3.6.6Inquiry Process
Observing Tropic Responses- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 146 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Designing the Investigation
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.
Section 2 of 5Controlled Variables
Seedling age
- Seedling age Use seedlings of the same age and height because growth rate and response vary with developmental stage.
Light intensity
- Light intensity Keep the distance from the lamp and its brightness constant, or tropisms differ due to stimulus strength.
Temperature
- Temperature Maintain the same temperature, since warmth changes growth rate and so the amount of bending.
Water supply
- Water supply Give equal amounts of water so the seedlings are not stressed by drying or flooding.
Light direction
- 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.
| Tropism | Stimulus | Typical shoot response |
|---|---|---|
| Phototropism | Light | Positive, grows towards light |
| Gravitropism | Gravity | Negative, grows up |
| Hydrotropism | Water | Roots grow towards moisture |
Section 3 of 5Analysing Results
Analysing Results
Mean and spread
- Mean and spread Calculate the mean angle for each group and show the range or standard deviation to judge variability.
Control comparison
- Control comparison The control seedlings in uniform light should grow straight, which confirms that the light direction caused the bending.
Limitations
- Limitations Seedlings differ naturally and measurement of angle from a photograph has errors, so conclusions must be cautious.
Improvements
- Improvements Use a clinostat, larger samples or time-lapse imaging to improve the quality of data.
Section 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.
- Sum = 22 + 25 + 19 + 28 + 26 = 120°.
- Mean = 120 ÷ 5 = 24°.
- Difference from control = 24 − 2 = 22°.
Answer Mean bend 24°, which is 22° more than the control, supporting positive phototropism.
Section 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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