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WJEC GCSE Additional Science Teacher’s Notes 5 The respiratory system _How do we breathe (pages 47–48)_______________ Questions 1. Explain why the balloons inflate when the sheet is pulled down, and deflate when it is pushed up. The balloons inflate when the sheet is pulled down because that creates more volume/space around the balloons which results in a lower pressure. The (now) higher pressure inside the balloons forces them outwards, causing inflation. When the sheet is raised, the process is reversed, resulting in a higher pressure outside the balloons than inside. 2. List the ways in which this model of the respiratory system is inaccurate. The lungs are not like balloons – they are more like sponges. The ‘ribcage’ on the model does not move. There is more space around the balloons than around the lungs. 3. Do these inaccuracies mean that it is not a useful model of the respiratory mechanism Explain your answer. It is still a useful model because it illustrates how diaphragm movement affects pressure and then how this alters the shape of the lungs. It is a qualitative model and the inaccuracies would only affect the scale of the effect. _How does the air we breathe in differ from that we___ breathe out (pages 49–50) Questions 4. Why is the exhaled air warmer than the inhaled air The body is warmer than the environment and so it warms the air. 5. Why does exhaled air have more water vapour The lining of the alveoli is damp which moistens the air. 6. Explain the change in the percentage of nitrogen in exhaled air (note: the body neither absorbs nor gives out nitrogen). The reduction of oxygen and the increase in carbon dioxide roughly balance each other. Water vapour is added which increases the overall volume. Therefore the percentage of the air that is nitrogen goes down, even though the actual amount of nitrogen stays the same. (There would also be a slight decrease in the percentage of other gases, but as the number is so small, the change is masked when figures are rounded.) Though demanding, this question is a good test of whether pupils have a full understanding of percentages. 14
WJEC GCSE Additional Science Teacher’s Notes PRACTICAL Demonstrating the difference in carbon dioxide content of inhaled and exhaled air (page 50) 1. What are your conclusions from this experiment There is more carbon dioxide in exhaled air than in inhaled air. 2. A student put forward a hypothesis that ‘There is carbon dioxide in exhaled air but not in inhaled air’. a Explain why the evidence from this experiment cannot support this hypothesis. The test is not sensitive to small amounts of carbon dioxide, so the fact that the ‘inhaled air’ lime water stays clear does not conclusively prove that there is no carbon dioxide in inhaled air. b Suggest how you could modify the procedure to test this hypothesis. The lime water should be switched for bicarbonate indicator (pupils may just say ‘a more sensitive indicator’). Discussion point Why does inhaled air come in through tube A and exhaled air go out through tube B The tube attached to the mouth in A has its end in air, whereas in B it’s in water. There is therefore less resistance to air flow in A, so air comes in from tube A. When air is breathed back into the tubes, it is actually equally easy for the air to travel down either tube, and indeed some will go back into tube A. However, the effect is more noticeable in B because the air goes directly into the lime water. _How does smoking damage the lungs (pages 51–53)_ Questions 7. Compare the data of a lifelong non-smoker with someone giving up at 30. Roughly how much more likely is the smoker to develop lung cancer before the age of 65 About 1% more likely. 8. Suggest a reason why, if you smoke, it is best to give up before the age of 40. Giving up later than 40 produces less decrease in risk than giving up at 40 or earlier. Discussion points Overall, smokers are about 15 times more likely to get lung cancer than non-smokers. In itself, however, this does not prove that smoking causes lung cancer. Why not, and what extra evidence is needed to show a causal link The point to stress here is that correlation does not prove a causal relationship. The extra evidence is a mechanism linking smoking and cancer, which has been found. The carcinogens in cigarette smoke provide clear evidence that smoking is likely to cause cancer. 15
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