Give the biological importance of :

(a) Nitrogen present in the air we breathe.

Explain how the percentage by volume of oxygen and carbon dioxide in the air is maintained at about $21\%$ and $0.03\%$ respectively.

Name the gases removed from the air:

respiration

Give a reason why dissolved air in water has a higher percentage of oxygen than that in the atmosphere?

Give the biological importance of Carbon dioxide dissolved in water.

Nitrogen prepared from the atmosphere is denser, than nitrogen prepared by chemical reactions. Give reasons.