The diagram shows a model that can be used in the laboratory to demonstrate how breathing works. A glass jar represents the chest, a rubber sheet stretched ...

(a) Part representing the diaphragm [1]. The rubber sheet stretched across the bottom of the jar.

(b) Parts representing the lungs [1]. The two balloons.

(c) What happens to the balloons when the sheet is pulled down [2]. The balloons inflate and get bigger, because air is drawn into them. One mark for the balloons inflating, one for air being drawn in.

(d) Explanation in terms of volume and pressure [3]. Pulling the rubber sheet down increases the volume inside the jar; the pressure inside the jar therefore falls below atmospheric pressure; air then moves in from the higher pressure outside to the lower pressure inside, inflating the balloons. All three linked steps are needed, and they mirror what happens when the diaphragm contracts and flattens during inhaling.

(e) One way the model is a good representation [2]. The rubber sheet behaves like the diaphragm: pulling it down increases the volume and lowers the pressure so that air moves in; this is just what happens in the body when the diaphragm contracts and flattens during breathing in. One mark for the sheet acting like the diaphragm to change volume and pressure, one for linking this to inhaling.

(f) Two ways the model is not accurate [4]. Give two limitations, each explained for two marks:

• The model has no ribs or intercostal muscles, whereas in a human these also move (the ribs swing up and out) to help change the volume of the chest.
• The balloons are simply hollow bags, whereas real lungs are divided into millions of tiny alveoli and actually carry out gaseous exchange, which the balloons do not.

Other acceptable points: the rubber sheet is pulled by hand rather than by a muscle contracting; the rigid glass jar cannot move, unlike the flexible chest wall.