CARBOHYDRATES 25 



polymer of D-glucose units with /3-{1^4) linkages. In cotton cellulose there are about 

 three thousand glucose units comprising a molecule that is about 16, OOOA long and 

 4x8A in cross-section. In the cell wall cellulose molecules are grouped together parallel 

 to each other to form micelles with a diameter of about 60A. In the micelles certain re- 

 gions show a crystalline structure where the cellulose molecules are arranged in an or- 

 derly way; other areas show a random arrangement. The micelles are in turn arranged 

 into microfibrils (diameter 200-250A) and these into fibrils, visible in the ordinary micro- 

 scope. Hydrolysis of cellulose by acid or enzymes yields first cellodextrins containing 

 30 or fewer glucose units, then cellobiose and finally glucose. The intermediates between 

 glucose and cellulose do not occur naturally. 



The hemicelluloses were originally named because they were found associated with 

 cellulose in cell walls and thought to be intermediates in its formation. They comprise 

 the polysaccharide material extractable from cell walls by 17. 5% sodium hydroxide, but 

 wherever possible it is advisable to avoid the term "hemicellulose" and refer to the com- 

 ponents of this group in terms of their specific structures (e.g. "xylans", "mannans" etc.). 



The most abundant polysaccharide cell wall materials after cellulose are the xylans. 

 There are several different types, occurring in almost all higher plants. They seem to 

 occur especially in association with lignin. In non-lignified tissues pectic substances 

 become more prominent. The basic unit of xylan structure is D-xylose; but, depending 

 on the plant source, they may be branched or unbranched and may or may not contain ad- 

 ditional units such as L-arabinose or D-glucuronic acid. Corn cob xylan contains about 

 200 sugar units as a straight chain with /i-(1^4) links. On the contrary wheat straw xylan 

 has only about 40 D-xylose units but in addition five L-arabinose units and three D-glu- 

 curonic acid units. Other hemicelluloses may have L-arabinose rather than D-xylose as 

 their principal monosaccharide component. The "acidic hemicelluloses" are those with 

 a relatively large proportion of glucuronic acid units connected to the xylose or arabinose 

 backbones. They are readily soluble in dilute (4%) sodium hydroxide. However, there is 

 no sharp dividing line between neutral and acidic hemicelluloses, so that extraction with 

 increasing concentrations of alkali may yield a series of fractions with gradually increas- 

 ing content of uronic acid. Hydrolysis of hemicelluloses which contain glucuronic acid 

 produce along with the monosaccharides some aldobiouronic acid, a disaccharide in which 

 the glycosyl group is a uronic acid. These appear in the hydrolysate because the glyco- 

 sidic bond is peculiarly resistant to hydrolysis when it is formed from a uronic acid. 



The pectic substances of plants are found in primary cell walls and intercellular ce- 

 ment. They are a mixture (and to some extent a chemical combination) of an araban, a 

 galactan, and the methyl ester of a galacturonan. The araban is a low molecular weight 

 branched chain of a(1^5) and a(1^3) L-arabinofuranose units. The galactan is a straight 

 chain of about 120 /S-(l-»4) D-galactopyranose units. Most of the properties of pectin, 

 however, and the name itself are referrable to the galactouran component which has about 

 200 (l-^4)-a'-D-galactopyranosyluronic acid units. Pectin actually occurs in plants as in- 

 soluble protopectin which contains bound calcium and phosphate and has a much larger 

 molecular weight (1000 or more units). With senescence or acid treatment of the plant 

 tissue water-soluble pectin is obtained by hydrolysis of some of the glycosidic bonds. 

 Alkaline hydrolysis of pectin removes the methyl ester leaving pectic acid. The so-called 

 pectinic acids are intermediate hydrolysis products with some carboxyl and some ester 

 groups. The calcium and magnesium salts of pectic acid are the important cementing sub- 

 stances of the middle lamella. 



Arabans have not been found except associated with pectin. However, a few galactans 

 are known to occur apart from pectin. The best-known source of these is the wood of 

 Western larch {Larix occidentalis) which may contain as much as 18% galactan. This 

 molecule is highly branched with 1, 6 and 1, 3 linkages. A small amount of arabinose may 

 also be present in the polymer, most likely as end-groups. 



