CIE iGCSE Biology-14.5 Tropic responses- Study Notes- New Syllabus
CIE iGCSE Biology-14.5 Tropic responses- Study Notes – New syllabus
CIE iGCSE Biology-14.5 Tropic responses- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe gravitropism as a response in which parts of a plant grow towards or away from gravity
- Describe phototropism as a response in which parts of a plant grow towards or away from the direction of the light source
- Investigate and describe gravitropism and phototropism in shoots and roots
Supplement
- Explain phototropism and gravitropism of a shoot as examples of the chemical control of plant growth
- Explain the role of auxin in controlling shoot growth, limited to:
(a) auxin is made in the shoot tip
(b) auxin diffuses through the plant from the shoot tip
(c) auxin is unequally distributed in response to light and gravity
(d) auxin stimulates cell elongation
Gravitropism in Plants
Definition
Gravitropism (also called geotropism) is a type of plant growth response to gravity.
📌 Key Points
- Roots grow towards gravity (positive gravitropism).
- Shoots grow away from gravity (negative gravitropism).
🔍 Why Does It Happen?
- Helps anchor the plant and absorb water and minerals (roots).
- Helps the shoot reach light for photosynthesis (shoots).
🧪 Controlled by Plant Hormones (Auxins)
- Auxins move in response to gravity.
- In roots, high auxin inhibits growth on the lower side ➝ root bends downward.
- In shoots, high auxin stimulates growth on the lower side ➝ shoot bends upward.
🧠 Summary:
Gravitropism helps plants orient their growth: roots grow down into the soil for support and nutrients, while shoots grow up to access light, both guided by the distribution of auxins.
Gravitropism helps plants orient their growth: roots grow down into the soil for support and nutrients, while shoots grow up to access light, both guided by the distribution of auxins.
🌿Note:
Roots → Positive gravitropism
Shoots → Negative gravitropism
Roots → Positive gravitropism
Shoots → Negative gravitropism
Phototropism in Plants
Definition
Phototropism is a growth response in plants where parts of the plant grow towards or away from a light source.
📌 Types of Phototropism
| Plant Part | Direction of Growth | Type |
|---|---|---|
| Shoots | Towards light | Positive phototropism |
| Roots | Away from light | Negative phototropism |
🔍 Why Is Phototropism Important?
- Shoots bend toward light to maximize photosynthesis.
- Helps the plant survive and grow efficiently by absorbing more sunlight.
🌿 Hormonal Control (Auxins)
- Auxins are plant hormones that control growth.
- When light hits one side of the plant, auxins move to the shaded side.
- This causes the shaded side to grow faster, bending the shoot toward the light.
🧠 Summary:
Phototropism allows plants to grow toward light, improving their ability to capture energy through photosynthesis. This response is controlled by auxins which redistribute to the darker side of the plant.
Phototropism allows plants to grow toward light, improving their ability to capture energy through photosynthesis. This response is controlled by auxins which redistribute to the darker side of the plant.
Plant Tropisms: Investigating Gravitropism and Phototropism
Definition of Tropism
A tropism is a directional growth response in which the direction of the stimulus determines the direction of growth.
🌍 Gravitropism (Geotropism)
| Plant Part | Response to Gravity | Type of Gravitropism | Purpose |
|---|---|---|---|
| Shoot | Grows away from gravity | Negative gravitropism | To reach light for photosynthesis |
| Root | Grows toward gravity | Positive gravitropism | To anchor the plant and absorb water/minerals |
🌞 Phototropism
| Plant Part | Response to Light | Type of Phototropism | Purpose |
|---|---|---|---|
| Shoot | Grows toward light | Positive phototropism | Maximize light for photosynthesis |
| Root | Grows away from light | Negative phototropism | Help roots stay underground |
🔬 How to Investigate Tropisms
📌 Phototropism Investigation (Shoots):
- Take 3 identical seedlings.
- Place them in 3 boxes:
- One with light from one side.
- One with overhead light.
- One kept in darkness.
- Leave for a few days and observe:
- One-sided light: Shoot bends towards the light.
- Overhead light: Shoots grow straight upward.
- Darkness: Shoots grow slowly and straight.
Conclusion: Shoots show positive phototropism.
📌 Gravitropism Investigation (Roots and Shoots):
- Place seedlings horizontally in a moist petri dish.
- After 2–3 days:
- Roots grow downward (towards gravity).
- Shoots grow upward (against gravity).
Conclusion:
- Roots = positive gravitropism
- Shoots = negative gravitropism
🌿 Role of Auxins in Both Tropisms
| Tropism | Auxin Movement | Effect |
|---|---|---|
| Phototropism (Shoot) | Moves to the shaded side | Causes faster growth, bending toward light |
| Gravitropism (Root) | Accumulates on the lower side | Inhibits growth, so root bends down |
| Gravitropism (Shoot) | Accumulates on lower side | Stimulates growth, so shoot bends up |
📌Note:
🌿 Shoots: + Phototropism, − Gravitropism
🌱 Roots: − Phototropism, + Gravitropism
Controlled by auxins (plant hormones)
🌿 Shoots: + Phototropism, − Gravitropism
🌱 Roots: − Phototropism, + Gravitropism
Controlled by auxins (plant hormones)
Tropisms in Plants
Definition:
- A tropism is a plant’s growth response to a directional stimulus.
- Positive tropism = growth towards the stimulus
- Negative tropism = growth away from the stimulus
☀️ Phototropism (Response to Light)
| Feature | Description |
|---|---|
| Stimulus | Light |
| Observed in | Shoots (mostly) |
| Type of response | Positive phototropism (shoots grow toward light) |
| Reason | To maximize light absorption for photosynthesis |
| Hormone involved | Auxin (a plant growth hormone) |
| Mechanism | – Auxin moves to the shaded side of the shoot tip – Auxin causes cell elongation on that side – This makes the shoot bend toward the light |
🌍 Gravitropism (Response to Gravity)
| Feature | Description |
|---|---|
| Stimulus | Gravity |
| Observed in | Shoots and roots |
| Type of response | – Shoots: Negative gravitropism (grow against gravity) – Roots: Positive gravitropism (grow towards gravity) |
| Reason | – Shoots grow upward to reach light – Roots grow downward for water and anchorage |
| Hormone involved | Auxin again plays a key role |
| Mechanism (in shoots) | – Auxin collects on the lower side of a horizontal shoot – This side grows faster, causing the shoot to bend upward |
🧠 Summary:
- Both phototropism and gravitropism are controlled by uneven distribution of auxin, which causes
- differential cell elongation, guiding plant growth toward or away from light and gravity.
📌 Auxin facts:
– Made in shoot tips
– Moves in response to light and gravity
– Controls directional growth
– Made in shoot tips
– Moves in response to light and gravity
– Controls directional growth
Role of Auxin in Shoot Growth
What is Auxin?
Auxin is a plant growth hormone that controls the direction and rate of growth in shoots and roots.
🌿 Key Points on Auxin in Shoots:
| Step | Description |
|---|---|
| (a) Auxin is made in the shoot tip | The shoot tip (apical meristem) produces auxin continuously. This is the primary site of hormone production. |
| (b) Auxin diffuses through the plant from the shoot tip | After production, auxin diffuses downward from the tip to other parts of the shoot, influencing the growth of cells further down. |
| (c) Auxin is unequally distributed in response to light and gravity | Auxin moves to the shaded side of the shoot if exposed to light from one direction (phototropism), and to the lower side when affected by gravity (gravitropism). |
| (d) Auxin stimulates cell elongation | Cells on the side with more auxin elongate faster, causing the shoot to bend toward light or grow upward against gravity. |
🧠 Remember:
In shoots, more auxin = more elongation
In roots, more auxin = less elongation (opposite effect)