THE CHEMICAL BASIS OF LIFE 237 



processes go on that are far more complex than those 

 which the chemist has been able to control. 



Animals differ from most plants in requiring organic 

 compounds for their food. Most plants are able to 

 manufacture their living substance from the inorganic 

 constituents of the air and soil, but animals are compelled 

 to live upon plants or other animals which furnish food 

 in the form of organic matter. Organic food substances 

 fall into three principal classes: proteins, carbohydrates 

 and fats. Proteins are complex compounds containing 

 C, H, O, N, and frequently other elements. The white 

 of egg, cheese, and the lean fiber of meat consist almost 

 entirely of protein. All living matter contains protein 

 material which alone can supply the nitrogen for the 

 animal body. Fats are more or less oily substances 

 containing C, O, and H. They are quite readily oxidized 

 and yield a considerable quantity of heat. Butter, olive 

 oil, suet, lard and tallow are common examples of fatty 

 substances. 



Carbohydrates are composed of C, H and O, there being 

 twice as many atoms of H as O in the molecule. They 

 include such substances as sugar, starch and cellulose. 

 Starch is commonly stored in the cells of plants in the form 

 of grains with concentric layers like the coats of an onion. 

 When treated with iodine it turns blue. It is insoluble in 

 water, but it may be converted into sugar by fermentation. 



This process of fermentation is one of the most common 

 kinds of chemical action that takes place in organisms. 

 It may be illustrated in the fermentation of sugar or 

 molasses. If a small amount of yeast is added to a solu- 

 tion of sugar, after a time small bubbles of gas (CC^) may 

 be seen to arise from it, and its temperature increases. 

 After the process has run its course the sugar in the solu- 

 tion disappears and in its place there is found a certain 



