Triple Science Only - Plant Hormones- Tropisms
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of response to stimuli and diffusion.
What is diffusion?
The net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.
What is a stimulus?
A change in the environment that is detected by receptors and can cause a response.
Why do plants need to respond to stimuli?
To increase their chances of survival by growing towards resources like light and water, and anchoring themselves properly in the soil.
Topic Explainer Video
Check out this @lauradoesbiology video that explains plant hormones- tropisms, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Plant Hormones- Tropisms
What are Tropisms?
- Tropisms are directional growth responses of plants to stimuli.
- The direction of growth is determined by the direction of the stimulus.
Types of Tropisms:
- Phototropism – response to light.
- Gravitropism (geotropism) – response to gravity.
A positive tropism is directional growth towards a stimulus.
A negative tropism is directional growth away from a stimulus.
Tropism in Different Plant Organs:
|
Organ |
Phototropism |
Gravitropism |
|
Shoots |
Positive- shoots grow towards light. |
Negative- shoots grow away from gravity. |
|
Roots |
Negative- roots grow away from light. |
Positive- roots grow towards gravity. |
The Role of Auxin
- Auxin is a plant hormone that controls growth near the tips of roots and shoots.
- It is produced in the shoot tips and root tips and moves by diffusion.
- Uneven distribution of auxin causes unequal growth rates on different sides of the root or shoot.
Phototropism (Shoots)
- In even light auxin will diffuse away from the shoot tip and spread evenly in the shoot, causing equal growth. The shoot will grow straight up.
- Light from one direction causes auxin to move to the shaded side of the shoot.
- Auxin accumulates on the shaded side.
- The shaded side grows faster, due to increased cell elongation, bending the shoot towards the light.
- This benefits the plant as growing towards light will increase the amount of light absorbed for photosynthesis.
Gravitropism (Roots)
- Gravity causes auxin to move to the lower side of the root.
- Auxin accumulates on the lower side.
- In roots, high auxin inhibits cell elongation, so the upper side grows more, and the root bends downwards.
- This benefits the plant as the roots will anchor the plant in the soil and be able to absorb water and mineral ions.
Key Terms & Definitions
|
Term |
Definition |
|---|---|
|
Tropism |
A directional growth response of a plant to a stimulus. |
|
Phototropism |
Growth in response to light. |
|
Gravitropism |
Growth in response to gravity. |
|
Auxin |
A plant hormone that regulates growth by affecting cell elongation. |
No answer provided.
Exam Tips
In shoots, more auxin = more growth.
In roots, more auxin = less growth.
Remember: Auxin works differently in roots and shoots!
No answer provided.
Practice Question
Explain how auxin causes a shoot to grow towards light from one direction. (4 marks)
Model Answer:
- Auxin is produced in the shoot tip and diffuses down the shoot.
- Auxin accumulates on the shaded side of the shoot.
- Auxin causes increased growth on the shaded side.
- This causes the shoot to bend towards the light.
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!