[Chap. XII THE FOOD OF PLANTS 105 



(C1-H22O11), otherwise known as cane sugar and saccharose, is the com- 

 mon household sugar. It is present in most plants and is especially abun- 

 dant in sorghum, sugar cane, sugar beet, and sugar maple. Each molecule 

 of sucrose is composed of a molecule of glucose chemically bound with 

 a molecule of fructose minus one molecule of water. When treated with 

 dilute acids, it is hydrolyzed to these two simple sugars. 



Classification of foods. For convenience we may think of all these 

 substances as belonging to a few large groups of chemical compounds: 

 the carbohydrates, fats, and proteins. In a detailed consideration of foods 

 this classification is incomplete unless numerous compounds that may be 

 derived from carbohydrates, fats, and proteins by chemical alteration 

 are assumed to be included. In any brief discussion of foods this assump- 

 tion is usually made. The chemical alterations of the carbohydrates, fats, 

 and proteins are brought about by such processes as partial oxidation, 

 reduction, and hydrolysis. These processes result in the fomiation of 

 alcohols, organic acids, organic bases, and other compounds which may 

 still be usable by organisms as food. The total number of such derived 

 compounds is undoubtedly very large, but aside from those formed in 

 digestion and fermentation their relative bulk is exceedingly small. Since 

 many of these derived compounds are acid or basic, they may form 

 numerous chemical combinations with each other and with the mineral 

 ions that pass from the soil into the plant. 



Carbohydrates, The carbohydrates are composed of carbon, hydrogen, 

 and oxygen. The hydrogen and oxygen occur in the proportion of 2 to 1 

 as in water (H2O) and glucose (CoHi20fi). To this group belong all the 

 sugars, starches, inulin, and cellulose. The pectic compounds are closely 

 related to the carbohydrates. A molecule of pectic acid, for instance, is 

 composed of molecules of certain sugars chemically combined with acids 

 derived from sugars by partial oxidation. A molecule of starch, or of 

 cellulose, is composed of many molecules of glucose in chemical com- 

 bination, and a molecule of inulin is composed of many molecules of 

 fructose similarly combined.^ When acted upon by certain enzymes or 

 acids these complex carbohydrates are hydrolyzed to the simple sugars 

 out of which they were made. These complex carbohydrates are usually 

 inert, insoluble compounds of no food value to the organism unless they 

 are digested to the simple sugars which are soluble and chemically 

 active. 



- For a more detailed statement of the formation of complex carbohydrates, see p. 135, 

 Chapter XV. 



