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WJEC GCSE Additional Science Teacher’s Notes • volume of yeast and glucose solution • time period for reading. Better pupils may suggest controlling pH if they have enough knowledge of enzymes. This would be relevant as carbon dioxide produced may lower pH. Risk assessment: the only hazard is the glass tubing, which could break when the bung is inserted. Precaution – hold bung and not tubing when inserting. Possibility of yeast allergy is irrelevant as the experiment does not involve ingesting the yeast. PRACTICAL What effects do respiration and photosynthesis have on the atmosphere (pages 43–44) 1. Explain the final colour seen in each tube. Tube A: Little or no change in colour, because the carbon dioxide produced by respiration in the snails is absorbed by photosynthesis in the pond weed. Tube B: Goes red because carbon dioxide is being absorbed by the pond weed during photosynthesis. Tube C: Goes yellow because the pond weed is respiring (producing carbon dioxide) but cannot photosynthesise because of the lack of light. Tube D: Goes yellow because the snails are respiring (producing carbon dioxide). Tube E: No change because nothing in it will either use or produce carbon dioxide – it’s a control. Note that the balance between respiration and photosynthesis in tube A is not likely to be perfect, so there may be a slight change in colour. 2. What was the purpose of: a tube C To show that photosynthesis was responsible for colour change in tube B (and therefore, by implication, in tube A) b tube E Control tube to show no colour change occurs without snails and pond weed. Also useful as a colour comparison for tube A. 3. Why were the tubes sealed with cotton wool rather than a cork or bung To allow gases in for photosynthesis and/or respiration (particularly in tubes B–D). Will also avoid build-up of pressure in tubes B–D (though this is unlikely to be a problem). Questions 1. Explain the trends shown in the graph (Figure 4.13). Between dawn and dusk, carbon dioxide levels decrease and oxygen levels increase due to photosynthesis exceeding respiration. Between dusk and dawn the reverse happens because photosynthesis does not occur due to lack of light, but respiration continues. Look for an understanding that both photosynthesis and respiration occur in the dawn to dusk period. 12
WJEC GCSE Additional Science Teacher’s Notes 2. The greenhouse contains very few animals. What changes would you expect to see in this graph if there was a mixed population of animals and plants In the dawn to dusk period, there would be less change in the levels of both carbon dioxide and oxygen. In the dusk to dawn period, the fall in oxygen and rise in carbon dioxide would both increase. 13
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