CHAPTER 12 • HOW DO LIVING THINGS GET RID OF WASTES? 



1 How does an organism come to produce substances that it does 



not need? 



2 Are tlie wastes produced by protoplasm poisonous? 



3 Does the excreted urine in animal manure make it injurious to 



plants ? 



4 Do plants excrete wastes? 



5 What kinds of wastes are excreted ? 



6 Is sweat a kind of waste? 



7 Have all animals kidneys? 



8 How do the kidneys make urine out of wastes in the blood ? 



9 Why do physicians sometimes analyze a patient's urine? 

 10 What other organs besides kidneys remove wastes? 



Living things are continually taking in fooci and oxygen. From the 

 oxidation of food within their living cells they derive energy. We know 

 that whenever fuel burns, there are formed ashes and hot gases that would 

 smother tlie flame unless they were removed. Would any of the substances 

 formed during metabolism in living organisms interfere with further metab- 

 olism? Are there any wastes produced besides the carbon dioxide and 

 water removed by the lungs? How do living things dispose of any such 

 wastes? How does the body remove waste fluids without losing essential 

 food constituents? 



What Kinds of Wastes Are Produced in Living Things? 



The Origin of Wastes in Living Things In every chemical process 

 substances are formed that did not exist before. Some of the substances pro- 

 duced in the metabolism of a complex organism are related to keeping the 

 protoplasm alive, as, for example, digestive ferments and chlorophyl. In- 

 cidentally, however, other substances are also produced, and these may be 

 of no use to the living body or to the living process. Some may even be 

 injurious. Such substances are wastes, like the sawdust of a mill or the 

 smoke that goes up the chimney or the coal-tar of a gas factory. 



Removal of Wastes from Cells Carbon dioxide, water, urea, and other 

 waste products of oxidation in protoplasm diflfuse out of cells by osmosis. Oxy- 

 gen, which is one of the wastes or by-products of photosynthesis (see page 138), 

 also diffuses out of the chlorophyl-containing cells through the cell-walls. 



In plants, water and carbon dioxide are usually eliminated in the form 

 of gas. The carbon dioxide discharged by the cells of the roots usually re- 

 mains in solution, forming so-called carbonic acid. 



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