UNIT THREE — REVIEW • HOW DO LIVING THINGS KEEP ALIVE? 



We may survey the world of life from the point of view of man or from 

 that of the ameba. In each case we are left uncertain whether the uniformi- 

 ties or the diversities are more impressive and remarkable. Hundreds of 

 thousands of plants and animals differ enough to be kept clearly apart by 

 the observing. Yet they are enough alike to carry on the same basic proc- 

 esses. Cabbages and kings both grow on proteins, fats and carbohydrates. 

 Both depend upon water and air. Both discharge their wastes into the outer 

 world. Both are beset by various parasites. And both, after death, become 

 the food of a million humbler beings. 



The naked protoplasm of the ameba is most intimately related to its 

 environment of changing fluid and floating particles. It swallows portions 

 of this environment and assimilates them. Other portions (water, dissolved 

 salts and gases) flow in and out, now faster, now slower, bringing in and 

 taking away. This inward and outward diffusion is determined in part by 

 the nature of the protoplasm and in part by the momentary state of the 

 surroundings. The material condition of living is an interaction of a living 

 unit and the rest of the world; it is a stream of events rather than a static 

 combination of substances at a particular temperature. 



Living protoplasm is a constant aggression against the environment. It 

 takes from the world a variety of materials which it makes its very own — 

 its very being. The life of an organism consists of building itself up into 

 more and more, and of dodging dangers. The earliest life forms were 

 probably even simpler than the ameba, and they must have been able to 

 transform inorganic compounds into more complex ones by absorbing free 

 energy, as chlorophyl-bearing cells store sunshine energy in carbohydrates. 



Getting stuff from the surroundings may be as simple as absorbing fluid 

 or gas by osmosis. We may consider the many thousands of plant species as 

 elaborations of protoplasm more and more specialized in the direction of 

 more efficient capture and storage of sunshine. The elaborations establish 

 communication between protoplasm far from the surface and the outside 

 world. The specializations include transportation systems and supporting 

 systems. A large tree will make tons of wood and bark in the course of 

 raising its leaves aloft and sending its roots afield. The living processes, 

 however, are confined to the protoplasmic contents of living cells. Further 

 elaborations are related to tiding-over periods not favorable to metabolism 

 and to resisting the constant threat of destruction by other living things. 

 Essentially, however, the plant is a system of processes and structures 

 through which the environment is selectively taken in and transformed into 

 more plant. It is a system of maintaining a constant stream of materials 

 through the protoplasm. 



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