6o FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY 



assume /5-glucosidic bonds, while it seems likely that to Konjak 

 mannan, being a reserve substance, a starch-like structure with a- 

 glucosidic bonds should be assigned. 



It is highly significant that the pectic substances, which are held to be 

 responsible for the coherence of plant tissues and which contain poly- 



OH 



> O^OH 



CH20H 



^-Glucose 



OH 

 OHy 1 OH 



CH2OH 

 cX-G/ucose 



OH. OH 



\VHOhN 



CH^OH 



C(-Monri06e 



OH 



oih-o"^ 



COOR 



CH2OH 



,^ty 



CH2OH 



0— 



CHpOH 



CH2OH 



'^:x°- 



CH2OH CH2OH 



Starch 



CH2OH 



-0- 



CH20H CH2OH CH2OH CH2OH 



c(-Mannan 



C(_-Galaciuronic acid 

 (R^CHs or H) 



COOR 



o—^^o^ \ — A-o 



COOR COOR 



Polygalocturonic acid 



^^^' 



O— 



Fig. 50. Polysaccharides 



galacturonic acid as a basic material, also have the structural principle 

 of polysaccharides. Here the -CHgOH side chain of the monose ring 

 is replaced by the carboxyl group -COOH. The pectins are therefore 

 capable of salt formation. Polygalacturonic acid is soluble in water, 

 but its Ca-salt is not, so that this polyacid can be precipitated by Ca 

 ions. Part of the carboxyl groups is esterified with methanol (Deuel, 

 1943). It is interesting to note that the methylation does not interfere 

 with the solubility in water, because methyl groups bound to oxonium 

 oxygen obtain an induced polarity so that they partly lose their 

 lipophilic character and become hydrophilic. 



The monomer of the pectic acid is a-galacturonic acid. As in a- 

 galactose, the hydroxyl groups of the first and fourth C-atoms are 

 not situated on the same side of the pyranose ring (Fig. 50); the a- 

 glucosidic linkage causes a rotation of succeeding chain members. 



