64 CARBOHYDRATES 



galactoaraban. Galactans are found in peas, beans, and certain other 

 legumes. They are not hydrolyzed by the enzymes of the digestive tract; 

 consequently their nutritive value can be only indirect. 



Mannosans 



These polysaccharides both in the simple form and in combination with 

 the anhydrides of other sugars, for example, fructose and galactose, have 

 been found in yeast, mushrooms, seeds, nuts, fruits, berries, leaves, and 

 practically all plant tissues. They are especially abundant in the ivory 

 nut, coffee bean, and carob bean. The best known members of this 

 class are ivory nut mannan and salep mannan. Wlien salep, a meal 

 obtained from the dried tuberous roots of various orchids, is extracted 

 with water and alcohol added to the extract, the mannan precipitates 

 as a white powder. On acid hydrolysis only D-mannose is formed. The 

 polysaccharide is made up of the D-mannose residues attached by /3-l,4- 

 linkages in a single, unbranched chain. Ivory nut mannan Js similarly 

 constituted, but is insoluble in water. 



Although certain molluscs and crustaceans secrete enzymes that hydro- 

 lyze mannans, no digestion of these carbohydrates is brought about by 

 the digestive enzymes of higher animals. 



Chitin 



The polysaccharide, chitin, is widely distributed in nature, being found 

 in the exoskeletons of many invertebrate animals, as well as in certain 

 plants and fungi. Typical examples of its occurrence are the cuticle 

 of insects and the shells of crabs and lobsters, where it makes up about 

 one-fourth to one-half of the dry weight. It functions as a highly re- 

 sistant protective substance and, together with protein and mineral matter, 

 gives strength and rigidity to the organism. Chitin is remarkably in- 

 soluble in all ordinary solvents and resistant to alkaline hydrolysis, 

 although it can be hydrolyzed by long heating wdth strong acid. 



Chemically, chitin consists of N-acetyl-D-glucosamine residues linked 

 through the 1,4-positions into a linear chain several hundred units long. 

 It is thus one of the very few major carbohydrates in nature which con- 

 tains nitrogen. 



Pectin 



This carbohydrate is contained in the water extract of many fleshy 

 fruits. On addition of acid and sugar in proper concentrations, pectin 

 forms a gelatinous mass well known as jelly. The mother substance 

 existing in the plant, designated as protopectin, is considered by many 



