Teacher's notes and answers to questions in the book - Hodder Plus ...

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.
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