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

WJEC GCSE Additional Science Teacher’s Notes
1. Why do you think the time taken for 50% of the leaf discs to float was measured in seconds rather
than in minutes and seconds
It is easier because you don’t have to calculate decimal fractions of a minute. It would be
bad practice to just measure to the nearest minute (inaccurate) or to record minutes and
seconds (mixed units).
2. It was felt that measuring the time taken for 50% of the leaf discs to float would provide a more
accurate measure of photosynthesis than waiting for all the discs to float. Why do you think this is
If some discs were not viable they may not carry out photosynthesis and so would never
float.
3. Do you think the variation in the results is acceptable to draw a conclusion from Explain your
answer.
Probably not. Although there is a clear trend, the variation between repeats is often greater
than the difference between the means at adjacent distances.
4. Do you think three repeats for this experiment is enough Explain your answer.
No. The repeatability of these results is poor and so many more repeats should be done.
5. Do you think that the differences in the results for the different distances are significant Explain
your answer.
Probably not, given the variation in the results. Even the difference between the results at 30
and 35 cm may not be significant.
6. What conclusion would you draw from these results
Increasing light intensity may increase the rate of photosynthesis, but further repeats are
necessary in order to say for certain whether this is the case.
NOTE: These are real results which illustrate the uncertainty often found in experimental data.
It should be emphasised that these uncertain results do not mean that light intensity has no
effect on rate of photosynthesis, but simply that the experiment is inconclusive. This is a good
exercise to see if pupils can look at data in an impartial way. They are likely to know that
increasing light intensity should increase rate of photosynthesis, and this may lead them to
exhibit bias when interpreting the results.
_ Why study respiration (pages 40–41)____________
PRACTICAL How can we measure respiration
(pages 40–41)
1. Explain your results for flask A.
The seeds will be respiring, and respiration produces heat, which is retained by the flask.
2. Explain the purpose of flask B.
Flask B is a control to show that the effect is due to a living process (respiration) in the
seeds.
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