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

What prevents backflow of blood inside the heart during contraction?

$(a)$ Name the organs which secrete the following enzymes in the human digestive system:
$(i)$ Pepsin
$(ii)$ Salivary amylase
$(iii)$ Trypsin
$(iv)$ Lipase
$(b)$ Name the organs which perform the following functions in the human alimentary canal:
$(i)$ Absorption of digested food
$(ii)$ Absorption of water

Lack of oxygen in muscles often leads to cramps among cricketers. This results due to:

Name the following with reference to the alimentary canal:
$(a)$ $(i)$ The largest gland.
$(ii)$ The gland that secretes digestive enzymes as well as hormones.
$(iii)$ The part where digested food is absorbed.
$(b)$ What are villi? Mention their function.

Which of the following statement$(s)$ is (are) true about respiration?
$(i)$ During inhalation,ribs move inward and diaphragm is raised
$(ii)$ In the alveoli,exchange of gases takes place i.e.,oxygen from alveolar air diffuses into blood and carbon dioxide from blood into alveolar air
$(iii)$ Haemoglobin has greater affinity for carbon dioxide than oxygen
$(iv)$ Alveoli increase surface area for exchange of gases