CHAPTER XIX 



ELIMINATION OF WASTES 



Questions. 1. Why does an organism produce substances that it does 

 not need ? 2. Are excretions of protoplasm injurious to living things ? 

 3. Have all animals kidneys ? 4. Why does a physician sometimes ask 

 for a specimen of the patient's urine ? 



159. The origin of wastes in living things. Every chemical 

 process results in the formation of substances that did not exist 

 before. In metabolism (the chemical processes of protoplasm) 

 some of the substances produced are related to keeping the pro- 

 toplasm alive, for example, digestive ferments, and chlorophyl 

 and other pigments. Incidentally, however, other substances 

 are also produced, which may be of no use to the living body or 

 to the living process. Some may even be injurious. Such sub- 

 stances are called wastes and may be compared to the sawdust 

 of a mill, or to the smoke that goes up the chimney, or to the 

 coal tar of a gas factory. 



160. Removal of wastes from cells. We have already seen 

 (Chap. XV) that oxidation in protoplasm gives rise to carbon 

 dioxid, water, urea, and other waste products. These diffuse 

 out of the cell by osmosis. In our study of photosynthesis 

 (sect. 83) we found that one of the wastes, or by-products, 

 is oxygen, which diffuses out of the chlorophyl-containing cells 

 through the cell walls. 



In plants water and carbon dioxid are thrown out, in the form 

 of gas or vapor, from the parts exposed to the air. The carbon 

 dioxid given off by the cells of the roots usually remains in solu- 

 tion, forming so-called carbonic acid. Other wastes produced 

 by plants are not generally eliminated from the body but are 

 more likely to be locked up in cells, where they can do no harm 

 to protoplasm. Among the waste substances thus accumulated 



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