CHAPTER IV 

 METABOLISM OF ORGANISMS 



Nature, which governs the whole, will soon change all things which 

 thou seest, and out of their substance will make other things ana 

 again other things from the substance of them, in order that the world 

 may be ever new. — Marcus Aurelius. 



Life is only known to us in the form of individual organisms, 

 so we turn now to concrete examples of unicellular plants and 

 animals which present, in relatively simple form within the con- 

 fines of a cell, the essentials of all the fundamental life processes, 

 many of which we shall later have occasion to study in their com- 

 plex expressions in the higher animals. Our present interest in 

 these simple forms is chiefly to illustrate the complex nutritional 

 interdependence of three great groups of organisms — green plants, 

 animals, and colorless plants. Animals cannot exist without plants. 



A. Green Plants 



Unicellular green plants are distributed all over the world and 

 adapted to a great variety of conditions. We find them, for ex- 

 ample, forming green coatings on the bark of trees, scums on 

 puddles and ponds, or being blown about in dust by wind. Of the 

 many hundreds of species we select Protococcus vulgaris because 

 of the simplicity of its structure and life history, and because it is 

 readily obtained for study. 



1. Protococcus 



A single Protococcus is invisible to the naked eye, but like many 

 another microscopic form, it makes up in numbers for the small 

 size of the individual, and frequently gives a greenish color to moist 

 surfaces of rocks, troe trunks, fence posts, and flower pots. 



Examined under the microscope, the organism is seen to consist 

 of a spherical mass of protoplasm with a nucleus centrally located. 

 Most of the cytoplasm surrounding the nucleus is specialized to 

 form a plastid which contains a green pigment. The whole organ- 

 ism is enclosed within a distinct, rigid cell wall which has been 



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