(a) (i) Respiration: Respiration is the process by which living things breakdown/oxidise food/nutrients/glucose/ organic substances; to release energy/ATP for metabolic activities of the body; with or without the use of oxygen.

Gaseous exchange	Aerobic respiration
oxygen is used to oxidise glucose	organism and CO² moves out
process takes place outside the cell	process occurs inside the cell
physical process	chemical process
No energy is released	energy is released
No enzymes involved	enzymes involved

(b) (I) Residual Air: During breathing out/expiration/exhalation; not all the air is removed from the lungs; the air left inside the lungs during breathing out is residual air.
(ii) Importance of Residual Air: Air is left in the lungs so that the lungs do not collapse; air is always left inside the lungs so that more oxygen can be removed (diffuse) into the blood stream.
(c) Characteristic features of respiratory structures: -

They have wide/large surface area
they are moist
thin walled membranes
they are highly vascularised/ richly supplied with blood
highly permeable to gases.

(d)(i) Oxygen debt: The build up of lactic acid; during strenuous exercise is poisonous and makes the muscles ache; oxygen is needed to break down/ get rid of the lactic acid, that is why athletes continue breathing heavily even after the exercise is over; the oxygen needed to get rid of the lactic acid is known as oxygen debt.
(ii) Activities that can result in oxygen debt: Running; boxing; swimming /any correct example of strenuous activities.