$(a)$ What is the role of mucus in the stomach?
$(b)$ How is the exit of food from the stomach regulated?
$(c)$ Where does food enter from the stomach?

$(a)$ 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.

Where does digestion of fat take place in our body?

Explain the process by which the energy requirements of autotrophic organisms are fulfilled. In which form are the unused carbohydrates stored?

Which of the following statements about the autotrophs is incorrect?

$(a)$ Name the following:
$(i)$ Part where air is filtered by fine hair and mucus.
$(ii)$ Part which terminates in balloon-like structures.
$(iii)$ Balloon-like structures where exchange of gases takes place.
$(iv)$ Part which separates the chest cavity from the abdominal cavity.
$(b)$ Why is the rate of breathing in aquatic organisms much faster than in terrestrial organisms?