CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 23 



— 5 arabinosc 1—5 arabinose 1 — 



3 



I 



1 



arabinose 



The gums and mucilages include occasional cell wall polymers, 

 and many close relatives. They follow the pattern exhibited by 

 other non-cellulosic compounds, having both neutral and acidic 

 types. Acidic components include L-glucuronic acid, D-galacturonic 

 acid, D-glucuronic acid, sulfate and phosphate groups. The neutral 

 sugars associated with neutral and acidic polymers include hexoses 

 and 6-deoxyhexoses, pentoses, sugar alcohols, and ethers of any 

 of these units. Gum arabic contains galactose, L-arabinose, L-rham- 

 nose, and D-glucuronic acid in the ratio of 4:2: 1 : 1. Each residue 

 in the 1,3-linked galactose backbone is a branch point, with two 

 different branch chains attached via 6,1 -linkage on alternate 

 galactose residues: 



arabinose 3 — 1 galactose 

 1 

 I 

 6 



— 3 galactose 1—3 galactose 1 — 



6 



1 

 glucuronic acid 

 4 



1 

 arabinose 



Arabic acid and its salts have molecular weights of 250,000- 

 300,000 and a diameter of about 100 A. Gum tragacanth contains 

 1,4-linked D-galacturonic acid, L-fucose and xylose; the molecular 

 weight is about 840,000. The molecule is elongate with dimensions 



