PROBLEM A Hoiv Do Their Activities Keep Cells Alive? 



What do living things do to remain ahve? 



This question might have been asked 

 another way: what is it that Hving things 

 do that makes them different from hfe- 

 less things? You can think at once of 

 many activities that distinguish the Hving 

 from the hfeless. If you think of animals 

 you will say they move from place to 

 place (without the help of an outside 

 agent) ; they take in food and grow; they 

 breathe; they produce many substances 

 useful to them; they get rid of wastes; 

 and they make more of their own kind, 

 or reproduce. Plants engage in many of 

 these activities, too, although sometimes 

 in ways different from animals. Trees 

 cannot move from place to place, but 

 they use food and grow, they make sub- 

 stances useful to them, and they cer- 

 tainly make more of their own kind. 



Activities of the organism are activities 

 of the cells. You learned in the last prob- 

 lem that all living things are made up of 

 cells — small masses of protoplasm. There- 

 fore, it should not surprise you that all 

 the activities of a living thing are also 

 the activities of its cells. Food enters the 

 cells in your body, and the cells grow; 

 cells make substances useful to cells, and 

 they reproduce. It is true that most of 

 the cells in your body do not move from 

 place to place but the protoplasm within 

 the cell moves. Some of these activities 

 are visible when you study cells with 



the microscope; others are not visible. 

 If microscopes are available, the move- 

 ment of protoplasm can be readily seen 

 in the ameba and also in some plant cells. 

 See Exercises i and 2. Living paramecia 

 can be seen engaging in various activi- 

 ties. See Exercise 3. But you must re- 

 member that the paramecium is a single- 

 celled animal and performs some of its 

 activities differently from the cells that 

 make up the body of a many-celled 

 animal or plant. Let us study in more 

 detail some of the more important cell 

 activities. 



Oxidation occurs in the cell. Some of 

 you may know the meaning of the word 

 oxidation. All of you can guess from the 

 sound of the word that it has somethinij 

 to do with oxygen. Oxidation is the 

 chemical union of a substance with oxy- 

 gen. If carbon unites with oxygen the 

 compound that results from the union 

 is normally carbon dioxide. If hydrogen 

 combines with oxygen, the compound 

 hydrogen oxide (water) results. If iron 

 unites with oxygen, iron oxide (com- 

 monly called rust) is produced. And so 

 with other substances; when they unite 

 with oxygen, oxides are formed. 



The union may be rapid or slow. You 

 constantly see examples of rapid oxida- 

 tion, for rapid oxidation is burning or 

 yombusrion. When you touch a lighted 

 match to a piece of paper, rapid oxida- 



