IB MYP 4-5 Biology-Tropism- Study Notes - New Syllabus
IB MYP 4-5 Biology-Tropism- Study Notes – New syllabus
IB MYP 4-5 Biology-Tropism- Study Notes – IB MYP 4-5 Biology – per latest IB MYP Biology Syllabus.
Key Concepts:
- Phototropism (response to light)
- Geotropism/Gravitropism (response to gravity)
- Hydrotropism (response to water)
- Role of auxins in plant responses
Tropism
What is Tropism?
Tropism is a plant’s growth response to a stimulus like light, gravity, or water.
- Plants can’t move, but they respond by growing toward or away from a stimulus.
- This helps them survive by finding light, water, or support.
Types of Tropism
| Type of Stimulus | Tropism Name | Example |
|---|---|---|
| Light | Phototropism | Shoot bending toward sunlight |
| Gravity | Gravitropism | Roots growing downward |
| Water | Hydrotropism | Roots growing toward moisture |
| Touch | Thigmotropism | Vines curling around support |
Phototropism
Phototropism is the plant’s growth in response to light.
Shoots: Show positive phototropism (grow toward light)
Roots: Show negative phototropism (grow away from light)
How Does Phototropism Work?
- Light hits one side of the plant shoot
- Auxins (plant hormones) move to the shaded side
- Cells on the shaded side grow faster, causing the shoot to bend toward light
Uneven auxin distribution causes bending
Summary Table – Phototropism
| Part of Plant | Type of Response | Result |
|---|---|---|
| Shoot | Positive phototropism | Bends toward light |
| Root | Negative phototropism | Grows away from light |
| Hormone involved | Auxin – moves to shaded side, causes bending | |
Why Is Phototropism Important?
- Leaves face sunlight for maximum photosynthesis
- Helps plants grow tall and outcompete others
- Increases chances of survival in crowded places
Geotropism / Gravitropism
What is Geotropism (Gravitropism)?
Geotropism (also called gravitropism) is a plant’s growth response to gravity.
- “Geo” = Earth / gravity
- “Tropism” = growth movement
- Even without senses, plants detect gravity and grow correctly.
How Do Plants Respond to Gravity?
| Part of Plant | Response Type | Growth Direction |
|---|---|---|
| Roots | Positive geotropism | Grow downward (with gravity) |
| Shoots | Negative geotropism | Grow upward (against gravity) |
How Does Geotropism Work?
- Gravity causes auxins (plant hormones) to move to the lower side of root or shoot.
- In roots: high auxin slows growth → top grows faster → root bends downward.
- In shoots: high auxin speeds growth → bottom grows faster → shoot bends upward.
Real-Life Observations
- Lay a plant on its side → roots bend downward, shoots bend upward.
- Even in total darkness, seedlings grow correctly – gravity guides them, not just light.
Summary Table – Geotropism
| Feature | Description |
|---|---|
| Stimulus | Gravity |
| Root response | Grows downward (positive geotropism) |
| Shoot response | Grows upward (negative geotropism) |
| Hormone involved | Auxin |
| Importance | Helps roots find water, shoots find light |
Why Is Geotropism Important?
- Roots grow deep to absorb water and nutrients
- Shoots grow upward to reach light for photosynthesis
- Ensures proper plant orientation even if tilted or disturbed
Hydrotropism
What is Hydrotropism?
Hydrotropism is a plant’s growth response to moisture or water in the soil.
- “Hydro” = water
- “Tropism” = growth movement
- Roots grow toward wetter areas to help the plant survive.
Types of Hydrotropism
| Direction of Growth | Name |
|---|---|
| Toward water | Positive hydrotropism |
| Away from water (rare) | Negative hydrotropism |
Where Does It Happen?
- Root tips – detect moisture gradients
- Root hairs – absorb water to keep the plant hydrated
How Does It Work?
- Root cells sense water concentration differences.
- Auxins move to the dry side of the root.
- Cells on the drier side elongate more → root bends toward water.
Real-Life Observation
- Place moist cotton on one side of a plant’s roots → roots bend toward moisture.
- This shows active water-seeking, not just passive growth.
Comparison – Types of Tropism
| Tropism Type | Stimulus | Plant Response |
|---|---|---|
| Phototropism | Light | Shoots grow toward light |
| Geotropism | Gravity | Roots grow down, shoots grow up |
| Hydrotropism | Water | Roots grow toward moisture |
Why Is Hydrotropism Important?
- Helps plants find water for survival in dry or patchy soils
- Supports photosynthesis, transport, and temperature control
- Makes plants more drought-resistant
Role of Auxins in Plant Responses
What Are Auxins?
Auxins are plant hormones that control the growth of cells, especially in shoots and roots. They play a major role in tropisms – growth responses to environmental stimuli like light, gravity, and water.
Where Are Auxins Produced?
- Produced in the tips of shoots and roots
- They move through the plant to regulate growth direction
How Auxins Affect Growth
| Location | Auxin Effect |
|---|---|
| Shoots | Auxins promote cell growth |
| Roots | Auxins inhibit growth (in high amounts) |
Auxins in Phototropism (Response to Light)
- Light from one side → auxins move to the shaded side of the shoot
- Shaded cells grow faster → shoot bends toward the light
Auxins in Geotropism (Response to Gravity)
- Gravity pulls auxins to the lower side of the organ
- In roots → high auxin slows growth → root bends downward
- In shoots → high auxin speeds growth → shoot bends upward
Auxins in Hydrotropism (Response to Water)
- Roots detect water → auxins shift to the dry side
- Dry side grows slower → root bends toward water source
Summary Table – Role of Auxins
| Stimulus | Plant Part | Auxin Action | Result |
|---|---|---|---|
| Light | Shoot | Moves to shaded side, promotes growth | Bends toward light |
| Gravity | Root | Gathers on lower side, inhibits growth | Bends downward |
| Shoot | Gathers on lower side, promotes growth | Bends upward | |
| Water | Root | Shifts to dry side, slows growth | Root bends toward water |
Why Are Auxins Important?
- Help plants grow toward light and water, and grow correctly in gravity
- Increase survival by guiding growth direction
- Used in agriculture (e.g., rooting powders, weed control, tissue culture)