Chapter 5 



What Are Living Units? 



Many authorities maintain that it is not possible to draw a line 

 of cleavage between living and non-living entities. The task is 

 complicated by the fact that nature is so rich in examples and so 

 gradual in most changes, that it is hard to fit all the data into the 

 rigid frame of any definition. Nevertheless, we must first try to 

 define what Ave mean by "living" units or entities. 



In its broadest sense a living unit or entity is one that can direct 

 chemical changes by catalysis,* and at the same time reproduce 

 itself by autocatalysis, that is, by directing the formation of units 

 like itself from other, and usually simpler chemical substances. 

 Among the simplest known living units are genes, and some of the 

 bacteriophages and ultrafiltrable viruses, which in size approxi- 

 mate molecular dimensions. The electron microscope indicates 

 that some bacteriophages ("the bacteria of bacteria") and some 

 viruses {e.g., that of psittacosis, or "parrot fever") are tiny organ- 

 isms. 



Figure 7, prepared by Dr. W. M. Stanley (Nobel Prize, 1946) 

 shows that some viruses and bacteriophages approach molecular 

 dimensions. See also Figure 8, an electron micrograph of tobacco 

 mosaic virus. 



Organisms are known which can synthesize their own organic com- 

 pounds from inorganic substances. These are called autotrophs. The 

 best known of this group are chlorophyll-containing plants, including 

 algae, which by photosynthesis form organic compounds from carbon 

 dioxide. The autotrophic bacteria may live in the absence of light, 

 are generally microscopic, and are not distinguishable morphologically 

 from other bacteria; but they derive their energy from the oxidation 

 of inorganic substances and utilize it to reduce carbon dioxide to 

 organic compounds. Intermediate between the autotrophic bacteria 



* Catalysis is discussed in the next chapter. In brief, it is the process whereby a 

 specific particulate unit or surface (the catalyst) continuously brings about chemical 

 union, breakdown, or structural change in other units as a result of very close 

 contact or approach, under suitable conditions. 



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