III.] THE CARBOHYDRATES. ! 



LESSON III. 

 THE CARBOHYDRATES. 



THE term Carbohydrate, first used by C. Schmidt, is applied to a 

 large and important group of substances, which occur especially in 

 plants, and some of which, such as starch and sugar, make up a 

 large part of their organs ; while cellulose, another member of the 

 group, forms the chief material from which many parts of plants 

 are constructed. Carbohydrates also occur, but to a much smaller 

 extent, in animals, in which they are chiefly represented by 

 glycogen and some forms of sugar. 



In elementary composition they are non-nitrogenous, and consist* 

 of C, H, and 0, with the H and in the same proportion as in 

 water, i.e., 2 atoms of H to i atom of 0. As this proportion 

 obtains in many other substances which certainly do not belong 

 to the carbohydrate group, e.g., acetic acid (C. 2 H 4 2 ), lactic acid 

 (C 3 H 6 3 ), the definition must be somewhat extended. The group 

 is understood to include those substances that do not contain less 

 than 6 atoms of carbon, although many carbohydrates contain 

 multiples of this. To every 6 atoms of C there are at least 5 

 atoms of 0, so that on the one hand acetic acid is excluded, and 

 pyrogallic acid (C 6 H 6 3 ) on the other. 



They have certain general characters. They are indifferent 

 bodies, with a neutral reaction, which form only loose combina- 

 tions with other bodies, specially with* bases. Other general 

 characters they possess directly, e.g., dextrose, or they can be 

 readily converted into bodies which have the following features in 

 common. One or other character may fail, but, as a group, they 

 have the following : 



(a.) The property of reducing alkaline metallic solutions, and of 

 being coloured yellow by alkalies. 



(b.) They rotate the plane of polarised light. 



(c.) In contact with yeast they split up into alcohol and carbon 

 dioxide, i.e., undergo fermentation. (Some do not undergo fer- 

 mentation.) 



(d.) On heating with HC1 or H 2 S0 4 they are decomposed with 

 the formation of Idvulinic acid, humin substance, and formic acid. 



(e.) They give a deposit of yellow needles with phenyl-hydrazin. 



(/.) Various colour reactions with acids and aromatic alcohols. 



(g.) Some, e.g., cellulose and starch, are quite insoluble in water, 

 while others are very soluble. Those which are very insoluble in 

 water can usually be rendered soluble by heating them with an 



