Life and Protoplasm - 5 



ways contributes both energy — the energy 

 that activates the vital system — and matter, 

 to form new components in the living struc- 

 ture. In other words, food in the living body 

 can serve not only as a fuel, but also as raw 

 material for chemical syntheses that provide 

 for maintenance and lead to growth. 



Many inanimate bodies utilize fuels, but 

 few, if any, can grow or even maintain their 

 existing structure. An automobile duplicates 

 almost all the destructive phases of animate 

 nutrition. It takes in fuel (food); it distrib- 

 utes the fuel to the carburetor; it sucks in 

 (breathes) oxygen through the carburetor, 

 which sends the fuel-oxygen mixture to the 

 cylinders; it chemically decomposes (oxi- 

 dizes) the fuel and utilizes the energy that is 

 liberated for the development of mechanical 

 power. Furthermore, the automobile must 

 eliminate (excrete) the end products (waste 

 products) of its chemical activities. But here 

 the analogy stops. The automobile cannot 

 grow. It cannot even replace the small struc- 

 tural losses that inevitably result from wear 

 and tear. All the constructive phases of nu- 

 trition, by which new substances are synthe- 

 sized and incorporated into the structure of 

 the living body, are absent in all inanimate 

 systems. 



Growth and Reproduction: The Most 

 Unique Activities of Living Systems. Biol- 

 ogists have tried to find a parallel to the char- 

 acteristic growth phenomena of living bodies 

 in the "growth" of crystals in a super-satu- 

 rated solution. But this phenomenon seems 

 much simpler than organic growth. Crystal 

 growth follows a precise and characteristic 

 pattern, but is altogether local and external. 

 The crystal enlarges by the addition of new 

 molecular layers at the surface only, and the 

 enlargement is at the expense of molecules 

 that exist as such in the surrounding solu- 

 tion. Organic growth, in contrast, pervades 

 the entire protoplasmic mass, and depends 

 upon an elaborate series of chemical changes 

 leading to the formation of new components 

 in the protoplasm. Or, from another point of 

 view, organic growth depends upon a pre- 



cisely patterned aggregation of many kinds 

 of molecules, whereas crystals grow by the 

 assemblage of one, or at most, two or three 

 kinds of molecules. 



The most unique characteristic of living 

 bodies is the capacity for reproduction: and 

 here it is hard to find any convincing inani- 

 mate examples. From the humblest bacterium 

 to the mightiest mammal, each living species 

 must maintain an unbroken line of descent, 

 if it is to avoid extinction. The processes of 

 reproduction are extremely complex and 

 delicate even in the simpler forms of life. 

 The formation of a new body, which is al- 

 most an exact replica of the old, presupposes 

 the existence of a delicate mechanism that 

 can sort out certain important components 

 in each living organism and pass these on to 

 each ensuing generation. These important 

 genie materials, as we shall see (Chap. 26), 

 possess not only the unique potential of self- 

 replication, but also the capacity of provid- 

 ing templates for the replication of other 

 essential components in each particular kind 

 of living thing. 



Certain inanimate bodies, namely crystals, 

 may show an extremely simple form of "re- 

 production." Occasionally, while a small crys- 

 tal is in the process of "growing," it will 

 fragment spontaneously and each of the frag- 

 ments will become the center around which 

 a perfect new crystal will form. However, 

 with the possible exception of the multiplica- 

 tion of the filtrable viruses (pp. 7-11), such 

 a "reproductive" process is incomparably 

 simpler than all cases of animate reproduc- 

 tion. 



LIFE AND PROTOPLASM 



To define life completely is scarcely pos- 

 sible, but the word can be used to designate 

 the sum total of all activities — responsive- 

 ness, nutrition, reproduction, etc. — that are 

 displayed by living bodies generally. Life, 

 according to this usage, simply specifies 

 "what living things do." It does not in any 

 sense explain what they do, or how they do it. 



