CARBON CYCLE 183 



25. How often must the air be changed in your school room? 

 In your living room? In your sleeping room? 



26. Draw a diagram of the ventilating system in your home. 



CARBON CYCLE 



Digestion. Foods contain: carbon and hydrogen in the form 

 of carbohydrates (sugars and starches) ; f ats (butter, oils, fats 

 of meat); proteids (white of eggs, lean meat, gluten of wheat), 

 which are carried to the stomach and digested. Digestion is the 

 process of changing a food from a material which cannot be dis- 

 solved to a food which can be dissolved (from non-soluble to 

 soluble.) 



Most of our foods do not dissolve in water, or in the juices of 

 the alimentary canal. The food we eat, such as butter, eggs, meat, 

 bread, etc., must be changed to a substance which will dissolve and 

 pass through the walls of the stomach and intestines into the blood. 



The principal fuel and source of energy in the body is sugar. 

 No one eats anything like the required amount of sugar in his food, 

 because we eat about three pounds of food a day, and since sugar 

 furnishes two-thirds of our entire working and warming powers, it 

 would mean that each person would have to eat from a pound and 

 a half to two puunds of sugar if the required amount were eaten as 

 sugar. 



All the starch which we eat must be changed into sugar, since 

 starch will not dissolve and pass into the blood. Our bread, crackers, 

 cakes, cereals, potatoes, rice, corn, wheat, etc., contain large amounts 

 of starch which must be changed into sugar by a simple process 

 known as hydration. This is Nature's method of adding water to 

 the starch, thus changing it into sugar. This process of digestion 

 begins in the mouth. In the saliva there is a substance called 

 ptyalin (ti-a-lin) which attacks the starch, and begins to change 

 it into malt sugar. The food then passes to the stomach and is 

 changed from malt sugar to grape sugar, so called because it was 

 first found in grapes, or glucose. This process of digestion continues 

 from the mouth all the way down the alimentary canal through 

 the small intestines. 



