BIOLOGY'S BUILDING MATERIALS 21 



from any other substance hence we can use it as a test. If a 

 substance supposed to contain any proteid is put into nitric acid 

 and heated gently, it will turn bright yellow. Then if the acid be 

 washed off and ammonia added the proteid, if present, will become 

 orange color. This is the test for any proteid for no other substance 

 will act in the same way. 



The proteids are the most useful of the nutrients for they make 

 up most of the active living substance of plant and animal; they 

 are called tissue builders on this account. Proteids are composed 

 of the elements carbon, hydrogen, oxygen, nitrogen, sulphur, 

 phosphorus, with sometimes mineral salts as well, so we see they 

 are very complex organic compounds. 



Carbohydrates. Next to proteids in importance to all living 

 things come the carbohydrates. They are composed of carbon, 

 hydrogen, and oxygen, with always twice as much hydrogen as 

 oxygen, and varying amounts of carbon. 



Carbohydrates are found almost entirely in plants, whose 

 tissues they largely compose. When animals eat them, they 

 either make them over into proteid tissue or else oxidize them as 

 fuel to produce heat and energy. Some are converted into fats 

 and stored as such. 



Some common carbohydrates are: 



The starches. 



Corn starch from corn 



Potato starch " potato 



Flour starch " wheat 



Tapioca starch " cassava root 



The sugars. 



Cane sugar ^ SUgarC K ane ) (saccharose) 



Beet sugar sugar beet J 



Grape sugar " fruits (Glucose) 



Milk sugar " milk (Lactose) 



Cellulose. 



Complicated forms found in wood, paper, cotton, linen. 



