90 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



(e) Galactans — They are frequently found in plants ; in the wood, in the 

 seeds and elsewhere. The galactan of lupin seeds is constructed on the 

 1:4-^ principle, and is made up of about 120 galactose units. 



(/) Mannans — These are often present in wood, especially of coni- 

 fers, and they are difficult to separate out. The mannans of pine and 

 spruce are built of mannose molecules joined in straight chains of about 

 200 residues. The mannan of yeast, on the other hand, is highly branched 

 and is not homogeneous. The number of molecules of mannose in these 

 chains varies from 90 to 830. 



(g) Fructosans (Levans) — The inulin found in the tubers of many of the 

 Compositae is a linear molecule containing about 30 D-fructose residues, 

 interspersed with about 6% of D-glucose units. Plants contain a multitude 

 of other fructosans (asparagosine, graminine, triticine, etc.) and so also do 

 bacteria. Unlike other fructosans, those in bacteria are highly branched 

 molecules. 



2. Pentosans 



(a) Xylans — Xylans are present in the lignified membranes of plants. 

 They are branched molecules made up of D-xylose units. 



(6) Other pentosans — Plants contain many other types among which 

 araban may be mentioned. 



{b) Polyuronides 



The units here are uronic acids. The most numerous group is that of the 

 "pectic substances" or "pectin", in which the principal constituent is 

 pectic acid — a chain of D-galacturonic acid molecules united chiefly by 

 l:4-a-linkages. The pectins are not at all homogeneous and certain of their 

 constituents are still ill-defined. 



Another polyuronide is alginic acid, present in marine algae and formed 

 of chains of about one hundred units of D-mannuronic acid, joined to- 

 gether by l:4-/8-glucoside linkages. 



COOH H OH COOM ^ ^^ 



H OH COOH. H OH 



COOH. H 



Pectic acid (polygalacturonic acid) 



