PROBLEM 2. How Cclls Keep Alive 



fats, or proteins that are oxidized and 

 their stored chemical energy is released 

 as heat energy. You may wonder how 

 the coal and the living cells got the en- 

 ergy which is hidden within themselves. 

 The understanding of this is an important 

 part of biology. You will learn in the 

 next Unit how energy gets into all living 

 cells. For the present you need only re- 

 member that all living cells contain stored 

 chemical energy; this is changed into 

 other forms in the process of oxidation. 



Work in living things depends on oxi- 

 dation in the cells. Have you ever 

 stopped to think that as long as an or- 

 ganism remains alive it is constantly re- 

 leasing energy? Heat is being released 

 and work is being done. To biologists 

 work means more than earning your 

 living or going to school. Playing ball is 

 much harder work than studying a les- 

 son. Moving your eyes across this page 

 is also work. To keep your body standing 

 upright you must do a considerable 

 amount of work. Even when you sleep 

 yout heart keeps working regularly; so 

 do the chest muscles and other parts of 

 the body. Millions of cells are always 

 carrying on oxidation and doing work. 

 In plants, too, the living cells are con- 

 stantly carrying on oxidation, releasing 

 energy, and doing work. All living things 

 do work as the result of the oxidation 

 which occurs in all their millions of cells. 



Why oxidation can go on continuously. 

 You read that carbohydrates, fats, and 

 to some extent proteins in protoplasm 

 serve as fuel in living cells. These com- 

 pounds are spoken of as food substances 

 for living things. They unite with oxy- 

 gen and thus they disappear and new 

 compounds are formed. In other words 



121 



Fig. 149 This resting cow is doing work. What 

 kind of work is being dojte? What ki?ids of 

 energy are being released? (schneider and 



SCHWARTZ) 



the food compounds and oxygen are 

 constantl)' being used up in oxidation. 

 But, under normal conditions, they are 

 constantlv^ being replaced. Evidently, 

 there must be a fairly constant passage of 

 oxygen and of these various compounds 

 into the living cell. They move into the 

 cell by the process known as diffusion. 

 If you do Exercise 9 you will see diffu- 

 sion occurring. Let us review it. 



Diffusion. You know that when you 

 put sugar into the bottom of a cup of 

 coffee and wait a short time the sugar 

 will sweeten all parts of the drink even 

 without your stirring it. That is, the 

 sugar moves through the coffee. But 

 how is its motion explained? Chemists 

 tell us that all substances consist of tiny 

 particles known as molecules (moll'e- 

 kewls). These molecules are in constant 

 motion. In gases the molecules are far 

 apart and they bounce about actively. 

 Each one moves, first in one direction. 



