LIFE AND PROTOl'LASM joy 



Some Signs of Manifestation of Life 

 One sign of life is the release of cnersy, which is a rosult of respir-i- 

 tion. It occurs in all living things, ho it a tree, a frog, or a man 

 when oxygen is taken into the body, whore it combines with oxidiz- 

 able materials to release energy. The by-products are carbon dioxide 

 and water, which are given off by the organism. 



Living things are sensitive to and respond to various stimuli in 

 their environment. The plant in the window, the earthworm in 

 the ground, the fish in the water, and the bird in the tree are all 

 sensitive to and respond in different ways to the stimulus of light. 

 Temperature, chemical substances, gravity, electricity, radiations, 

 and mechanical factors, all are stimuli which affect living things in 

 different ways. It is this characteristic of li\ing things that we call 

 irritability or sensitivity. 



One direct outcome of the ability of living things to respond to 

 stimuli in their environment is their adaptiveness. Thus li\ing 

 organisms ha\e the capacity to adjust themseh-es to changes in con- 

 ditions. Some plants throw out new roots, or suckers, or trailing 

 stems, by means of which they can get a foothold in slightly different 

 environments from those in which they are accustomed to live. 

 Certain low forms of plants have even become adapted to li\e in the 

 hot springs as those in the Yellowstone National Park, in a habitat 

 many degrees warmer than that of their near relatives found in 

 adjoining pools. Fishes, and certain small crustaceans, may similarly 

 adapt themselves to life in water containing a high concentration of 

 salts. This power of adaptation is a quality of the organism as a 

 whole, and results in adjustment between the external environment 

 and the internal body material. 



The Production and Use of Enzymes by Living Things 



In recent years a good deal of work has been done by physiologists 

 to see how the cell is able to perform the cycl(\ in which material 

 is taken into the organism as food or is made into food as in the case 

 of green plants. Food is changed into a j^oluble form so that it may 

 pass through the delicate living membrane of every coll. Meantime 

 each cell is using oxygen, which also has to be taken in through the 

 cell membrane, while wastes are given off by the same road. Physi- 

 ologists seem agreed that those living processes, called digestion, 

 absorption, respiration, and excretion, are made possible by the 



