

 and illustrates how this metabolism, like that of each organism, consists 

 of an entry and an exit of energy and material, but now situated at the 

 frontier between the biosphere and the inorganic world. The metabolism 

 of the biosphere is conditioned by manifestations of biochemical variation, 

 without which life would disappear. In fact, the unity of the biochemical 

 plan of organisms comes down finally to a continuity of the biosphere 

 in time and space and the accompanying biochemical diversity which has 

 appeared as biochemical evolution has progressed with the extension of 

 the biosphere. This extension has only been possible when, by means of 

 new ecosystems, the exchanges between the inorganic surroundings and 

 the biosphere, which are a condition for the survival of the latter, have 

 been maintained. 



