Question

Cows can digest cellulose, but humans can’t. Why not?

Short answer

Humans lack the β-glucosidase enzyme that’s why they can’t digest cellulose.

Step-by-step solution

Cows can digest cellulose

Cellulose is composed of D-glucose units linked by $\beta -1,4$glycosidic bonds. This bonding arrangement is rigid and very stable.

Long cellulose molecules, called microfibrils, are held in bundles by hydrogen bonding between the many hydroxyl groups of the glucose rings.

Step 2: humans can’t digest cellulose

Humans and other mammals lack the -glucosidase enzyme needed to hydrolyze cellulose, so they cannot use it directly for food.

Cellulose can be hydrolysed by several groups of bacteria and protozoa. Termites and ruminants maintain colonies of these bacteria in their digestive tracts. This is, in fact, the explanation of how cows can eat and digest cellulose. Hence Humans lack the β-glucosidase enzyme and cannot digest cellulose.