Why can humans not digest cellulose found in vegetables?

Cellulose is a type of carbohydrate found in the cell walls of plants and vegetables. It is made up of sugar molecules linked together by special bonds called beta bonds. Humans cannot digest cellulose because the enzymes in our digestive system cannot break these beta bonds. Our enzymes can only break alpha bonds, which are found in starches like potatoes and rice. Since cellulose passes through our stomach and intestines without being broken down, it acts as fiber in our diet. This fiber helps keep our digestive system healthy by adding bulk to the stool and helping food move smoothly through the intestines. Although we cannot use cellulose for energy, it is important for good digestion and preventing constipation. Some animals like cows and termites can digest cellulose because they have special bacteria in their stomachs that break the beta bonds, but humans do not have these bacteria.