Triple Science Only - Required Practical- Tropisms
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of tropisms in plants.
What is auxin and what does it do in plant shoots?
Auxin is a plant hormone that controls growth. In shoots, auxin causes increased growth, especially on the shaded side, leading to bending towards light.
What is phototropism?
Phototropism is a plant’s growth response to light. Shoots show positive phototropism (grow towards light), while roots show negative phototropism (grow away from light).
How does gravity affect auxin distribution in roots?
Gravity causes auxin to accumulate on the lower side of roots. In roots, high auxin levels inhibit growth, so the root bends downwards (positive gravitropism).
Topic Explainer Video
Check out this @lauradoesbiology video that explains the required practical- tropisms, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Required Practical- Tropisms
Aim: To investigate the effect of light or gravity on the growth direction of newly germinated seedlings.
Suggested Apparatus and Method
Apparatus
- Petri dishes
- Cotton wool
- Ruler
- Seeds (e.g. cress)
- Light source
- Cardboard boxes (for directional light)
- Water
- Marker pen
- Forceps
Method – Phototropism Investigation (Light)
1. Line a Petri dish with moist cotton wool.
2. Place 10 seeds evenly spaced on the surface.
3. Cover the dish with a box that has a hole on one side (to allow light from one direction only).
4. Place it in a warm place and allow the seeds to germinate.
5. After 5 days, measure the length of shoots and observe the direction of growth.
- You can use the forceps to carefully straighten the shoots to help you measure them accurately with the ruler. Or, you can use a piece of string placed next to the shoots and then measure the string with a ruler.
6. Draw labelled biological diagrams of the seedlings to show curvature.
7. Repeat with control conditions such as: no cardboard box so light is even and a cardboard box with no hole so it is completely dark.
In this experiment we started with seeds that hadn’t yet germinated. You may instead be provided with seedlings that have already started to grow. In this case you would measure the length of the seedlings before and after the experiment to calculate the increase in length.
Scientific Explanation
- Auxin is produced by the shoot tip.
- Auxin diffuses and moves to the shaded side of the shoot.
- Cells on the shaded side elongate more, causing the shoot to bend towards the light.
- This is positive phototropism.
Key Terms & Definitions
|
Term |
Definition |
|---|---|
|
Tropism |
A plant’s directional growth response to a stimulus (light or gravity). |
|
Phototropism |
Growth response to light (shoots grow towards it). |
|
Gravitropism |
Growth response to gravity (roots grow downwards). |
|
Auxin |
A plant hormone that regulates growth by controlling cell elongation. |
|
Positive tropism |
Growth towards the stimulus. |
|
Negative tropism |
Growth away from the stimulus. |
No answer provided.
Exam Tips
When describing this practical in an exam, consider your independent, dependent and controlled variables.
Examiners look for:
- Independent variable- different light conditions- for example -one directional light, even light and complete darkness.
- Dependent variable- Measurement of shoot lengths using a ruler and the direction of growth.
- Controlled variables- Such as: volume of water given to seedlings, type of seeding, age of seedlings at the start of the experiment, temperature and time the seedlings are left to grow. Always try to give at least three controlled variables in an investigation plan.
No answer provided.
Practice Questions
Plan an investigation to show the effect of light from one direction on the growth of plant seedlings. You can use lamps, pots of seedlings, cardboard boxes, scissors, rulers and any other standard laboratory equipment. (6 marks)
Model Answer:
• get three pots of seedlings and measure the height of the shoots.
• place one pot under a cardboard box in the dark, have one pot in full sunlight, have one pot under a cardboard box with a hole cut in one side (so light is from one direction).
• control variable(s) e.g. same temperature / volume of water / soil type / type of seedling
• after a set time (at least several hours).
• re-measure heights of shoots.
• calculate the mean height increase for each group.
• observe and record the direction of growth / use a protractor to estimate angle of bending.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!