26 CARBOHYDRATES 



Although chitin, the structural polysaccharide of fungi and invertebrates, has not 

 been reported in higher plants, its monomer, N-acetylglucosamine, has been found as its 

 uridine diphosphate derivative in mung bean seedlings (Phaseolits muiigo) (15). Free glu- 

 cosamine has been reported in soy beans (Glycine max) (16). 



FOOD RESERVE POLYSACCHARIDES 



Starch, like cellulose is composed only of D-glucose units, but joined by a rather 

 than i3 glycosidic linkages. Starch is normally a mixture of two types of polysaccharide, 

 amylose and amylopectin. The former is a straight chain molecule of about 300 units 

 joined 1-^4. The latter contains a thousand or more units of which a majority are also 

 1^4 but with about 4% 1—6 linkages so that there is on an average about one branch for 

 every 25 glucose residues. In most starches there is about 25% amylose to 75% amy- 

 lopectin; but the ratio may be reversed in some varieties. In some starches (e. g. potato) 

 the amylopectin is partially esterified with phosphates at C-6 positions. Starch is found 

 widely in the plant kingdom where it serves as food storage material. However, some 

 higher plants do not contain starch and use other carbohydrates as food reserves. 



Fructans (polymeric fructosides) take the place of starch in a wide variety of plants 

 and supplement starch as food reserves in others. The best known of these is inulin which 

 contains about 25-28 2-*l'-linked fructofuranoside units per molecule and is soluble in hot 

 but not cold water. It occurs especially in the Compositae. Other shorter chain fructans 

 are also known, especially in the grasses (14). In contrast to inulin these contain only 

 about 10 units, are linked 2—6 and are soluble in cold water. Many of the fructans, in- 

 cluding inulin, seem to contain glucose, probably as terminal groups. Some fructans are 

 linear, while others are highly branched. Of taxonomic interest is the observation of 

 Quillet (17) that among the liverworts the Jungermanniales have fructans while the Mar- 

 chantiales have starch. 



Mannans are also distributed in groups as widely separated as grasses and conifers. 

 At least in some cases they serve as reserve carbohydrate but may be structural material 

 in other cases. Some plant mucilages also contain mannans. The mannans of palm seeds 

 have been most studied. They contain about seventy-five mannose residues linked with 

 /3-(l-4) bonds. 



Galactomannans, polysaccharides containing both D-galactose and D-mannose, serve 

 as a food reserve in many legume endoperms and seeds of palm and coffee trees. The 

 carob bean (Ceratonia siliqua), in particular, is a commercial source of these galacto- 

 mannans which are used as thickeners. They have a linear chain of /3 (1— 4)-mannopy- 

 ranoside units with galactopyranose units linked a-(l-»6) as side chains. Because of their 

 physical properties and occurrence in seeds the galactomannans have sometimes been 

 classed with the mucilages. If a distinction is to be made, it must be in terms of func- 

 tion i. e. , the galactomannans seem to be food reserves, while the mucilages are more 



concerned with binding water. 



An interesting polysaccharide has been reported in the fruit of the chicle plant 

 (Achras sapota) by Venkataramen and Reithel (18). It is apparently composed of glucose 

 and galactose since as the fruit ripens, lactose appears as a breakdown product. This 

 accounts for one of the rare appearances of lactose in the plant kingdom. 



PLANT GUMS AND MUCILAGES 



Both of these constituents are polymers containing more than one type of monosac- 

 charide, but uronic acids are generally present. Traditionally, mucilages have been de- 

 fined as normal plant constituents; and since they occur in xerophytes, seeds, and young 



