THE CARBOHYDRATES. 3 



In addition to the above classes there are a number of 

 compounds containing three, four, five, seven, eight, or 

 nine atoms of carbon in a molecule. Their chemical names 

 are compounded from a prefix which indicates the number 

 of carbon atoms with the suffix ose, thus, triose, tetrose, 

 pentose, etc. As yet they are not known to be of any great 

 importance in physiological chemistry. 



The first class, the monosaccharids, is so named be- 

 cause they contain one of the groups of six carbon atoms. 

 They are mostly crystalline, easily soluble in water, and 

 have a sweet taste. Chemically they are aldehyde or ketone 

 compounds of the hexatomic alcohols. The former are in- 

 dicated by the prefix aldo-, the latter by keto-; thus aldo- 

 pentose, keto-hexose, etc. They have the power of reduc- 

 ing the oxygen compounds of the metals and of forming 

 compounds with phenyl-hydrazin. They will also undergo! 

 fermentation with yeast. 



The disaccharids contain in a molecule twelve atoms of 

 carbon. They may be conceived of as composed of two 

 molecules of a monosaccharid minus one molecule of water: 



C 6 H 12 6 + C G H 12 6 = C 12 H 22 1 + H 2 0. 



By the action of acids or ferments they take up a 

 molecule of water and form two molecules of a mono- 

 saccharid. This formation of one or more simple sugars 

 from a molecule of a disaccharid is called inversion, and 

 the resulting sugar is known as invert-sugar. The di- 

 saccharids do not undergo fermentation with yeast until 

 they have been inverted. They are all soluble in water and 

 have a sweet taste. 



The polysaccharids contain more than two groups of 

 six carbon atoms, though the number is, in most cases, not 



