$(a)$ 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?

On which type of food does salivary amylase act in the buccal cavity,and what is the name of the initial product formed due to the action of this enzyme?

$(i)$ Why are the valves needed in the heart?
$(ii)$ Leakage of blood from vessels reduces the efficiency of the pumping system. How is the leakage prevented?

Choose the forms in which most plants absorb nitrogen.
$(i)$ Proteins
$(ii)$ Nitrates and Nitrites
$(iii)$ Urea
$(iv)$ Atmospheric nitrogen

$(a)$ Name the hormone which is released into the blood when its sugar level rises. Name the organ which produces this hormone and its effect on blood sugar level. Also,mention the digestive enzymes secreted by this organ with one function of each.
$(b)$ Explain the need for chemical communication in multicellular organisms.

Define enzyme. Name one enzyme in humans which digests fats.