DSE生物試題-年份
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2017年1月6日 星期五
2017年1月4日 星期三
Form 5 EXP 01 suggested answers
Practical
15.4
Investigation of the phototropic responses
of shoots
Objective: To study the phototropic response
of the shoot of the seedling(1 mark)
Biological principle and experiment
design: (3
marks)
Plants can detect light (stimulus) and respond to them. However, the
responses of plants are very slow, and usually involve growth of certain parts
of the body towards or away from light.
To investigate the phototropic responses of the plant shoots, young
seedlings receive the illumination of unilateral light (1).
To ensure that the responses of the plant are due to the unilateral
stimulus, same numbers of same batch of
seedlings are placed on a clinostat (1), so that
the effect of the unilateral light is removed. In this set-up, the seedlings
receive light from different direction. This set-up is referred as control set-up (1)
The following labeled diagram shows the experimental set-up and the
control set-up:
Experimental
set-up
Correct experimental set-up (1)
Correct control set-up (1)
Lateral light (1)
Labels (1): seedlings, shoots, petri
dish,
cotton wool etc
|
Control
set-up
|
Materials and
instruments:
Clinostat X 1 Culture medium (for growth of seedlings) appropriate amounts
Petri dish X 2 seedlings (germinating mung beans) appropriate
amounts
Cotton wool appropriate amounts
Procedures: (1
mark)
1. Prepare two petri dishes with same amounts of
cotton wool and culture medium. Place 10 mung beans on each of the petri dish
respectively.
2.
Fix one petri
dish on a clinostat which rotates horizontally. Switch on the clinostat so that
it rotates at
about 4 revolutions per hour. This is the control set-up.
Results: (4
marks)
The table below shows the growth of seedlings in
different set-up
Set-ups
|
Ways of growth of shoot of the seedlings
|
Experimental set-up /
Set-up without clinostat
|
The shoots grow towards the light.
|
Control set-up /
Set-up with clinostat
|
The shoots grow vertically without
bending.
|
Discussion:
(4 marks)
1 Based on the experiment
results, describe the growth response of shoots to light. What is the
significance of such response to plants? (3
marks)
The growth of shoot is positively phototropic (1).
This allows the leaves to obtain more light (1)
to carry out photosynthesis more
efficiently to produce food (1)
2 Both set-ups should be maintained at the same
height. Why? (1 mark)
To ensure that the seedlings in these two
set-ups receive same the amounts of light (1).
Conclusion: (1 mark)
Growth of the shoot of seedlings is positively phototropic (1)
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