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The glucose residues are all in the
4
C
1
conformation. Consequently, all
linkages become
diequatorial.
Note the similarity with mannuronan and
chitin/chitosan.
1.4.3. Biosynthesis
Many properties of cellulose can be
understood by studying how cellulose is
synthesized in bacteria and plants. Briefly, the
cellulose synthase complex is organized as a
six hexagonal ‘rosettes’ ( d= 25-30 nm) from
which a long (up to 7
µ
m) microfibril of 36
cellulose chains (6 from each rosette)
emerge. Microtubuli assist the deposition of
the microfibril to encircle the plant cells,
providing physical strength and direction of
cell expansion (Figure take from Science, 312
(2006).
1.4.4. Solubility and
crystallinity
Cellulose is generally insoluble in water and
organic solvents. Only short oligomers are
soluble in water.
Cellulose has high degree of crystallinity (50-85%, depending on type of
cellulose and the method for determining the crystallinity), originating mostly
from the highly organized microfibril. The crystallinity (strong chain-chain
interactions) is responsible for the poor solubility. It also makes cellulose very
resistant to degradation because other molecules cannot penetrate the crystal
at sufficiently high rate.
The non-crystalline part of cellulose is termed ‘amorphous’. Amorphous
cellulose is chemically more reactive and swell better.
Microcrystalline cellulose is cellulose where the amorphous part has been
removed by acid hydrolysis, from which the crystalline part is more or less
protected.
O
O
OH
HO
O
CH
2
OH