PROBLEM 2. How Cclls Keep Alive 



tion (burning) usually takes place. The 

 paper unites with the oxygen which is 

 present in the air. In uniting, it forms 

 a variety of oxides and produces heat 

 and light. It is true that the paper does 

 not burn until you touch a lighted match 

 to it; the match serves to heat the paper 

 to its kindling temperature. This is gen- 

 erally necessary for rapid oxidation. Slow 

 oxidation occurs at lower temperatures. 



There is another difference between 

 slow and rapid oxidation. In slow oxida- 

 tion no light is produced. But heat is 

 always produced whenever oxidation 

 takes place; the slower the oxidation, 

 the less the heat. In fact the amount of 

 heat may be so small that delicate instru- 

 ments are needed to detect it. At this 

 point, unless you have done these ex- 

 periments before, you will find it profit- 

 able to do Exercises 4, 5, and 6, Also try 

 Exercise 7. 



Let us sum up what we have learned 

 about oxidation: Oxygen must be pres- 

 ent if oxidation is to take place; an oxide, 

 or^compound of oxygen with another 

 substance, is always formed; heat is re- 

 leased; and if the oxidation is rapid, 

 light is also produced. 



Oxidation occurs in all living cells. 

 Some of the compounds in the proto- 

 plasm, particularly carbohydrates and 

 fats, unite with oxygen. Oxides are 

 formed in the cell and heat is produced. 

 Among these oxides is carbon dioxide. 

 Can you devise an experiment to show 

 that oxidation goes on somewhere in 

 your body? Do Exercise 8. Ordinarily, 

 in this oxidation within the cell no light 

 is produced. Oxidation within the cell 

 is of great importance. The whole proc- 

 ess is also called cellular respiration. 



119 



Fig. 147 Tlois l.yirdlfr is using energy. Where 

 does it come from? (public schools of evans- 



VILLE, INDIANA) 



Energy. What is energy? Energy can 

 be defined as the ability to do work, 

 that is, make something move. Energy 

 makes work possible. You just read that 

 when burning takes place heat and light 

 are produced. Heat is one form of en- 

 ergy; light is another. There are other 

 forms of energy, many of which are of 

 less interest to us in biology. 



Let us consider for a moment why 

 heat is thought of as a form of energy. 

 In a simple machine like a steam engine 

 the engine cannot do work unless there 

 is steam to push the piston. You say, 

 therefore, that steam has energy, the 

 ability^ to do work. The energy in the 

 steam is heat energy. When the steam 

 loses its heat and becomes water again, 

 it can no longer push the pistons; that 

 is, it can no longer do work; it has lost 

 its energy. Light energy is not often 

 used by man for running machines; but 



