38o CARBOHYDRATE METABOLISM 



methi'l groups in the molecule. All or a portion of the carboxyl groups may- 

 be methylated. 



Household fruit jellies owe their "gelling" property to the pectin derived 

 from the fruits. Neither protopectin nor pectic acid can be used in making 

 such jellies. Because of its commercial exploitation as an aid in jelly-making 

 pectin is the best known of the pectic compounds. The use of commercial 

 pectin is of the greatest value in preparing jellies from fruit products which 

 do not themselves contain sufficient pectin to gel readily. Pectin gels for 

 household purposes require the presence of three components: pectin (usually 

 from 0.2 to 0.7 per cent), sugar (usually 65 to 70 per cent), and sufficient 

 acid to maintain a pH of about 3.0 to 3.2. 



Protopectin apparently differs from pectin in the greater length of the 

 molecular chain. Protopectin occurs most abundantly in the primary cell 

 walls, in which it is present in the intermicellar material. 



The cell walls of many fleshy fruits, such as apples, are especially rich 

 in protopectin. During storage of apples and other fruits protopectin is 

 gradually converted into "soluble" pectin (Carre and Home, 1927)- As 

 fruits become overripe further degradation of the pectin to galacturonic acid, 

 galactose, and arabinose may occur. Because of dissolution of the pectic 

 compounds of the middle lamella the cells of overripe fruits gradually separate 

 from each other and the fruit becomes soft and flabby. 



As already noted in Chap. XVH root hairs also seem to be coated with 

 pectic substances, but their exact chemical nature is unknown. 



The gums are complex compound carbohydrates that are somewhat simi- 

 lar in composition to the pectic compounds. Upon hydrolysis they yield 

 hexoses, or pentoses, or both, and a complex organic acid. Gum arabic is an 

 example of such compounds. It forms as an exudate from the branches of 

 some of the tropical species of acacia. Upon hydrolysis gum arabic produces 

 arabic acid, galactose, and arabinose. Gum tragacanth is exuded from the 

 stems of species of the genus Astragalus and is similar in its properties to 

 gum arabic. The gummy exudates formed on the stems of cherry, plum, 

 and peach trees also belong to this group of substances. Some of the gums 

 readily form colloidal sols in water, as for example gum arabic, others such 

 as cherry gum merely swell in water. 



The ?nucilages are complex compound carbohydrates which form slimy 

 colloidal systems with water. Mucilages are found in the cells of cacti and 

 other succulents, are exuded by the epidermal hairs of many plants, coat the 

 surface of the seeds of some species (flax), and are abundant on the outside 

 walls of many aquatic species. Agar-agar and other similar products of the 

 marine algae are usually classed as mucilages. Knowledge of the chemistry 



