Q.Starch and cellulose are both polymers of glucose, yet humans can digest starch but not cellulose. Explain why.

Digestive enzymes, like all enzymes, are highly specific for the particular bond geometry they can act on. The human digestive system produces amylase enzymes that are specifically shaped to recognise and hydrolyse $\alpha$-1,4-glycosidic bonds -- exactly the linkage found throughout starch (in both amylose and the main chains of amylopectin) -- breaking starch down into smaller sugars that can then be absorbed.

Cellulose is built from the very same glucose monomer, but joined instead by $\beta$-1,4-glycosidic bonds. Because this is a different bond geometry from the one amylase is shaped to act on, human amylase cannot hydrolyse it -- and humans produce no separate 'cellulase' enzyme capable of breaking $\beta$-glycosidic bonds at all. As a result, cellulose passes through the human digestive tract largely intact (as dietary fibre) rather than being digested and absorbed as a sugar source. Cert …

Compare the specific bond geometry each polysaccharide uses ($\alpha$ for starch, $\beta$ for cellulose) against which geometry the rel …

Assuming the difference is about the sugar itself (as if cellulose were made of a 'different' sugar) -- both are pure glucose polymers; the digestibility difference comes entirely fr …