CARBOHYDRATES 2 1 



acidic nature to the two enolic hydrogens since it has a lactone ring rather than a free 

 carboxyl group. Ascorbic acid occurs in certain plants as an indole derivative ascorbigen, 

 (cf. Chapter 14). Many aspects of ascorbic acid are reviewed in a symposium publication 

 (11). 



OLIGOSACCHARIDES 



The oligosaccharides are polymers formed by the linking together of several mono- 

 saccharide units through glycosidic bonds. Although oligosaccharides are arbitrarily 

 limited to molecules containing less than ten monosaccharide units, the commonest have 

 only 2, 3, or 4 units; and hexose units are by far the most frequent. One awkward case is 

 the water-soluble glucan (polyglucoside) of barley roots which contains 7-11 units and may 

 therefore be classed as either an oligosaccharide or a polysaccharide (12). 



Aldobiouronic acids are oligosaccharides containing uronic acid units. They are 

 found as hydrolysis products of certain gums, mucilages, and polysaccharides (q. v. ) but 

 apparently do not occur as natural plant constituents. 



The oligosaccharides are water-soluble, optically active compounds, distinguished 

 most readily from the monosaccharides by their hydrolysis to the monomers. They may 

 be reducing or non-reducing, depending on whether or not all the potential carbonyl groups 

 are tied up in glycosidic linkages. Oligosaccharides also occur as components of glyco- 

 sides. For example, rutinose is a part of the flavonoid compounds rutin and hesperidin 

 but apparently does not occur in the free state. Other rare oligosaccharides are compo- 

 nents of steroid and triterpene glycosides. Some fructosyl sucroses (trisaccharides) 

 which occur in monocots may be intermediates in fructan biosynthesis. These and oligo- 

 fructosides generally have been discussed by Bacon (13, 14). 



Structures and occurrence of some of the natural oligosaccharides are given in 

 Table 1. 



POLYSACCHARIDES GENERALLY 



Polysaccharides or glycans are arbitrarily defined as polymers of monosaccharides 

 (and their derivatives) containing 10 or more units. However, most natural polysaccha- 

 rides contain many more than 10 units and may have several thousand. Despite the vast 

 number of polysaccharides that would be possible, the known representatives account for 

 only a few of the structural possibilities. Where a polysaccharide is composed of more 

 than one monosaccharide, the units fall into an orderly sequence; and only certain ones of 

 the available hydroxyl groups are utilized in forming the glycosidic bonds. Generally, 

 the structural polysaccharides are straight-chain compounds, while reserve food poly- 

 saccharides tend to be branched. Branched molecules are more easily dispersed in water 

 to form hydrophilic colloid systems that may be very viscous. The straight-chain poly- 

 saccharides, on the other hand, are slightly soluble or insoluble. Polysaccharides are 

 usually obtained as amorphous rather than crystalline solids, although a degree of crystal 

 order may be detected by x-ray diffraction methods. Some of the different classes of 

 polysaccharides will be discussed separately. Ideally a classification should be based on 

 structure, but this is possible to only a limited extent with present knowledge. 



STRUCTURAL POLYSACCHARIDES 



Cellulose is one of the main constituents of plant cell walls, in particular the sec- 

 ondary cell walls which are most important for structural strength. Cellulose is a linear 



