THE SCOPE OF BIOLOGY 9 



to the group contained in the starch molecules. H^O is a 

 molecule of water; and hence we say that if a molecule of water 

 is added to a starch molecule, it will convert it into a sugar 

 molecule. It must not be understood, however, that this 

 can be done by simply adding water to starch, for the two 

 will not combine. There are methods (see page 306), however, 

 by which they can be made to combine, and under these cir- 

 cumstances starch can very easily be converted into sugar. 



Among the different types of sugars, there are two of espe- 

 cial importance. One of these is grape sugar, also called glu- 

 cose or dextrose. These three names are closely related, al- 

 though not exactly identical. The formula for these is also 

 CeH^Oe. The other type is cane sugar, obtained from sugar 

 cane or the sugar beet. The formula for this is C^H^On, 

 which, as will be noticed, is nearly, but not quite, twice the 

 formula of the grape-sugar molecule. By the addition of a 

 molecule of water it is possible to break a molecule of the cane 

 sugar into two molecules of the grape-sugar type, according 

 to the following equation: Ci 2 H22Oii+H2O = 2C6Hi20 6 . This 

 is commonly spoken of as inverting the sugar. 



Sources of carbohydrates. Carbohydrates come almost wholly 

 from the vegetable world. Green plants manufacture starch 

 in their leaves by combining the carbon dioxid gas which 

 they absorb from the air with the water which they absorb 

 from the soil. This starch is very easily converted into 

 sugar within the plant, and then carried to various parts 

 where it may be stored, either in the form of starch or sugar. 

 It is subsequently used by the plant as food, or, if the plant 

 is consumed by animals, it serves as their food. So far as 

 known, there is no other source of carbohydrates in nature 

 besides the green plants, and as all animals and all plants 

 consume carbohydrates, it is plain that the whole living world 

 is dependent upon the green plants for carbohydrates. 



Hydrocarbons (Fats). Good examples of fats are found in but- 

 ter, in mutton tallow, in lard, in olive oil, etc., and in many other 



