Question

humans are unable to digest

starch or cellulose or glucose

Answer 1

That's because we humans lack the enzyme that digests cellulose. In fact , all mammals do.  But ruminants, those animals which can digest cellulose (you know, cows , camels and such ) do so by harbouring cellulose- digesting microorganisms in their digestive tract.

Answer 2

Cellulose.......we lack the enzyme cellulase which breaks the cellulose..