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3.6.6Inquiry Process

Observing Tropic Responses- Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 146 of 160

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Part 1 of 5Designing the Investigation

3.6.6 Observing Tropic Responses- Skills
1
State a testable hypothesis, such as that shoots bend towards a light source from one side.
2
Identify the independent variable (direction of light), the dependent variable (angle of curvature) and the controlled variables.
3
Grow seedlings of the same species and age in identical conditions, with a control group in uniform light or darkness.
4
Use enough seedlings in each group, and repeat the experiment to improve reliability.
5
Photograph or draw the seedlings at the start and finish, then measure angles with a protractor.

Part 2 of 5Controlled Variables

Seedling age
  • Use seedlings of the same age and height because growth rate and response vary with developmental stage.
Light intensity
  • Keep the distance from the lamp and its brightness constant, or tropisms differ due to stimulus strength.
Temperature
  • Maintain the same temperature, since warmth changes growth rate and so the amount of bending.
Water supply
  • Give equal amounts of water so the seedlings are not stressed by drying or flooding.
Light direction
  • Light must come from one side only, so the rest of the apparatus should be shielded from stray light that would mask the response.
TropismStimulusTypical shoot response
PhototropismLightPositive, grows towards light
GravitropismGravityNegative, grows up
HydrotropismWaterRoots grow towards moisture

Part 3 of 5Analysing Results

Analysing Results
Mean and spread
  • Calculate the mean angle for each group and show the range or standard deviation to judge variability.
Control comparison
  • The control seedlings in uniform light should grow straight, which confirms that the light direction caused the bending.
Limitations
  • Seedlings differ naturally and measurement of angle from a photograph has errors, so conclusions must be cautious.
Improvements
  • Use a clinostat, larger samples or time-lapse imaging to improve the quality of data.

Part 4 of 5Worked Example

Question

  • Five shoots bend by 22°, 25°, 19°, 28° and 26° towards a lamp, while controls in uniform light bend by an average of 2°.
  • Calculate the mean bend of the test group and the difference from the controls.

  1. Sum = 22 + 25 + 19 + 28 + 26 = 120°.
  2. Mean = 120 ÷ 5 = 24°.
  3. Difference from control = 24 − 2 = 22°.

Answer Mean bend 24°, which is 22° more than the control, supporting positive phototropism.

Part 5 of 5Exam Checklist & Tips

3.6.6 – Key Points for the Exam

  • Tropism A directional growth response of a plant to a directional stimulus.
  • Phototropism Growth in response to the direction of light.
  • Gravitropism Growth in response to gravity.
  • Control A group treated identically except for the variable being tested.

Common Mistakes

  • Calling a tropism a movement of the whole plant; it is a growth response.
  • Forgetting a control group with uniform light.
  • Using different species or ages and then concluding the light caused the difference.

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