Humans digest starch but pass cellulose largely undigested even though both are long glucose polymers. Which statement best accounts for the difference?

Core Concept

Both polymers consist of glucose monomers joined by glycosidic bonds formed during dehydration synthesis. In starch the bonds connect alpha-glucose subunits, producing helical chains that human amylase recognizes and hydrolyzes into smaller sugars. Cellulose instead links beta-glucose subunits, and successive glucose units flip orientation, producing straight chains that hydrogen bond into rigid microfibrils. Human digestive enzymes have active sites shaped for alpha linkages, so they cannot bind or cleave the beta geometry. Grazing animals obtain glucose from cellulose only through symbiotic microbes that secrete cellulase.