72 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



where the valence forces are some eight times greater than with methane, 

 the substance is liquid at room temperature. By an extension of this 

 argument it is easy to see why higher members of the series finally 

 become solid — the molecules are firmly bound together by the co-opera- 

 tion of many small forces. Similarly with cellulose. The anhydroglucose 

 units in the chain are held together by C — O — C links whose binding 

 energy (i.e. the energy required to separate the molecules) is of the 

 order of 70 kilocals. per gm. mol. Laterally, however, the chains are 

 held together by hydrogen bonds between neighbouring — OH groups, 

 whose energy is in the neighbourhood of only 5 kilocals. per gm. mol. 

 Correspondingly, single molecules in a sugar crystal can readily be 

 separated from each other, i.e. the crystals are readily soluble (due to 

 the mutual attraction of the hydroxyls in glucose and in water) and are 

 fusible. In cellulose, however, where very many hydrogen bonds may 

 be expected to cooperate in holding two chains together, and where 

 many of the hydroxyls "satisfy" each other within the crystalline 

 lattice, the substance is swellable but not soluble; and, since the energy 

 required to separate many hydrogen bonds uniting two chains is greater 

 than the energy of a single C — O — C link, the substance burns or chars 

 (involving a breakdown of C — O — C) before it melts. In these and 

 in other physical properties, cellulose thus behaves as it does because it 

 consists of long molecular chains. Let us review very briefly the various 

 other lines of evidence which indicate long chain length, and the various 

 attempts which have been made to estimate chain length. 



The molecular weight of cellulose 



With polymeric substances, among which cellulose must be classed, 

 it is not always easy to say precisely what is meant by the molecule, 

 and therefore the allocation of a molecular weight is correspondingly 

 somewhat arbitrary. With two-dimensional lattices such as occur, for 

 instance, in graphite, the "molecule" should properly refer to the whole 

 sheet of atoms in a plane; and with three-dimensional lattices such as 

 diamond or silica the "molecule" is equally clearly the whole of the 

 atoms within any one region of perfect crystallinity, i.e. the crystallite. 

 In neither of such cases is it easy, or often even possible, to define the 

 molecule. With linear polymers, on the other hand, the problem is 

 much easier and with cellulose the molecule can readily be defined as 

 one single chain of anhydroglucose units, however difficult the deter- 

 mination of the corresponding molecular weight may prove to be. That 

 is not to say, of course, that the chains of cellulose are likely to be of 

 the same length; on the contrary it is highly probable, on a priori 



