$(a)$ Why does a piece of bread taste sweet when chewed for some time?
$(b)$ Cellulose acts as roughage in humans but serves as a source of nutrients in cows. Justify the statement.

(N/A) The saliva present in the mouth contains an enzyme called salivary amylase,which breaks down the starch present in the bread into simple sugars. This is why a piece of bread tastes sweet when chewed for some time.
$(b)$ Cellulose is a complex carbohydrate that requires specific enzymes and a longer time for digestion. Herbivorous animals like cows possess a specialized stomach (rumen) and a longer small intestine,which allow for the fermentation and digestion of cellulose by symbiotic bacteria. In contrast,human beings lack the necessary enzymes and the specialized digestive tract to break down cellulose. Therefore,it remains undigested in humans and acts as roughage,which helps in bowel movement.