Organic Substances 51 



Carbohydrates occur in four types of substances in a form 

 insoluble enough to be demonstrated even by relatively slow 

 reactions. The chemical nature and physiological role of 

 these substances vary widely; the only reason for their 

 being dealt with in one section is the fact that they all con- 

 tain sugar and that they can be demonstrated by the re- 

 actions of the sugar moiety. For the sake of brevity, they 

 will be called "saccharides." Some of these substances have 

 additional identifying reactions, due to other chemical or 

 physicochemical features of their molecules. 



Classification of the Saccharides 



1. Simple polysaccharides are built up from sugar mole- 

 cules only. In animals the building stones are glucose ( glyco- 

 gen) or galactose ( galactogen ) ; in plants they may be glu- 

 cose (cellulose) or pentoses. Substances belonging in this 

 group are either insoluble or soluble in a colloidal form only. 

 Polymer linkages holding the individual molecules together 

 go from Ci to C4 of the next molecule, although in some cases 

 the linkages may connect to C3. This last type of compounds 

 shows a histochemical behavior different from that of the 

 other members of the group. 



2. Mucoid substances are characterized by their content 

 of aminosugar (glucosamine, acetylglucosamine, or acetyl- 

 galactosamine) as the most typical component. K. Meyer^ 

 divides this group into three classes. 



A. Mucopolysaccharides (protein-free) 



a) Neutral, containing no acid groups (example: chitin, composed 

 of acetylglucosamine only) 



b) Acid 



1. Simple; acid component, uronic acid (example: hyaluronic 

 acid, composed of acetylglucosamine and glucuronic acid) 



2. Complex; acid component, uronic acid and sulfmic acid or 

 phosphoric acid (examples: corneal mucoid, hog gastric 

 mucin, heparin, all composed of glucosamine or acetylglu- 

 cosamine, some uronic acid, and sulfuric acid; chondroitin- 

 sulfuric acid, composed of acetylgalactosamine, glucuronic 



2. Meyer, K.: Adv. Protein Chem., 2:249, 1945. 



