Chemistry · Ch 14 — Biomolecules
Cellulose
Cellulose
Cellulose, the principal structural polysaccharide of plants, is -- like amylose -- an unbranched, straight-chain polysaccharide, but built from beta-glucose units (rather than starch's alpha-glucose units) linked by beta-1,4-glycosidic bonds (rather than starch's alpha-1,4 bonds). This single stereochemical difference at the glycosidic linkage -- alpha versus beta -- has an enormous practical consequence: the beta-1,4-glycosidic bond is chemically far more resistant to hydrolysis than the corresponding alpha-1,4 bond of starch. Breaking it chemically requires concentrated strong acid at high temperature and high pressure, conditions far harsher than the dilute-acid, moderate-temperature hydrolysis that suffices for starch (section 14.2.3b). More importantly biologically, the human digestive system possesses NO enzyme capable of hydrolysing the beta-1,4-glycosidic bond (human digestive enzymes, such as amylase, are specific for the alpha-1,4 linkage of starch) -- consequently, cellulose passes through the human gut completely undigested. Far from being wasted, this undigested cellulose serves an important dietary role as fibrous roughage, which is useful for healthy bowel movement, even though it contributes no direct nutritional/caloric value to humans the wa …
Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.
What this figure shows. A long unbranched chain of beta-D-glucopyranose rings (numbered C-1 to C-6, with the exocyclic -CH2OH groups drawn), successive rings joined C-1 to C-4 of the next by beta-1,4-glycosidic linkages -- the ring at each junction alternately flipped relative to its neighbour (a consequence of the beta-linkage geometry, unlike amylose's alpha-linked rings which all point the same way), giving cellulose its fully extended, straight, ri …