Investigating Plant Responses

1. 1 State [1 mark]

   State the independent variable in an investigation of the effect of light on the growth of seedlings.

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   Model answer

   The direction of the light.

   Mark scheme

   • Direction of the light — 1 mark
2. 2 Name [2 marks]

   Name two control variables in this investigation.

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   Model answer

   Any two from: type of seeds, age of the seedlings, number of seeds, temperature, volume of water, distance from the lamp, time.

   Mark scheme

   • First — 1 mark
   • Second — 1 mark
3. 3 Explain [2 marks]

   In an investigation of gravitropism, the seedlings were kept in the dark. Explain why.

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   Model answer

   Light would also cause the shoots to grow towards it. In the dark, gravity is the only stimulus, so the result is valid.

   Mark scheme

   • Light would affect the growth — 1 mark
   • Gravity is then the only stimulus — 1 mark
4. 4 Calculate [3 marks]

   The lengths of four shoots were 30, 34, 31 and 33 mm. Calculate the mean length.

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   Model answer

   30 + 34 + 31 + 33 = 128. 128 ÷ 4 = 32 mm.

   Mark scheme

   • Sum of 128 — 1 mark
   • Division by 4 — 1 mark
   • 32 mm — 1 mark
5. 5 Explain [3 marks]

   Explain why a clinostat is used in an investigation of gravitropism.

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   Model answer

   A clinostat turns the seedlings, so gravity acts equally on all sides. If the seedlings on the clinostat do not bend, but those that are not turned do, it shows that gravity causes the bending.

   Mark scheme

   • Turns the seedlings so gravity acts equally — 1 mark
   • It is a control — 1 mark
   • Shows that gravity causes the bending — 1 mark
6. 6 Evaluate [6 marks]

   A student measured the length of shoots with a ruler to the nearest millimetre, and drew them. The mean angle of bending of seedlings in light from one side was 40 degrees, but the seedlings in the dark also had a mean bend of 12 degrees. Evaluate the investigation.

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   Model answer

   The seedlings in light from one side bent much more than those in the dark, so the direction of light does cause the bending. However, the seedlings in the dark also bent by 12 degrees, so another factor, such as light getting into the box, was affecting them, which makes the investigation less valid. The student should make the box light-proof. The results could be made more repeatable by using more seedlings and finding a mean, and by repeating the investigation. A ruler with a resolution of 1 mm limits the accuracy of the lengths, and a protractor and photographs would give better measurements of the bending. The drawings are a useful record but cannot easily be used to compare results. With these improvements the conclusion could be trusted more.

   Mark scheme

   • Light from one side caused a much larger bend — 1 mark
   • Dark seedlings bent, so another factor — 1 mark
   • Suggest a cause and how to stop it — 1 mark
   • More seedlings, a mean and repeats for repeatability — 1 mark
   • Accuracy of the measuring — 1 mark
   • A reasoned conclusion — 1 mark

Quick check

1. 1

   What is required practical 8?

   1. AOsmosis in potato
   2. BReaction time
   3. CEffect of light or gravity on seedlings
   4. DFood tests
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   C: Effect of light or gravity on seedlings

   It investigates the effect of light or gravity on the growth of newly germinated seedlings.

2. 2

   What must be recorded in this practical?

   1. AOnly photographs
   2. BLength measurements and labelled drawings
   3. COnly drawings
   4. DOnly the colour of the seedlings
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   B: Length measurements and labelled drawings

   Length measurements and labelled biological drawings.

3. 3

   What does a clinostat do?

   1. ACancels the effect of gravity by turning
   2. BProvides light
   3. CMeasures length
   4. DWaters the seedlings
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   A: Cancels the effect of gravity by turning

   It turns the seedlings so gravity acts on all sides equally.

4. 4

   Which of these is a control variable?

   1. AThe direction of the light
   2. BThe length of shoots
   3. CThe angle of bending
   4. DThe temperature
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   D: The temperature

   The temperature must be the same for all the seedlings.

5. 5

   Why are the dishes in the gravity investigation kept in the dark?

   1. ATo make the seeds germinate
   2. BTo keep them warm
   3. CSo that light does not affect the growth
   4. DTo grow faster
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   C: So that light does not affect the growth

   So that light does not change the direction of growth.

6. 6

   What is meant by a resolution of 1 mm?

   1. AThe ruler is 1 mm long
   2. BThe ruler detects changes of 1 mm
   3. CThe ruler is accurate
   4. DThe ruler has no error
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   B: The ruler detects changes of 1 mm

   The ruler can detect a change of 1 mm.

7. 7

   The shoot lengths were 20, 22 and 24 mm. What is the mean?

   1. A22 mm
   2. B66 mm
   3. C24 mm
   4. D20 mm
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   A: 22 mm

   \((20 + 22 + 24) \div 3 = 22\) mm.

8. 8

   Which feature should a biological drawing have?

   1. AShading
   2. BColour
   3. CNo labels
   4. DClear lines and labels
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   D: Clear lines and labels

   Clear single lines, with labels and no shading.

9. 9

   Seedlings in the dark are long and pale. What does this suggest?

   1. AThey have more chlorophyll
   2. BThey grow quickly to find light and have no chlorophyll
   3. CThey are healthier
   4. DThey are more reproducible
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   B: They grow quickly to find light and have no chlorophyll

   With no light they grow tall quickly to find light, and without light they cannot make chlorophyll.