22 BIOLOGY FOR BEGINNERS 



(Glycogen is an animal carbohydrate found in the liver of some 

 animals and called " liver starch." It seems to be stored there for 

 later use.) 



It is a little strange to think of cotton and starch, or wood and 

 sugar as being so nearly related, but they consist of the same three 

 elements, and are produced by the plants from water and carbon 

 dioxide. It would be a cheap diet, if we could take water from 

 a reservoir and carbon dioxide from the air and make them into 

 flour. 'Man has to depend on plants for this wonderful process, 

 and can only begin where the plants leave off, using the plant- 

 made carbohydrates for his food. 



The Test for Starches. No one test can be used for all the carbo- 

 hydrates, but we can test for any starch by dissolving the substance 

 supposed to contain it in hot water and then adding a drop of 

 iodine. The solution will turn blue if starch be present. No 

 substance other than starch will act this way under these 

 conditions. 



The Test for Grape Sugar. There is no one test for all sugars, 

 but grape sugar (glucose) is very common and can be easily dis- 

 tinguished from our household (beet or cane) sugar by what is 

 known as the Fehling Test so named from the man who devised it. 



Two solutions are used in the Fehling test, one colorless, and one 

 blue. When these are added in equal amounts to a similar amount 

 of the substance to be tested, and the mixture heated, a yellow- 

 brown solid will form if grape sugar be present. Cane or beet sugar 

 will not act this way. 



Fats. The last class of nutrients is the fats and oils, which are 

 also composed of carbon, hydrogen, and oxygen. They differ 

 from carbohydrates in having less oxygen. Hence they oxidize 

 more readily and as a result their chief use is to produce energy. 



Plants store fats in their seeds to supply energy for growth; 

 animals store fats in various places and use them for the same 

 purpose. 



Kinds. Cotton-seed oil, olive oil, and the oils from various 

 nuts are examples of vegetable fats; while lard, butter, and fat 

 meats are familiar examples of fat from animals. 



