Page 14 - 新思维科学学生用书8 样章

1.2 Gas exchange

Think like a scientist

Why are air sacs so small?

In this activity, you will use some agar jelly to represent the lungs, and some coloured
liquid to represent oxygen in the air.

You will need:
• two Petri dishes filled with agar jelly
• two cork-borers, one with a diameter of 10 mm and the other with a

diameter of 5 mm
• some coloured dye
• a dropper pipette

Method

1 Use the larger cork-borer to make eight holes in the jelly
in one of the dishes. Space the holes evenly in the dish.
2 Now use the smaller cork-borer to make 32 holes in the
jelly in the other dish. Try to space the holes evenly in the
dish.

3 Using the dropper pipette, carefully fill each hole in
both dishes with the coloured dye. Try to put the same
quantity of dye into each hole. It’s really important not to

get any dye on the jelly!
4 Leave both dishes for at least 15 minutes.
5 Predict what you think will happen.
6 After 15 minutes (or a little bit longer if things are
happening slowly) record your observations.

Questions
1 The holes that you made in the jelly represent the air sacs in the lungs. The

coloured dye represents oxygen in the air sacs. The holes in the jelly are an
analogy for the air sacs, and the dye is an analogy for oxygen.
Explain how your observations help to show what happens to oxygen in the lungs.
2 The total volume of the 32 small holes is the same as the total volume of the

eight large holes. Use your observations to suggest why it is better to have a lot
of very small air sacs in the lungs, rather than just a few large ones.
3 Do you think that the agar jelly with holes is a good model for what happens in
the lungs? Explain your answer.

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