i68 



Carbohydrates 

 Fats 



Proteins 



How a Complex Aji'wml Uses Food unit iv 



Minerals 



Water 



Oxidation 



#%■• 



Assimilation 



Fig. 189 Hoiv food coiiipoiinds are used by a livhig cell, hi what two ways are food 

 compounds jisedF Which coiiipotmds are used hi each process? Why are two arrows 

 drawn as darker lines? 



were to eat the proper food in proper 

 amounts. Some are actually ill because 

 their diet does not contain the proper 

 foods. Others may succumb to infection 

 because poor diets do not make them as 

 resistant as they should be. 



Since the beginning of this century 

 scientists have analyzed all our common 

 foods in the laboratory. They can tell us 

 what compounds are in the food and the 

 proportion of each. They can tell us 

 how much energy there is in a given 

 amount of each food. They can tell us 

 how each substance is used in the body 

 and how much of each is needed to keep 

 the body in good health. All of us can 

 now obtain this information. In this 

 book and in many others there are tables 

 showing the composition of some com- 

 mon foods. Use the table on page 172 to 

 answer the questions in Exercise i. 



Measuring heat energy. To measure the 

 energy in a food substance it is necessary 

 to burn the substance. By oxidation 

 (burning) the chemical energy in the 

 substance is changed to heat energy. It 

 is then possible to measure the amount 

 of heat energy produced. The idea of 



measuring heat energy may be new to 

 you. Do not confuse measuring temper- 

 ature with measuring heat energy. Heat 

 and temperature are not the same thing. 

 The difference between temperature 

 and heat can be understood if you con- 

 sider two cubes of iron, a small one that 

 measures one inch each way and a larger 

 cube that measures one foot each way. 

 If both cubes are at room temperature 

 and both are placed in the same hot oven, 

 the small cube will reach a high tempera- 

 ture long before the large one does. By 

 the time the small cube has reached a 

 temperature of 100° centigrade (boiling 

 temperature of water), the large cube 

 will only be warm. It takes much more 

 time and much more heat to raise the 

 temperature of the large cube to 100° C. 

 When both are at the same temperature 

 the large cube, therefore, contains much 

 more heat. You need not try to define 

 the words heat and temperature as long 

 as you understand this paragraph. Just 

 remember that heat is a form of energy; 

 it can be added to or taken away from 

 bodies, and as that is done the tempera- 

 ture of the body changes. 



