24 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



nature, a trick which is repeated again and again — in the proteins, 

 for instance, as well as with other polysaccharides — and which man is 

 now at last learning to copy and even to improve upon. A clue to the 

 kind of association involved in cellulose was already at hand in the 

 appreciation that under certain conditions a disaccharide, cellobiose, 



can be detected amongst the products of hydrolysis, 

 for it could readily be assumed that the glucose 



Fig. 6. Diagrammatic representation of the stereochemical 

 formulae for the two types of glucose in the 6-membered 

 ring (pyranose) form. As usual, the corners of the ring are 

 occupied by one carbon atom each, which is not shown, 

 with the exception of that carrying oxygen. The carbon 

 atoms are numbered in the figure following the usual con- 

 vention. Carbon 1 carries the potentially reducing group. 

 The rings are drawn in perspective and the upper of each 

 pair of radicles attached to a carbon is supposed to lie 

 above the plane of the ring, and the other one below this 

 plane. Upper Figure. The formula of a-glucose. This does 

 not occur in cellulose but does form the basis of the starch 

 molecule. Lx>wer Figure. /3-glucose, differing from a-glucose 

 only in the relative positions of the — H and — OH attached 



to carbon 1. 



Lli.OIl 



.CH,UH 



pre-existed already in the cellulose in the form of cellobiose residues. 

 This has indeed turned out to be the case. Following the elegant work 

 of Haworth and his collaborators, it is known that the glucose 

 units in cellulose occur in the six-ringed modification and exclusively 

 in the /3-form (Fig. 6). Since the reducing power of cellobiose 



H,OH 



Fig. 7. Cellobiose. The molecule could obviously be 

 increased in size by further condensation, on each end 

 of the cellobiose molecule, of the other glucose units. 

 The resulting long-chain compound would be cellulose. 



is doubled on hydrolysis then it follows that the union between the 

 two constituent glucose residues must be through the potentially 

 reducing group of one of them (carbon 1 in Fig. 6) and it remains 

 to determine to which of the four possible non-reducing groups 

 on the other sugar molecule this union is made. This has been done by 

 chemical methods into which we need not go here (4) and it has turned 

 out that the attachment is to carbon 4, giving the 1 :4 or glucosidic link 

 between the two glucoses (Fig. 7). It will be noticed that this link is 

 made by the separation from the two glucose molecules of the elements 



