Question

1. why is cellulose not digestable in human beings?

Answer 1

Cellulose is a polysaccharide, and every polysaccharide holds or binds many small monosaccharide units together, by glycosidic bonds.

Glycosidic bonds are of two types, A(alpha)-1,4 bond and B(beta)-1,4 bond. Cellulose contains the B bond.

Now enzymes (for example, amylase) are concerned with breaking the glycosidic bonds of the big polysaccharide, thus releasing the smaller saccharides (glucose, fructose) as product. This is what we call as Digestion.

But our enzymes cannot break the B bond, present in cellulose, whereas it can break A bonds, as present in starch and glycogen. So we, humans, cannot digest cellulose.

PS- Herbivores such as cows has a special digestive enzyme, cellulase (present in their rumen) that can break the B bond of cellulose, hence digesting it.