INTRODUCTION TO BIOLOGY 



and from the brain cells. The differentiated cells are organized into 

 tissues and the tissues into organs. The functioning of each cell is 

 controlled by the organism as a whole. The cells of a multicellular 

 organism are the integrated, dependent, and interdependent parts of 

 the organism. In this way, cells of a multicellular organism differ 

 from the microbial cell which is an organism itself, an independent 

 systejti of reproduction. One category, the cells of an organism, 

 corresponds to parts of an organism; the other, the microbial cell, 

 to a complete organism. 



Organisms contain macromolecules endowed with a specific 

 structure and a specific function. In order that life can be perpetu- 

 ated, these macromolecules have to work in co-operation. The 

 smallest unit of integration, co-operation, and reproduction is the 

 cell. Both the cell of a multicellular organism and the microbe are 

 the ultimate units of integration and reproduction. As such, they 

 share so many essential features that the common traits have tran- 

 scended the differences. The term cell is now used for the metazoal 

 cell as well as for the microbe. But it is essential to know that the 

 word cell applies to two different categories of systems: the de- 

 pendent part of the multicellular organism and the independent 

 unicellular organism. 



Life, Organism, Reproduction, 

 and Assimilation 



Assimilation is sometimes listed among the discriminative features 

 of life. From a biochemical point of view, assimilation, "the action 

 of making like," is the assemblage of small building blocks — amino 

 acids or nucleic bases — common to all living beings, into specific 

 macromolecules — proteins or nucleic acids — that are characteristic 

 for each species. Assimilation is essentially the organization of 

 complex specific sequences. Assimilation is a patternization. 



Only a living being is able to s\'nthesize a few thousand specific 

 macromolecules, putting each of the twenty amino acids and each 

 of the four nucleic bases where it belongs. This can be accomplished 

 only because of the presence and activity of specific templates that 

 are parts of the living system. 



[5] 



