234 ELEMENTARY SCIENCE 



Carbohydrates. You have learned that photosynthesis 

 produces carbohydrates, of which starch and sugar are 

 examples. Carbohydrates are substances composed of 

 carbon, hydrogen, and oxygen only, and in their molecules 

 the atoms of hydrogen and oxygen always occur in the 

 proportion of two to one, as in water. Thus the molecule 

 of a common kind of sugar (there are many kinds) has the 

 formula CeHi^e, which indicates that a molecule of this 

 kind of sugar is composed of six atoms of carbon, twelve of 

 hydrogen, and six of oxygen. The proportion of carbon in 

 different carbohydrates varies, but that of hydrogen and 

 oxygen always remains the same. Now in photosynthesis, 

 for every molecule of water (H2O) used it is believed that 

 a molecule of carbon dioxide (C02) is also used. Since the 

 proportion of hydrogen to oxygen in these two kinds of 

 molecules taken together is two of hydrogen to three of 

 oxygen, it is evident that in the manufacture of carbohy- 

 drates from these substances (photosynthesis), there will be 

 a considerable surplus of oxygen. This you have already 

 learned in considering the maintenance of the oxygen- 

 supply in the atmosphere. 



Most of our food contains more or less of carbohydrate. 

 Carbohydrate foods are of especial importance as sources 

 of the energy that we expend. 



Fats. The fats constitute another great class of foods. 

 You have already learned in connection with your study 

 of combustion that fat is composed of just the same ele- 

 ments as carbohydrate. The chief difference is that the 

 proportion of oxygen in them is a great deal less than 

 to is in carbohydrates. So fats have a greater oxygen- 

 hunger than have the other kinds of foods; they burn 



