Investigating Tropic Responses

1. 1 State [1 mark]

   State the independent variable in an investigation of phototropism.

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   Model answer

   The direction of the light.

   Mark scheme

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

   State two variables that should be kept the same in an investigation of the effect of light direction on seedlings.

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   Model answer

   Any two from: the type and age of seedlings, the temperature, the volume of water, the distance from the lamp, the size of the pot, and the time.

   Mark scheme

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

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

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   Model answer

   Light would also affect the direction of growth, because shoots show phototropism. Keeping the dishes in the dark means that gravity is the only stimulus.

   Mark scheme

   • Light would also affect the direction of growth — 1 mark
   • So that gravity is the only stimulus — 1 mark
4. 4 Calculate [3 marks]

   The angles of bending of five seedlings were 20, 25, 30, 25 and 35 degrees. Calculate the mean angle.

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   Model answer

   20 + 25 + 30 + 25 + 35 = 135. 135 ÷ 5 = 27 degrees.

   Mark scheme

   • Sum of 135 — 1 mark
   • Division by 5 — 1 mark
   • 27 degrees — 1 mark
5. 5 Explain [3 marks]

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

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   Model answer

   It turns the seedlings slowly, so that gravity acts equally on all sides. If the seedlings then grow straight, and those which are not turned bend, it shows that gravity is the stimulus which causes the bending.

   Mark scheme

   • Turns seedlings so gravity acts equally on all sides — 1 mark
   • Acts as a control — 1 mark
   • Shows that gravity is the stimulus — 1 mark
6. 6 Evaluate [6 marks]

   A student found that the shoots in the pot with light from one side bent towards it by 38 degrees, but that the shoots in the dark also bent by 10 degrees. Evaluate the investigation and suggest improvements.

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   Model answer

   The shoots with light from one side bent far more than those in the dark, so the light direction does seem to cause the bending. However, the shoots in the dark also bent, which suggests that something else, such as light leaking into the box, a difference in temperature, or the seedlings being uneven, affected them. This reduces the validity of the results. The student should make the dark box properly light-proof, use more seedlings, use seedlings of the same age and height, and repeat the investigation to find a mean. A protractor and photographs would make the measurement more accurate. With these improvements the investigation would be more reliable, and the conclusion could be accepted with more confidence.

   Mark scheme

   • Light-side bending much larger than dark — 1 mark
   • Dark pot bent, so another factor — 1 mark
   • Possible cause such as light leak or temperature — 1 mark
   • Make the dark box light-proof — 1 mark
   • More seedlings, repeats and a mean — 1 mark
   • Better measurement, and a reasoned conclusion — 1 mark

Quick check

1. 1

   What is the dependent variable when investigating phototropism?

   1. AThe type of seed
   2. BThe direction of the light
   3. CThe angle of bending
   4. DThe temperature
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   C: The angle of bending

   The angle of bending of the shoots is what is measured.

2. 2

   What does a clinostat do?

   1. ATurns seedlings slowly
   2. BMeasures angles
   3. CProvides light
   4. DWaters the seedlings
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   A: Turns seedlings slowly

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

3. 3

   Why is a pot with light from above included?

   1. ATo make the shoots grow
   2. BTo warm the seedlings
   3. CTo check the lamp
   4. DAs a control
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   D: As a control

   As a control, to show that shoots grow straight when there is no difference between the sides.

4. 4

   In a dish on its side, what happens to the roots?

   1. AThey curve upwards
   2. BThey curve downwards
   3. CThey stay straight
   4. DThey die
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   B: They curve downwards

   They curve downwards, towards gravity.

5. 5

   Which instrument is used to measure the angle of bending?

   1. AA ruler
   2. BA thermometer
   3. CA protractor
   4. DA balance
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   C: A protractor

   A protractor.

6. 6

   The angles of bending were 10, 20 and 30 degrees. What is the mean?

   1. A20 degrees
   2. B30 degrees
   3. C60 degrees
   4. D10 degrees
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   A: 20 degrees

   \((10 + 20 + 30) \div 3 = 20\) degrees.

7. 7

   Why should the gravity dishes be kept in the dark?

   1. ATo make the seeds germinate
   2. BTo keep them warm
   3. CTo grow faster
   4. DSo that light does not affect the growth direction
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   D: So that light does not affect the growth direction

   So that light does not affect the direction of growth.

8. 8

   Why can the heat from a lamp be a problem?

   1. AIt dries the air
   2. BIt may warm one side of the seedlings
   3. CIt makes the lamp brighter
   4. DIt kills the roots
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   B: It may warm one side of the seedlings

   It can warm one side, which may change the growth.

9. 9

   Seedlings on a clinostat grow straight in all directions. What does this show?

   1. ALight causes the bending
   2. BWater causes the bending
   3. CGravity causes the bending in the stationary dish
   4. DThe seedlings are dead
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   C: Gravity causes the bending in the stationary dish

   Turning cancels the effect of gravity, so the bending seen in the other dish is caused by gravity.