1.2.1—
Cellulose

Cellulose occurs as a crystalline, fibrillar aggregate of b -1,4-linked glucan chains (Frey-Wyssling, 1969). Cellulose fibrils give plant cell walls most of their enormous strength, much as glass fibres embedded in an epoxy resin give strength to a fibreglass composite (Northcote, 1972).

The basic structure of the b -1,4-linked glucan chains of cellulose is illustrated in Fig. 1.2 by conformational line drawings and in Fig. 1.3 by molecular models. Residues of b -D -glucose (Fig. 1.1) are glycosidically linked to each other, from carbon 1 of one residue to carbon 4 of the adjacent residue. The upside-down inversion of every second residue in the chain minimizes contact between atoms of adjacent residues. Close inspection of the models in Fig. 1.3 shows that the –OH groups at carbon 3 are in very close proximity to the ring oxygens (O₅ ) of adjacent residues. Hydrogen bonds between O₃ and O'₅ help to stabilize the flat, straight, ribbon-like structure of b -1,4-linked glucan chains.

The flat, ribbon-like structure allows the chains to fit closely together, one on top of the other, over their entire lengths. These interchain associations are stabilized by hydrogen bonds between O₆ of a glucose residue in one chain and

Figure 1.2
b -1,4–linked glucan chains of cellulose.
Portions of two associated chains are illustrated by
conformational line drawings. Distances between atoms
are not  accurately indicated in this illustration, but see Fig. 1.3.

Figure 1.3
Hemicellulosic xyloglucan associated with cellulose.
The repeating subunit of a hemicellulosic xyloglucan is shown in
association with a portion of a b  –1,4 –linked glucan chain of cellulose
(Bauer et al.,  1973). Molecular models have been used to accurately indicate
interatomic distances and bond angles. Hydrogen bonds from the cellulosic
glucan chain to the glucan backbone of the hemicellulose are indicated by arrows.

the oxygen of the glycosidic bond (O₁ ) between glucose residues in an adjacent chain. Since the glucan chains of cellulose are very long (8,000 to 15,000 residues), the number of hydrogen bonds between adjacent chains is very large. The resultant crystal is extremely stable and so tightly packed that there is no room for water molecules in the crystal structure.

Although there is some controversy as to whether native cellulose fibrils are 3.5 nm or 10 nm in diameter, it is clear that the glucan chains of cellulose

aggregate to form stiff crystalline rods of very considerable length and mechanical strength. The molecular structure of b -1 ,4-linked glucose thus neatly determines the secondary and tertiary structures of cellulosic glucan chains, and establishes the quaternary, interchain associations—although not the dimensions—of the microfibrils. The stiff, crystalline rods of cellulose are clearly well suited to their biological function in the plant cell wall.